Article

Dark matter annihilation effects on the first stars

May 2008
Monthly Notices of the Royal Astronomical Society 390(4)

DOI:10.1111/j.1365-2966.2008.13853.x

Authors:

Alessandro Bressan

International School for Advanced Studies

Emanuele Ripamonti

Università degli Studi di Milano-Bicocca

Raffaella Schneider

Sapienza University of Rome

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We study the effects of weakly interacting massive particles (WIMPs) dark matter (DM) on the collapse and evolution of the first stars in the Universe. Using a stellar evolution code, we follow the pre-main-sequence (pre-MS) phase of a grid of metal-free stars with masses in the range 5 ≤M*≤ 600 M⊙ forming in the centre of a 106M⊙ halo at z= 20. DM particles of the parent halo are accreted in the protostellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of O(1012 GeV cm−3) at radii of the order of 10 au. Energy release from annihilation reactions can effectively counteract the gravitational collapse, in agreement with results from other groups. We find this stalling phase (known as a dark star) is transient and lasts from 2.1 × 103yr (M*= 600 M⊙) to 1.8 × 104yr (M*= 9 M⊙). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, , (ii) density, ρ, (iii) elastic scattering cross-section with baryons, σ0. For our fiducial set of parameters we find that the evolution of stars of mass M* < 40 M⊙‘freezes’ on the HR diagram before reaching the zero-age main sequence (ZAMS). Stars with M*≥ 40 M⊙ manage to ignite nuclear reactions; however, DM ‘burning’ prolongs their lifetimes by a factor of 2 (5) for a 600 M⊙ (40 M⊙) star. For ρ≳ 1012GeV cm−3, and same values of the other parameters, we find that all our models are entirely supported by DM annihilation and ‘freeze’ on the HR diagram before igniting nuclear reactions.

Analytic Approximations for the Velocity Suppression of Dark Matter Capture

Article

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Jun 2022

Compact astrophysical objects have been considered in the literature as dark matter (DM) probes, via the observational effects of annihilating captured DM. In this paper we investigate the role of stellar velocity on multiscatter-capture rates and find that the capture rates of DM by a star moving with respect to the DM halo rest frame are suppressed by a predictable amount. We develop and validate an analytical expression for the capture rate suppression factor. This suppression factor can be used to directly reevaluate projected bounds on the DM–nucleon cross section, for any given stellar velocity, as we explicitly show using Population III stars as DM probes. These objects (Population III stars) are particularly interesting candidates, since they form at high redshifts, in very high DM-density environments. We find that previous results, obtained under the assumption of a star at rest with respect to the DM rest frame, are essentially unchanged when considering the possible orbital velocities for those central stars.

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Dark stars: A review

Article

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May 2016
REP PROG PHYS

Dark Stars (DS) are stellar objects made (almost entirely) of ordinary atomic material but powered by the heat from Dark Matter (DM) annihilation (rather than by fusion). Weakly Interacting Massive Particles (WIMPs), among the best candidates for DM, can be their own antimatter and can accumulate inside the star, with their annihilation products thermalizing with and heating the DS. The resulting DSs are in hydrostatic and thermal equilibrium. The first phase of stellar evolution in the history of the Universe may have been dark stars. Though DM constituted only

<0.1\%

of the mass of the star, this amount was sufficient to power the star for millions to billions of years. Depending on their DM environment, early DSs can become very massive (

>10^6 M_\odot

), very bright (

>10^9 L_\odot

), and potentially detectable with the James Webb Space Telescope (JWST). Once the DM runs out and the dark star dies, it may collapse to a black hole; thus DSs can provide seeds for the supermassive black holes observed throughout the Universe and at early times. Other sites for dark star formation exist in the Universe today in regions of high dark matter density such as the centers of galaxies. The current review briefly discusses DSs existing today but focuses on the early generation of dark stars.

Neutrino point source searches for dark matter spikes

Article

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Aug 2022
J COSMOL ASTROPART P

Any dark matter spikes surrounding black holes in our Galaxy are sites of significant dark matter annihilation, leading to a potentially detectable neutrino signal. In this paper we examine 10 - 10 ⁵ M ⊙ black holes associated with dark matter spikes that formed in early minihalos and still exist in our Milky Way Galaxy today, in light of neutrino data from the ANTARES [1] and IceCube [2] detectors. In various regions of the sky, we determine the minimum distance away from the solar system that a dark matter spike must be in order to have not been detected as a neutrino point source for a variety of representative dark matter annihilation channels. Given these constraints on the distribution of dark matter spikes in the Galaxy, we place significant limits on the formation of the first generation of stars in early minihalos — stronger than previous limits from gamma-ray searches in Fermi Gamma-Ray Space Telescope data. The larger black holes considered in this paper may arise as the remnants of Dark Stars after the dark matter fuel is exhausted; thus neutrino observations may be used to constrain the properties of Dark Stars. The limits are particularly strong for heavier WIMPs. For WIMP masses ∼ 5TeV, we show that ≲ 10 % of minihalos can host first stars that collapse into BHs larger than 10 ³ M ⊙ .

Dark stars powered by self-interacting dark matter

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Aug 2022

Dark matter annihilation might power the first luminous stars in the Universe. These types of stars, known as dark stars, could form in (106–108) M⊙ protohalos at redshifts z∼20, and they could be much more luminous and larger in size than ordinary stars powered by nuclear fusion. We investigate the formation of dark stars in the self-interacting dark matter (SIDM) scenario. We present a concrete particle physics model of SIDM that can simultaneously give rise to the observed dark matter density, satisfy constraints from astrophysical and terrestrial searches, and address the various small-scale problems of collisionless dark matter via the self-interactions. In this model, the power from dark matter annihilation is deposited in the baryonic gas in environments where dark stars could form. We further study the evolution of SIDM density profiles in the protohalos at z∼20. As the baryon cloud collapses due to the various cooling processes, the deepening gravitational potential can speed up gravothermal evolution of the SIDM halo, yielding sufficiently high dark matter densities for dark stars to form. We find that SIDM-powered dark stars can have similar properties, such as their luminosity and size, as dark stars predicted in collisionless dark matter models.

Constraining dark matter properties with the first generation of stars

Article

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Dec 2021

Dark matter (DM) can be trapped by the gravitational field of any star, since collisions with nuclei in dense environments can slow down the DM particle below the escape velocity (vesc) at the surface of the star. If captured, the DM particles can self-annihilate, and, therefore, provide a new source of energy for the star. We investigate this phenomenon for capture of DM particles by the first generation of stars [Population III (Pop III) stars], by using the multiscatter capture formalism. Pop III stars are particularly good DM captors, since they form in DM-rich environments, at the center of ∼106 M⊙ DM minihalos, at redshifts z∼15. Assuming a DM-proton scattering cross section (σ) at the current deepest exclusion limits provided by the XENON1T experiment, we find that captured DM annihilations at the core of Pop III stars can lead, via the Eddington limit, to upper bounds in stellar masses that can be as low as a few M⊙ if the ambient DM density (ρX) at the location of the Pop III star is sufficiently high. Conversely, when Pop III stars are identified, one can use their observed mass (M⋆) to place bounds on ρXσ. Using adiabatic contraction to estimate the ambient DM density in the environment surrounding Pop III stars, we place projected upper limits on σ, for M⋆ in the 100 M⊙–1000 M⊙ range, and find bounds that are competitive with, or deeper than, those provided by the most sensitive current direct detection experiments for both spin-independent and spin-dependent (SD) interactions, for a wide range of DM masses. Most intriguingly, we find that Pop III stars with mass M⋆≳300 M⊙ could be used to probe the SD proton-DM cross section below the “neutrino floor,” i.e. the region of parameter space where DM direct detection experiments will soon become overwhelmed by neutrino backgrounds.

Multiscatter capture of superheavy dark matter by Pop III stars

Article

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Dec 2019
J COSMOL ASTROPART P

If captured by the gravitational field of stars or other compact objects, dark matter can self-annihilate and produce a potentially detectable particle flux. In the case of superheavy dark matter ( mX ≳ 10⁸ GeV), a large number of scattering events with nuclei inside stars are necessary to slow down the dark matter particles below the escape velocity of the stars, at which point the Dark Matter (DM) particle becomes trapped, or captured. Using the recently developed analytical formalism for multiscatter capture, combined with the latest results on the constraints of dark-matter-baryon scattering cross-section, we calculate upper bounds on the capture rates for superheavy dark matter particles by the first (Pop. III) stars. Assuming that a non-zero fraction of the products of captured superheavy dark matter (SHDM) annihilations can be trapped and thermalized inside the star, we find that this additional heat source could influence the evolutionary phase of Pop. III stars. Moreover, requiring that Pop. III stars shine with sub-Eddington luminosity, we find upper bounds on the masses of the Pop. III stars. This implies a DM dependent cutoff on the initial mass function (IMF) of Pop. III stars, thus opening up the intriguing possibility of constraining DM properties using the IMF of extremely metal-poor stars.

Multiscatter capture of superveavy dark matter by Pop III stars

Preprint

Full-text available

Aug 2019

If captured by the gravitational field of stars or other compact objects, dark matter can self-annihilate and produce a potentially detectable particle flux. In the case of superheavy dark matter (

m_{X} \gtrsim 10^{8} GeV

), a large number of scattering events with nuclei inside stars are necessary to slow down the dark matter particles below the escape velocity of the stars, at which point the Dark Matter (DM) particle becomes trapped, or captured. Using the recently developed analytical formalism for multiscatter capture, combined with the latest results on the constraints of dark-matter-baryon scattering cross-section, we calculate upper bounds on the capture rates for superheavy dark matter particles by the first (Pop. III) stars. Assuming that a non-zero fraction of the products of captured superheavy dark matter (SHDM) annihilations can be trapped and thermalized inside the star we find that this additional heat source could influence the evolutionary phase of Pop. III stars. Moreover, requiring that Pop. III stars shine with sub-Eddington luminosity, we find upper bounds on the masses of the Pop. III stars. This implies a DM dependent cutoff on the initial mass function (IMF) of Pop. III stars, thus opening up the intriguing possibility of constraining DM properties using the IMF of extremely metal-poor stars.

Can Dark Stars Account for the Star Formation Efficiency Excess at Very High Redshifts?

Article

Full-text available

Feb 2025
ASTROPHYS J

The James Webb Space Telescope (JWST) has recently conducted observations of massive galaxies at high redshifts, revealing a notable anomaly in their star formation efficiency (SFE). Motivated by the recent identification of three ~10 ⁶ M ⊙ dark star candidates, we investigate whether dark stars can be the origin of the SFE excess. It turns out that the excess can be reproduced by a group of dark stars with M ≳ 10 ³ M ⊙ , because of their domination in generating primary UV radiation in high-redshift galaxies. The genesis of these dark stars is attributed to the capture of weakly interacting massive particles within a mass range of tens of gigaelectronvolts to a few teraelectronvolts. However, if the top-heavy initial mass function of dark stars holds up to ~10 ⁵ M ⊙ , the relic black holes stemming from their collapse would be too abundant to be consistent with the current observations of massive compact halo objects. We thus suggest that just a small fraction of SFE excess may be contributed by the very massive dark stars, with the majority likely originating from other sources, such as the Population III stars, in view of their rather similar UV radiation efficiencies.

Can Dark Stars account for the star formation efficiency excess at very high redshifts?

Preprint

Full-text available

Jan 2025

\sim 10^{6}M_\odot

dark star candidates, we investigate whether dark stars can be the origin of the SFE excess. It turns out that the excess can be reproduced by a group of dark stars with

M \gtrsim 10^{3}\, \rm M_{\odot}

, because of their domination in generating primary UV radiation in high-redshift galaxies. The genesis of these dark stars is attributed to the capture of Weakly Interacting Massive Particles (WIMPs) within a mass range of tens of GeV to a few TeV. However, if the top-heavy initial mass function of dark stars holds up to

\sim 10^{5}M_\odot

, the relic black holes stemming from their collapse would be too abundant to be consistent with the current observations of Massive Compact Halo Objects (MACHOs). We thus suggest that just a small fraction of SFE excess may be contributed by the very massive dark stars and the majority likely originated from other reasons such as the Population III stars in view of their rather similar UV radiation efficiencies.

Black hole formation from axion stars

Preprint

Sep 2016

The classical equations of motion for an axion with potential

V(\phi)=m_a^2f_a^2 [1-\cos (\phi/f_a)]

possess quasi-stable, localized, oscillating solutions, which we refer to as "axion stars". We study, for the first time, collapse of axion stars numerically using the full non-linear Einstein equations of general relativity and the full non-perturbative cosine potential. We map regions on an "axion star stability diagram", parameterized by the initial ADM mass,

M_{\rm ADM}

, and axion decay constant,

f_a

. We identify three regions of the parameter space: i) long-lived oscillating axion star solutions, with a base frequency,

m_a

, modulated by self-interactions, ii) collapse to a BH and iii) complete dispersal due to gravitational cooling and interactions. We locate the boundaries of these three regions and an approximate "triple point"

(M_{\rm TP},f_{\rm TP})\sim (2.4 M_{pl}^2/m_a,0.3 M_{pl})

. For

f_a

below the triple point BH formation proceeds during winding (in the complex U(1) picture) of the axion field near the dispersal phase. This could prevent astrophysical BH formation from axion stars with

f_a\ll M_{pl}

. For larger

f_a\gtrsim f_{\rm TP}

, BH formation occurs through the stable branch and we estimate the mass ratio of the BH to the stable state at the phase boundary to be

\mathcal{O}(1)

within numerical uncertainty. We discuss the observational relevance of our findings for axion stars as BH seeds, which are supermassive in the case of ultralight axions. For the QCD axion, the typical BH mass formed from axion star collapse is

M_{\rm BH}\sim 3.4 (f_a/0.6 M_{pl})^{1.2} M_\odot

Heating of galactic gas by dark matter annihilation in ultracompact minihalos

Preprint

Nov 2016

The existence of substructure in halos of annihilating dark matter would be expected to substantially boost the rate at which annihilation occurs. Ultracompact minihalos of dark matter (UCMHs) are one of the more extreme examples of this. The boosted annihilation can inject significant amounts of energy into the gas of a galaxy over its lifetime. Here we determine the impact of the boost factor from UCMH substructure on the heating of galactic gas in a Milky Way-type galaxy, by means of N-body simulation. If

1\%

of the dark matter exists as UCMHs, the corresponding boost factor can be of order

10^5

. For reasonable values of the relevant parameters (annihilation cross section

3\times10^{-26} ~\textrm{cm}^3~ \textrm{s}^{-1}

, dark matter mass 100 GeV, 10% heating efficiency), we show that the presence of UCMHs at the 0.1% level would inject enough energy to eject significant amounts of gas from the halo, potentially preventing star formation within

\sim

1 kpc of the halo centre.

Impact of Sub-MeV Dark Matter on the Cooling of Pulsating White Dwarfs

Preprint

Full-text available

Nov 2024

In our galaxy, white dwarfs inevitably undergo scattering and capture processes with the interstellar diffuse dark matter. The captured dark matter forms a dark halo that eventually evaporates or annihilates. Theoretical pulsation modes and observations of pulsating white dwarfs provide predictions about their evolution. This motivates us to study the impact of sub-MeV interstellar dark matter on the cooling processes of white dwarfs. In this work, we consider the collisions between dark matter and relativistic degenerate electrons inside white dwarfs, numerically calculating the energy input and output results from scattering, capture, evaporation, and annihilation processes. Based on observational data from G117-B15, we conclude that the maximum cooling luminosity of the interstellar sub-MeV dark matter is approximately

10^{22} \, \text{erg}/\text{s}

, which is insufficient to provide an effective cooling mechanism for white dwarfs. Finally, if future observations detect a pulsating white dwarf in the Galactic center, the potential sensitivity of this scenario could extend to the region

10^{-3}\,\text{MeV} < m_\chi < 10 \, \text{MeV}

and

6.02 \times 10^{-38}\,\text{cm}^2 > \sigma_0 \geq 1.5 \times 10^{40} \, \text{cm}^2

Dark matter limits from the tip of the red giant branch of globular clusters

Article

Full-text available

Nov 2024

Capture and annihilation of weakly interacting massive particle (WIMP)-like dark matter in red giant stars can lead to faster-than-expected ignition of the helium core, and thus a lower tip of the red giant branch (TRGB) luminosity. We use data to place constraints on the dark matter-nucleon cross section using 22 globular clusters with measured TRGB luminosities, and place projections on the sensitivity resulting from 161 clusters with full phase space distributions observed by . Although limits remain weaker than those from Earth-based direct detection experiments, they represent a constraint that is fully independent of dark matter properties in the solar neighborhood, probing its properties across the entire Milky Way galaxy. Based on our findings, it is likely that the use of the TRGB as a standard candle in H 0 measurements is very robust against the effects of dark matter. Published by the American Physical Society 2024

Dark matter limits from the tip of the red giant branch of globular clusters

Preprint

Jul 2024

Capture and annihilation of WIMP-like dark matter in red giant stars can lead to faster-than-expected ignition of the helium core, and thus a lower tip of the red giant branch (TRGB) luminosity. We use Gaia data to place constraints on the dark matter-nucleon cross section using 22 globular clusters with measured TRGB luminosities, and place projections on the sensitivity resulting from 161 clusters with full phase space distributions observed by Gaia. Although limits remain weaker than those from Earth-based direct detection experiments, they represent a constraint that is fully independent of dark matter properties in the Solar neighbourhood, probing its properties across the entire Milky Way galaxy. Based on our findings, it is likely that the use of the TRGB as a standard candle in

H_0

measurements is very robust against the effects of dark matter.

Detectability of Supermassive Dark Stars with the Roman Space Telescope

Article

Full-text available

Apr 2024

Supermassive dark stars (SMDS) are luminous stellar objects formed in the early Universe at redshift z ∼ 10–20, made primarily of hydrogen and helium, yet powered by dark matter. We examine the capabilities of the Roman Space Telescope (RST), and find it able to identify ∼10 ⁶ M ⊙ SMDSs at redshifts up to z ≃ 14. With a gravitational lensing factor of μ ∼ 100, RST could identify SMDS as small as ∼10 ⁴ M ⊙ at z ∼ 12 with ∼10 ⁶ s exposure. Differentiating SMDSs from early galaxies containing zero metallicity stars at similar redshifts requires spectral, photometric, and morphological comparisons. With only RST, the differentiation of SMDS, particularly those formed via adiabatic contraction with M ≳ 10 ⁵ M ⊙ and lensed by μ ≳ 100, is possible due to their distinct photometric signatures from the first galaxies. Those formed via dark matter capture can be differentiated only by image morphology: i.e., point object (SMDSs) versus extended object (sufficiently magnified galaxies). By additionally employing James Webb Space Telescope (JWST) spectroscopy, we can identify the He ii λ 1640 absorption line, a smoking gun for SMDS detection. Although RST does not cover the required wavelength band (for z emi ≳ 10), JWST does; hence, the two can be used in tandem to identify SMDS. The detection of SMDS would confirm a new type of star powered by dark matter and may shed light on the origins of the supermassive black holes powering bright quasars observed at z ≳ 6.

Probing Dark Sectors with Neutron Stars

Article

Full-text available

Feb 2024

Tensions in the measurements of neutron and kaon weak decays, such as of the neutron lifetime, may speak to the existence of new particles and dynamics not present in the Standard Model (SM). In scenarios with dark sectors, particles that couple feebly to those of the SM appear. We offer a focused overview of such possibilities and describe how the observations of neutron stars, which probe either their structure or dynamics, limit them. In realizing these constraints, we highlight how the assessment of particle processes within dense baryonic matter impacts the emerging picture—and we emphasize both the flavor structure of the constraints and their broader connections to cogenesis models of dark matter and baryogenesis.

Detectability of Supermassive Dark Stars with the Roman Space Telescope

Preprint

Full-text available

Jun 2023

The first bright objects to form in the Universe at redshift

z \sim 10-20

might have been Dark Stars, made primarily of hydrogen and helium but powered by dark matter. In this study, we investigate the detectability of Supermassive Dark Stars (SMDS) by the Roman Space Telescope. RST will be able to detect SMDSs at redshifts as high as

z\simeq 14

. In cases with gravitational lensing factors of

\mu\sim 100

, RST will be able to find SMDS as small as

\sim10^4 M_{\odot}

z\sim 12

with

\sim 10^6

s of exposure. To differentiate SMDS from early galaxies containing zero metallicity stars at similar redshifts, we compare their spectra, photometry in RST bands, color indexes and image morphology. With RST alone, the differentiation is possible only for limited cases: SMDS formed via "adiabatic contraction" (DM pulled into the star via gravity alone) with

M\gtrsim 10^5M_{\odot}

and lensed by

\mu\gtrsim 30

have distinct photometric signatures from those of the first galaxies. For SMDSs formed via "dark matter capture," their spectra are degenerate to those of many galaxies with little to no nebular emission. Thus with RST alone, the only way to tell them apart from first galaxies would be via image morphology: i.e. point object (SMDSs) vs. extended object (sufficiently magnified galaxies). However, if the same objects are further examined by JWST spectroscopy, a "smoking gun" for detection of SMDS is the HeII

\lambda

1640 absorption line. While RST does not cover the wavelength band required to find this line (for

z_{\rm emi}\gtrsim 10

), JWST does. Hence the two detectors can be used together in identifying SMDS. The confirmed detection of any SMDSs will provide evidence for a new type of star, powered by dark matter. Moreover, such massive stars can also be natural progenitors of the supermassive black holes powering the extremely bright quasars observed at

z\gtrsim 6

Neutrino point source searches for dark matter spikes

Preprint

Feb 2022

Any dark matter spikes surrounding black holes in our Galaxy are sites of significant dark matter annihilation, leading to a potentially detectable neutrino signal. In this paper we examine

10-10^5 M_\odot

black holes associated with dark matter spikes that formed in early minihalos and still exist in our Milky Way Galaxy today, in light of neutrino data from the ANTARES and IceCube detectors. In various regions of the sky, we determine the minimum distance away from the solar system that a dark matter spike must be in order to have not been detected as a neutrino point source for a variety of representative dark matter annihilation channels. Given these constraints on the distribution of dark matter spikes in the Galaxy, we place significant limits on the formation of the first generation of stars in early minihalos -- stronger than previous limits from gamma-ray searches in Fermi Gamma-Ray Space Telescope data. The larger black holes considered in this paper may arise as the remnants of Dark Stars after the dark matter fuel is exhausted; thus neutrino observations may be used to constrain the properties of Dark Stars. The limits are particularly strong for heavier WIMPs. For WIMP masses

\sim 5 \,

TeV, we show that

\lesssim 10 \%

of minihalos can host first stars that collapse into BHs larger than

10^3 M_\odot

The Effect of Stellar Velocity on Dark Matter Capture and Detection with Population III Stars

Preprint

Jul 2021

Compact astrophysical objects have been considered in the literature as dark matter (DM) probes, via the observational effects of annihilating captured DM. In this respect, Population III (Pop III) stars are particularly interesting candidates, since they form at high redshifts, in very high DM density environments. It is customary to assume such a star would form roughly at the center of a DM halo, and, as such, have no rotational velocity. In this paper, we break from this assumption and explore the effects we can expect to observe if a Pop III star forms at some distance away from the center of the halo and thus has a non-zero rotational velocity. The capture rate of DM in such a star is suppressed by a predictable amount. We develop and validate an analytical expression for the capture rate suppression factor and re-evaluate the bounds placed on the DM-nucleon cross section as a result of DM capture. We find that our previous results, obtained under the assumption of star formed within the central 10 AU of the DM mini-halo are essentially unchanged, even when considering the possible rotational velocities for those central stars.

Multi-component multiscatter capture of Dark Matter

Preprint

May 2021

In recent years, the usefulness of astrophysical objects as Dark Matter (DM) probes has become more and more evident, especially in view of null results from direct detection and particle production experiments. The potentially observable signatures of DM gravitationally trapped inside a star, or another compact astrophysical object, have been used to forecast stringent constraints on the nucleon-Dark Matter interaction cross section. Currently, the probes of interest are: at high red-shifts, Population III stars that form in isolation, or in small numbers, in very dense DM minihalos at

z\sim 15-40

, and, in our own Milky Way, neutron stars, white dwarfs, brown dwarfs, exoplanets, etc. Of those, only neutron stars are single-component objects, and, as such, they are the only objects for which the common assumption made in the literature of single-component capture, i.e. capture of DM by multiple scatterings with one single type of nucleus inside the object, is valid. In this paper, we present an extension of this formalism to multi-component objects and apply it to Pop III stars, thereby investigating the role of He on the capture rates of Pop III stars. As expected, we find that the inclusion of the heavier He nuclei leads to an enhancement of the overall capture rates, further improving the potential of Pop III stars as Dark Matter probes.

Towards a more rigorous treatment of uncertainties on the velocity distribution of dark matter particles for capture in stars

Preprint

Jul 2020

Dark matter (DM) capture in stars offers a rich phenomenology that makes it possible to probe a wide variety of particle DM scenarios in diverse astrophysical environments. In spite of decades of improvements to refine predictions of capture-related observables and better quantify astrophysical and particle-physics uncertainties, the actual impact of the Galactic phase-space distribution function of DM has been overlooked. In this work, we tackle this problem by making use of self-consistent equilibrium phase-space models based on the Eddington inversion formalism and an extension of this method to a DM halo with some degree of anisotropy in velocity space. We demonstrate that incorrectly accounting for the variation of the DM velocity distribution with position in the Galaxy leads to a systematic error between a factor two and two orders of magnitude, depending in particular on the target star, the DM candidate mass and the type of interaction involved. Moreover, we show that underlying phase-space properties, such as the anisotropy of the velocity tensor, actually play an important part---previously disregarded---and can have a sizable impact on predictions of capture rates and subsequent observables. We argue that Eddington-like methods, which self-consistently account for kinematic constraints on the components of the Galaxy, actually provide a reliable next-to-minimal approach to narrow down uncertainties from phase-space modeling on predictions of observables related to DM capture in stars.

Formation of the first stars

Preprint

Jul 2018

Ralf S. Klessen

From studying the cosmic microwave background, we know our Universe started out very simple. It was by and large homogeneous and isotropic, with small fluctuations that can be described by linear perturbation theory. In stark contrast, the Universe today is highly structured on a vast range of length and mass scales. In the evolution towards increasing complexity, the formation of the first stars marks a primary transition event. The first generation of stars, the so-called Population III (or Pop. III) build up from truly metal-free primordial gas. They have long been thought to live short, solitary lives, with only one massive star forming per halo. However, in recent years this simple picture has undergone substantial revision, and we now understand that stellar birth in the early Universe is subject to the same complexity as star formation at present days. In this chapter, I review the current state of the field. I begin by introducing the basics concepts of star-formation theory and by discussing the typical environment in which Pop. III stars are thought to form. Then I argue that the accretion disk that builds up in the center of a halo is likely to fragment, resulting in the formation of a cluster of stars with a wide range of masses, and I speculate about how this process may be influenced by stellar feedback, the presence of magnetic fields, the energy input from dark matter annihilation, and the occurrence of large- scale streaming velocities between baryons and dark matter. Finally, I discuss direct and indirect constraints on Pop. III star formation from high-redshift observations and from the search for extremely metal-poor stars in the Milky Way and its satellites.

Heating of galactic gas by dark matter annihilation in ultracompact minihalos

Article

Full-text available

May 2017
J COSMOL ASTROPART P

1\%

of the dark matter exists as UCMHs, the corresponding boost factor can be of order

10^5

. For reasonable values of the relevant parameters (annihilation cross section

3\times10^{-26} ~\textrm{cm}^3~ \textrm{s}^{-1}

\sim

1 kpc of the halo centre.

Black hole formation from axion stars

Article

Full-text available

Mar 2017
J COSMOL ASTROPART P

The classical equations of motion for an axion with potential

V(\phi)=m_a^2f_a^2 [1-\cos (\phi/f_a)]

M_{\rm ADM}

, and axion decay constant,

f_a

. We identify three regions of the parameter space: i) long-lived oscillating axion star solutions, with a base frequency,

m_a

(M_{\rm TP},f_{\rm TP})\sim (2.4 M_{pl}^2/m_a,0.3 M_{pl})

. For

f_a

f_a\ll M_{pl}

. For larger

f_a\gtrsim f_{\rm TP}

, BH formation occurs through the stable branch and we estimate the mass ratio of the BH to the stable state at the phase boundary to be

\mathcal{O}(1)

M_{\rm BH}\sim 3.4 (f_a/0.6 M_{pl})^{1.2} M_\odot

Generalised form factor dark matter in the Sun

Article

Full-text available

Aug 2015
J COSMOL ASTROPART P

We study the effects of energy transport in the Sun by asymmetric dark matter with momentum and velocity-dependent interactions, with an eye to solving the decade-old Solar Abundance Problem. We study effective theories where the dark matter-nucleon scattering cross-section goes as

v_{\rm rel}^{2n}

and

q^{2n}

with

n = -1, 0, 1

or 2, where

v_{\rm rel}

is the dark matter-nucleon relative velocity and q is the momentum exchanged in the collision. Such cross-sections can arise generically as leading terms from the most basic nonstandard DM-quark operators. We employ a high-precision solar simulation code to study the impact on solar neutrino rates, the sound speed profile, convective zone depth, surface helium abundance and small frequency separations. We find that the majority of models that improve agreement with the observed sound speed profile and depth of the convection zone also reduce neutrino fluxes beyond the level that can be reasonably accommodated by measurement and theory errors. However, a few specific points in parameter space yield a significant overall improvement. A 3-5 GeV DM particle with

\sigma_{SI} \propto q^2

is particularly appealing, yielding more than a

6\sigma

improvement with respect to standard solar models, while being allowed by direct detection and collider limits. We provide full analytical capture expressions for q- and

v_{\rm rel}

-dependent scattering, as well as complete likelihood tables for all models.

Studies of dark matter in and around stars

Article

Sofia Magdalena Sivertsson

Thermal conduction by dark matter with velocity and momentum-dependent cross-sections

Article

Full-text available

Apr 2014
J COSMOL ASTROPART P

We use the formalism of Gould and Raffelt [1] to compute the dimensionless thermal conduction coefficients for scattering of dark matter particles with standard model nucleons via cross-sections that depend on the relative velocity or momentum exchanged between particles. Motivated by models invoked to reconcile various recent results in direct detection, we explicitly compute the conduction coefficients

\alpha

and

\kappa

for cross-sections that go as

v_{\rm rel}^2

v_{\rm rel}^4

v_{\rm rel}^{-2}

q^2

q^4

and

q^{-2}

, where

v_{\rm rel}

is the relative DM-nucleus velocity and q is the momentum transferred in the collision. We find that a

v_{\rm rel}^{-2}

dependence can significantly enhance energy transport from the inner solar core to the outer core. The same can true for any q-dependent coupling, if the dark matter mass lies within some specific range for each coupling. This effect can complement direct searches for dark matter; combining these results with state-of-the-art Solar simulations should greatly increase sensitivity to certain DM models. It also seems possible that the so-called Solar Abundance Problem could be resolved by enhanced energy transport in the solar core due to such velocity- or momentum-dependent scatterings.

Updated constraints on velocity and momentum-dependent asymmetric dark matter

Preprint

May 2016

We present updated constraints on dark matter models with momentum-dependent or velocity-dependent interactions with nuclei, based on direct detection and solar physics. We improve our previous treatment of energy transport in the solar interior by dark matter scattering, leading to significant changes in fits to many observables. Based on solar physics alone, DM with a spin-independent

q^{4}

coupling provides the best fit to data, and a statistically satisfactory solution to the solar abundance problem. Once direct detection limits are accounted for however, the best solution is spin-dependent

v^2

scattering with a reference cross-section of 10

^{-35}

^2

(at a reference velocity of

v_0=220

km s

^{-1}

), and a dark matter mass of about 5 GeV.

Dark matter annihilation and pair-instability supernovae

Article

May 2024

We study the evolution of heavy stars (M≥40M⊙) undergoing pair-instability in the presence of annihilating dark matter. Focusing on the scenario where the dark matter is in capture-annihilation equilibrium, we model the profile of energy injections in the local thermal equilibrium approximation. We find that significant changes to masses of astrophysical black holes formed by (pulsational) pair-instability supernovae can occur when the ambient dark matter density ρDM≳109 GeV cm−3. There are two distinct outcomes, depending on the dark matter mass. For masses mDM≳1 GeV the DM is primarily confined to the core. The annihilation increases the lifetime of core helium burning, resulting in more oxygen being formed, fueling a more violent explosion during the pair-instability-induced contraction. This drives stronger pulsations, leading to lighter black holes being formed than predicted by the standard model. For masses mDM≲0.5 GeV there is significant dark matter in the envelope, leading to a phase where the star is supported by the energy from the annihilation. This reduces the core temperature and density, allowing the star to evade the pair-instability allowing heavier black holes to be formed. We find a mass gap for all models studied.

Compatibility of JWST results with exotic halos

Article

Apr 2024

Different Methods to Constrain Dark Matter

Thesis

Jan 2022

Youjia Wu

The currently favored cosmological model suggests that over 85% of the matter in our universe is dark, yet the existence of dark matter is still to be confirmed by detecting it through interactions with normal matter. Direct detection experiments hope to observe signals from the scattering of dark matter particles off of cryogenic target nuclei. A null result from direct detection leads to an exclusion curve in the cross section-dark matter particle mass parameter space. Theoretical predictions for exclusion curves involve standard halo model, in which three astrophysical parameters are assumed to control the distribution of dark matter in the Milky Way. This thesis first discusses the uncertainties in these three parameters on the exclusion curve from the XENON1T experiment. Our estimate done with Monte Carlo simulations shows that at a low WIMP mass, the uncertainty in cross section can span six orders of magnitude. Dark matter self-annihilation might power the first-generation stars and form Dark Stars. The possibility of Dark Stars was originally proposed in the context of Weakly Interacting Massive Particle (WIMP) model. Although the WIMP model is successful in explaining large structures in the universe, it faces difficulties when applied to structures as small as dwarf galaxies. To overcome the small-structure problems, self-interactions between dark matter particles are introduced and the Self-Interacting Dark Matter (SIDM) model was proposed. In the second part of this thesis, we evaluate the probability that Dark Stars can be powered by SIDM. We first propose a simple particle physics model of SIDM that satisfies all the current constraints, and work out the phase space region in which Dark Stars can form. Then we investigate the gravothermal evolution of SIDM minihalos in the presence of a gas potential, and investigate whether it can lead to a sufficiently high dark matter density for Dark Stars to form. Finally, we present the first study of the properties of Dark Stars assuming they can reach hydrostatic equilibrium. Dark matter is a major player in the formation of Milky Way-like galaxies. Different dark matter models lead to a different accretion history of Milky Way-like galaxies. This thesis finally studies the recent accretion history of Milky Way-like galaxies using statistical cluster analysis. Stars from the same accreted satellite galaxy are clustered in action space. Since actions are conserved in slow enough gravitational evolution, the accreted satellites should remain clustered until today. We apply the cluster analysis algorithm Enlink to accreted star particles in action space from the halos of three simulated Milky Way-like galaxies in the FIRE-2 simulations. We compare the groups found by our cluster analysis with the actual accreted satellites from these galaxies, and find the well-recovered satellites. The results show that the member stars of satellites which fell into the galaxy less than 7.1 Gyr ago and were more massive than 4.0 times 10^8 solar mass can be well recovered by cluster analysis.

Evaporation of dark matter from celestial bodies

Article

Full-text available

May 2022
J COSMOL ASTROPART P

Scatterings of galactic dark matter (DM) particles with the constituents of celestial bodies could result in their accumulation within these objects. Nevertheless, the finite temperature of the medium sets a minimum mass, the evaporation mass, that DM particles must have in order to remain trapped. DM particles below this mass are very likely to scatter to speeds higher than the escape velocity, so they would be kicked out of the capturing object and escape. Here, we compute the DM evaporation mass for all spherical celestial bodies in hydrostatic equilibrium, spanning the mass range [10 ⁻¹⁰ - 10 ² ] M ⊙ , for constant scattering cross sections and s -wave annihilations. We illustrate the critical importance of the exponential tail of the evaporation rate, which has not always been appreciated in recent literature, and obtain a robust result: for the geometric value of the scattering cross section and for interactions with nucleons, at the local galactic position, the DM evaporation mass for all spherical celestial bodies in hydrostatic equilibrium is approximately given by E c /T χ ∼ 30, where E c is the escape energy of DM particles at the core of the object and T χ is their temperature. In that case, the minimum value of the DM evaporation mass is obtained for super-Jupiters and brown dwarfs, m evap ≃ 0.7 GeV. For other values of the scattering cross section, the DM evaporation mass only varies by a factor smaller than three within the range 10 ⁻⁴¹ cm ² ≤ σ p ≤ 10 ⁻³¹ cm ² , where σ p is the spin-independent DM-nucleon scattering cross section. Its dependence on parameters such as the galactic DM density and velocity, or the scattering and annihilation cross sections is only logarithmic, and details on the density and temperature profiles of celestial bodies have also a small impact.

Multicomponent multiscatter capture of dark matter

Article

Full-text available

Oct 2021

In recent years, the usefulness of astrophysical objects as dark matter (DM) probes has become more and more evident, especially in view of null results from direct-detection and particle-production experiments. The potentially observable signatures of DM gravitationally trapped inside a star, or another compact astrophysical object, have been used to forecast stringent constraints on the nucleon–dark matter interaction cross section. Currently, the probes of interest are at high redshifts, Population III (Pop III) stars that form in isolation or in small numbers, in very dense DM minihalos at z∼15–40, and, in our own Milky Way, neutron stars, white dwarfs, brown dwarfs, exoplanets, etc. None of these objects are truly single component and, as such, capture rates calculated with the common assumption made in the literature of single-component capture, i.e., capture of DM by multiple scatterings with one single type of nucleus inside the object, are not accurate. In this paper, we present an extension of this formalism to multicomponent objects and apply it to Pop III stars, thereby investigating the role of He in the capture rates of Pop III stars. As expected, we find that the inclusion of the heavier He nuclei leads to an enhancement of the overall capture rates, further improving the potential of Pop III stars as dark matter probes.

Towards a more rigorous treatment of uncertainties on the velocity distribution of dark matter particles for capture in stars

Article

Full-text available

Jan 2021
J COSMOL ASTROPART P

Dark matter (DM) capture in stars offers a rich phenomenology that makes it possible to probe a wide variety of particle DM scenarios in diverse astrophysical environments. In spite of decades of improvements to refine predictions of capture-related observables and better quantify astrophysical and particle-physics uncertainties, the actual impact of the Galactic phase-space distribution function of DM has been overlooked. In this work, we tackle this problem by making use of self-consistent equilibrium phase-space models based on the Eddington inversion formalism and an extension of this method to a DM halo with some degree of anisotropy in velocity space. We demonstrate that incorrectly accounting for the variation of the DM velocity distribution with position in the Galaxy leads to a systematic error between a factor two and two orders of magnitude, depending in particular on the target star, the DM candidate mass and the type of interaction involved. Moreover, we show that underlying phase-space properties, such as the anisotropy of the velocity tensor, actually play an important part—previously disregarded—and can have a sizable impact on predictions of capture rates and subsequent observables. We argue that Eddington-like methods, which self-consistently account for kinematic constraints on the components of the Galaxy, actually provide a reliable next-to-minimal approach to narrow down uncertainties from phase-space modeling on predictions of observables related to DM capture in stars.

Formation of the First Stars and Black Holes

Article

Full-text available

Apr 2020
SPACE SCI REV

We review the current status of knowledge concerning the early phases of star formation during cosmic dawn. This includes the first generations of stars forming in the lowest mass dark matter halos in which cooling and condensation of gas with primordial composition is possible at very high redshift (

z > 20

), namely metal-free Population III stars, and the first generation of massive black holes forming at such early epochs, the so-called black hole seeds. The formation of black hole seeds as end states of the collapse of Population III stars, or via direct collapse scenarios, is discussed. In particular, special emphasis is given to the physics of supermassive stars as potential precursors of direct collapse black holes, in light of recent results of stellar evolution models, and of numerical simulations of the early stages of galaxy formation. Furthermore, we discuss the role of the cosmic radiation produced by the early generation of stars and black holes at high redshift in the process of reionization.

Formation of the first stars and black holes

Preprint

Mar 2020

z > 20

Impacts of WIMP dark matter upon stellar evolution: main-sequence stars

Conference Paper

Full-text available

Aug 2009

Características de la producción científica de instituciones de Puebla indexada en la Web of Science

Article

Full-text available

Jan 2018

This article describes the scope and characteristics of the research developed in the state of Puebla, Mexico, whose results have been published in journals indexed in the Web of Science. The analyzed corpus was compiled by the search procedure with the word “puebla” in the organization and address fields, restricted to the period 2008-2012. We observed from the data obtained that the Universidad Autónoma de Puebla and the Instituto Nacional de Astrofísica, Óptica y Electrónica are the most productive institutions of Puebla. Our focus was on the 102 most cited articles of the period. Scientific production of Puebla institutions focuses on the physical and biomedical sciences and this production derives mostly from international collaborations. The multiauthor articles (at least three authors) are the most cited publications, while the individual research papers have little impact according to their number of citations. Finally, we analyze the differences in impact factor across scientific fields.

Updated constraints on velocity and momentum-dependent asymmetric dark matter

Article

Full-text available

Nov 2016
J COSMOL ASTROPART P

q^{4}

v^2

scattering with a reference cross-section of 10

^{-35}

^2

(at a reference velocity of

v_0=220

km s

^{-1}

), and a dark matter mass of about 5 GeV.

News on indirect and direct dark matter searches

Article

Jan 2011

Marco Taoso

We review the current status of direct and indirect Dark Matter searches, focusing in particular on those observations which have been interpreted as possible hints of dark matter. We discuss about the uncertainties affecting these interpretations and highlights the complementarity between different methods.

Indirect Probes of Dark Matter and Globular Cluster Properties From Dark Matter Annihilation within the Coolest White Dwarfs

Article

Oct 2014

White Dwarfs (WD) capture Dark Matter (DM) as they orbit within their host halos. These captured particles may subsequently annihilate, heating the stellar core and preventing the WD from cooling. The potential wells of WDs are considerably deeper and core temperatures significantly cooler than those of main sequence stars. Consequently, DM evaporation is less important in WDs and DM with masses M_{\chi} \gtrsim 100\, \kev and annihilation cross-sections orders of magnitude below the canonical thermal cross-section (\sigmav \gtrsim 10^{-46}\, \cm^3/s) can significantly alter WD cooling in particular astrophysical environments. We consider WDs in globular clusters (GCs) and dwarf galaxies. If the parameters of the DM particle are known, then the temperature of the coolest WD in a GC can be used to constrain the DM density of the cluster's halo (potentially even ruling out the presence of a halo if the inferred density is of order the ambient Galactic density). Recently several direct detection experiments have seen signals whose origins might be due to low mass DM. In this paper, we show that if these claims from CRESST, DAMA, CDMS-Si, and CoGeNT could be interpreted as DM, then observations of NGC 6397 limit the fraction of DM in that cluster to be

f_{\mathrm{DM}} \lesssim 10^{-3}

. This would be an improvement over existing constraints of 3 orders of magnitude and clearly rule out a significant DM halo for this cluster...

The Mutual Interaction Between Population III Stars and Self-Annihilating Dark Matter

Article

Dec 2013
MON NOT R ASTRON SOC

We use cosmological simulations of high-redshift minihalos to investigate the effect of dark matter annihilation (DMA) on the collapse of primordial gas. We numerically investigate the evolution of the gas as it assembles in a Population III stellar disk. We find that when DMA effects are neglected, the disk undergoes multiple fragmentation events beginning at ~ 500 yr after the appearance of the first protostar. On the other hand, DMA heating and ionization of the gas speeds the initial collapse of gas to protostellar densities and also affects the stability of the developing disk against fragmentation, depending on the DM distribution. We compare the evolution when we model the DM density with an analytical DM profile which remains centrally peaked, and when we simulate the DM profile using N-body particles (the 'live' DM halo). When utilizing the analytical DM profile, DMA suppresses disk fragmentation for ~ 3500 yr after the first protostar forms, in agreement with earlier work. However, when using a 'live' DM halo, the central DM density peak is gradually flattened due to the mutual interaction between the DM and the rotating gaseous disk, reducing the effects of DMA on the gas, and enabling secondary protostars of mass ~ 1 M_sol to be formed within ~ 900 yr. These simulations demonstrate that DMA is ineffective in suppressing gas collapse and subsequent fragmentation, rendering the formation of long-lived dark stars unlikely. However, DMA effects may still be significant in the early collapse and disk formation phase of primordial gas evolution.

The Final Stages of Massive Star Evolution and Their Supernovae

Article

Jan 2012

Alexander Heger

In this chapter I discuss the final stages in the evolution of massive stars - stars that are massive enough to burn nuclear fuel all the way to iron group elements in their core. The core eventually collapses to form a neutron star or a black hole when electron captures and photo-disintegration reduce the pressure support to an extent that it no longer can hold up against gravity. The late burning stages of massive stars are a rich subject by themselves, and in them many of the heavy elements in the universe are first generated. The late evolution of massive stars strongly depends on their mass, and hence can be significantly effected by mass loss due to stellar winds and episodic mass loss events - a critical ingredient that we do not know as well as we would like. If the star loses all the hydrogen envelope, a Type I supernova results, if it does not, a Type II supernova is observed. Whether the star makes neutron star or a black hole, or a neutron star at first and a black hole later, and how fast they spin largely affects the energetics and asymmetry of the observed supernova explosion. Beyond photon-based astronomy, other than the sun, a supernova (SN 1987) has been the only object in the sky we ever observed in neutrinos, and supernovae may also be the first thing we will ever see in gravitational wave detectors like LIGO. I conclude this chapter reviewing the deaths of the most massive stars and of Population III stars.

First Stars and WIMP Dark Matter: a state of the art review

Article

Sep 2012

Fabio Iocco

It has been suggested that Weakly Interacting Massive Particles (WIMPs) may significantly alter the formation and evolution of the First Stars, if the Dark Matter (DM) is actually made of such particles. In these proceedings I summarize the state of the art of the last five years of research in this field.

Observing supermassive dark stars with James Webb Space Telescope

Article

May 2012
MON NOT R ASTRON SOC

We study the capability of theJames Webb Space Telescope (JWST) to detect supermassive dark stars (SMDSs). If the first stars are powered by dark matter (DM) heating in triaxial DM haloes, they may grow to be very large (>106 M⊙) and very bright (>109 L⊙). These SMDSs would be visible in deep imaging with JWST and even Hubble Space Telescope (HST). We use sensitivity limits from previous HST surveys to place bounds on the numbers of SMDSs that may be detected in future JWST imaging surveys. We showed that SMDS in the mass range 106–107 M⊙ are bright enough to be detected in all the wavelength bands of the NIRCam on JWST (but not in the less sensitive MIRI camera at higher wavelengths). If SMDSs exist at z∼ 10, 12 and 14, they will be detectable as J-, H- or K-band dropouts, respectively. With a total survey area of 150 arcmin2 (assuming a multiyear deep parallel survey with JWST), we find that typically the number of 106 M⊙ SMDSs found as H- or K-band dropouts is ∼105fSMDS, where the fraction of early DM haloes hosting DS is likely to be small, fSMDS≪ 1. If the SMDS survive down to z= 10 where HST bounds apply, then the observable number of SMDSs as H- or K-band dropouts with JWST is ∼1–30. While individual SMDS are bright enough to be detected by JWST, standard Population III stars (without DM annihilation) are not, and would only be detected in first galaxies with total stellar masses of 106–108 M⊙. Differentiating first galaxies at z > 10 from SMDSs would be possible with spectroscopy: the SMDS (which are too cool produce significant nebular emission) will have only absorption lines, while the galaxies are likely to produce emission lines as well. Of particular interest would be the He ii emission lines at m as well as Hα lines which would be signatures of early galaxies rather than SMDSs. The detection of SMDSs with JWST would not only provide alternative evidence for weakly interacting massive particles, but also provide a possible pathway for the formation of massive (104–106 M⊙) seeds for the formation of supermassive black holes that power quasi-stellar objects at z= 6.

Selected Topics on the Dark Side

Article

Full-text available

Jan 2011

Cosmin Ilie

Although significant progress has been made in understanding the nature and history of our universe there are still many open questions in cosmology. The nature of the dark energy (DE) and dark matter (DM) are still elusive. Phantom Cosmology provides an unique opportunity to ``connect'' the phantom driven DE phase to the inflationary era. We present a concrete model where the energy density cycles through several phases. The model predicts transitions from a standard radiation/matter dominated regime to a dark energy/inflationary phases, in a repetitive pattern. An interesting feature of this formalism is that once we include interactions between the ``phantom fluid'' and ordinary matter, the phantom phase naturally gives way to a near exponential inflationary expansion, avoiding the Big Rip singularity. The first phase of stellar evolution in the history of the Universe may be Dark Stars (DS), powered by DM heating. We investigated the properties of DS assuming DM annihilations explain recently found cosmic ray anomalies. Our results show that the final stellar properties, once the star enters the main sequence, are always roughly the same. However the lifetime, final mass, and final luminosity of the DS show moderate dependence on boost factor and concentration parameter. We propose two mechanisms that could explain the growth of Dark Stars to become supermassive (SMDS). The launch of the James Webb Space Telescope (JWST) opens up the possibility of detecting SMDS. Using various dropout redshift selection functions we show that JWST could detect SMDS in a typical deep field survey. Specifically, at z ~ 10 there could be several million SMDS detected as JBand dropouts. The detection of ten-million SMDS is relatively slim at the same z~10. The 1e7 SMDS have a significant chance of being observed as a HBand dropout at z~12, whereas the million-sollarmass SMDS could show up in extremely large numbers (~200) in a typical deep field survey as an F150W dropout. The most promising technique to use in searching for SMDS would be the HBand dropouts. Such an observational discovery would confirm the existence of a new phase of stellar evolution powered by dark matter.

Searches for Particle Dark Matter: Dark stars, dark galaxies, dark halos and global supersymmetric fits

Article

Full-text available

May 2010

Pat Scott

The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.

Population III Star Formation in a ΛCDM Universe. I. The Effect of Formation Redshift and Environment on Protostellar Accretion Rate

Article

Full-text available

Jan 2007

We perform 12 extremely high resolution adaptive mesh refinement cosmological simulations of Population III star formation in a ΛCDM universe, varying the box size and large-scale structure, to understand systematic effects in the formation of primordial protostellar cores. We find results that are qualitatively similar to those of previous groups. We observe that in the absence of a photodissociating ultraviolet background, the threshold halo mass for formation of a Population III protostar does not evolve significantly with time in the redshift range studied (33 > z > 19) but exhibits substantial scatter (1.5 < Mvir/105M☉ < 7) due to different halo assembly histories: halos that assembled more slowly develop cooling cores at lower mass than those that assemble more rapidly, in agreement with previous work. We do, however, observe significant evolution in the accretion rates of Population III protostars with redshift, with objects that form later having higher maximum accretion rates ( 10-4M☉ yr-1 at z = 33 and 10-2M☉ yr-1 at z = 20). This can be explained by considering the evolving virial properties of the halos with redshift and the physics of molecular hydrogen formation at low densities. Our result implies that the inferred mass distribution of Population III stars is broader than previously thought and may evolve with redshift. Finally, we observe that our collapsing protostellar cloud cores do not fragment, consistent with previous results, which suggests that Population III stars that form in halos of mass 105-106M☉ always form in isolation.

Neutralino annihilation into Î³ rays in the Milky Way and in external galaxies

Article

Full-text available

Nov 2004

We discuss the gamma-ray signal from dark matter annihilation in our Galaxy and in external objects, namely, the Large Magellanic Cloud, the Andromeda Galaxy (M31), and M87. We derive predictions for the fluxes in a low energy realization of the minimal supersymmetric standard model and compare them with current data from EGRET, CANGAROO-II, and HEGRA and with the capabilities of new-generation satellite-borne experiments, like GLAST, and ground-based Cerenkov telescopes, like VERITAS. We find fluxes below the level required to explain the possible indications of a Î³-ray excess shown by CANGAROO-II (toward the galactic center) and HEGRA (from M87). As far as future experiments are concerned, we show that only the signal from the galactic center could be accessible to both satellite-borne experiments and to atmospheric Cerenkov telescopes (ACTs), even though this requires very steep dark matter density profiles.

Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for Cosmology

Article

Full-text available

Dec 2008

A simple cosmological model with only six parameters (matter density, Ωmh2, baryon density, Ωbh2, Hubble constant, H0, amplitude of fluctuations, σ8, optical depth, τ, and a slope for the scalar perturbation spectrum, ns) fits not only the 3 year WMAP temperature and polarization data, but also small-scale CMB data, light element abundances, large-scale structure observations, and the supernova luminosity/distance relationship. Using WMAP data only, the best-fit values for cosmological parameters for the power-law flat Λ cold dark matter (ΛCDM) model are (Ωmh2,Ωbh2,h,ns,τ,σ8) = (0.1277,0.02229 ± 0.00073,0.732,0.958 ± 0.016,0.089 ± 0.030,0.761). The 3 year data dramatically shrink the allowed volume in this six-dimensional parameter space. Assuming that the primordial fluctuations are adiabatic with a power-law spectrum, the WMAP data alone require dark matter and favor a spectral index that is significantly less than the Harrison-Zel'dovich-Peebles scale-invariant spectrum (ns = 1, r = 0). Adding additional data sets improves the constraints on these components and the spectral slope. For power-law models, WMAP data alone puts an improved upper limit on the tensor-to-scalar ratio, r0.002 < 0.65 (95% CL) and the combination of WMAP and the lensing-normalized SDSS galaxy survey implies r0.002 < 0.30 (95% CL). Models that suppress large-scale power through a running spectral index or a large-scale cutoff in the power spectrum are a better fit to the WMAP and small-scale CMB data than the power-law ΛCDM model; however, the improvement in the fit to the WMAP data is only Δχ2 = 3 for 1 extra degree of freedom. Models with a running-spectral index are consistent with a higher amplitude of gravity waves. In a flat universe, the combination of WMAP and the Supernova Legacy Survey (SNLS) data yields a significant constraint on the equation of state of the dark energy, w = -0.967. If we assume w = -1, then the deviations from the critical density, ΩK, are small: the combination of WMAP and the SNLS data implies Ωk = -0.011 ± 0.012. The combination of WMAP 3 year data plus the HST Key Project constraint on H0 implies Ωk = -0.014 ± 0.017 and ΩΛ = 0.716 ± 0.055. Even if we do not include the prior that the universe is flat, by combining WMAP, large-scale structure, and supernova data, we can still put a strong constraint on the dark energy equation of state, w = -1.08 ± 0.12. For a flat universe, the combination of WMAP and other astronomical data yield a constraint on the sum of the neutrino masses, mν < 0.66 eV (95%CL). Consistent with the predictions of simple inflationary theories, we detect no significant deviations from Gaussianity in the CMB maps using Minkowski functionals, the bispectrum, trispectrum, and a new statistic designed to detect large-scale anisotropies in the fluctuations.

Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results

Article

Full-text available

Feb 2009

We present new full-sky temperature and polarization maps in five frequency bands from 23 to 94 GHz, based on data from the first five years of the Wilkinson Microwave Anisotropy Probe (WMAP) sky survey. The new maps are consistent with previous maps and are more sensitive. The five-year maps incorporate several improvements in data processing made possible by the additional years of data and by a more complete analysis of the instrument calibration and in-flight beam response. We present several new tests for systematic errors in the polarization data and conclude that W-band polarization data is not yet suitable for cosmological studies, but we suggest directions for further study. We do find that Ka-band data is suitable for use; in conjunction with the additional years of data, the addition of Ka band to the previously used Q- and V-band channels significantly reduces the uncertainty in the optical depth parameter, τ. Further scientific results from the five-year data analysis are presented in six companion papers and are summarized in Section 7 of this paper. With the five-year WMAP data, we detect no convincing deviations from the minimal six-parameter ΛCDM model: a flat universe dominated by a cosmological constant, with adiabatic and nearly scale-invariant Gaussian fluctuations. Using WMAP data combined with measurements of Type Ia supernovae and Baryon Acoustic Oscillations in the galaxy distribution, we find (68% CL uncertainties): Ω b h 2 = 0.02267+0.00058 –0.00059, Ω c h 2 = 0.1131 ± 0.0034, ΩΛ = 0.726 ± 0.015, ns = 0.960 ± 0.013, τ = 0.084 ± 0.016, and at k = 0.002 Mpc-1. From these we derive σ8 = 0.812 ± 0.026, H 0 = 70.5 ± 1.3 km s-1 Mpc–1, Ω b = 0.0456 ± 0.0015, Ω c = 0.228 ± 0.013, Ω m h 2 = 0.1358+0.0037 –0.0036, z reion = 10.9 ± 1.4, and t 0 = 13.72 ± 0.12 Gyr. The new limit on the tensor-to-scalar ratio is r < 0.22(95%CL), while the evidence for a running spectral index is insignificant, dns /dln k = –0.028 ± 0.020 (68% CL). We obtain tight, simultaneous limits on the (constant) dark energy equation of state and the spatial curvature of the universe: –0.14 < 1 + w < 0.12(95%CL) and –0.0179 < Ω k < 0.0081(95%CL). The number of relativistic degrees of freedom, expressed in units of the effective number of neutrino species, is found to be N eff = 4.4 ± 1.5 (68% CL), consistent with the standard value of 3.04. Models with N eff = 0 are disfavored at >99.5% confidence. Finally, new limits on physically motivated primordial non-Gaussianity parameters are –9 < f local NL < 111 (95% CL) and –151 < f equil NL < 253 (95% CL) for the local and equilateral models, respectively.

Constraining dark matter through 21‐cm observations

Article

Full-text available

May 2007

Beyond reionization epoch cosmic hydrogen is neutral and can be directly observed through its 21-cm line signal. If dark matter (DM) decays or annihilates, the corresponding energy input affects the hydrogen kinetic temperature and ionized fraction, and contributes to the Ly background. The changes induced by these processes on the 21-cm signal can then be used to constrain the proposed DM candidates, among which we select the three most popular ones: (i) 25-keV decaying sterile neutrinos, (ii) 10-MeV decaying light dark matter (LDM) and (iii) 10-MeV annihilating LDM. Although we find that the DM effects are considerably smaller than found by previous studies (due to a more physical description of the energy transfer from DM to the gas), we conclude that combined observations of the 21-cm background and of its gradient should be able to put constrains at least on LDM candidates. In fact, LDM decays (annihilations) induce differential brightness temperature variations with respect to the non-decaying/annihilating DM case up to ΔδTb= 8 (22) mK at about 50 (15) MHz. In principle, this signal could be detected both by current single-dish radio telescopes and future facilities as Low Frequency Array; however, this assumes that ionospheric, interference and foreground issues can be properly taken care of.

Radiative atomic Rosseland mean opacity tables

Article

Full-text available

Apr 1992

For more than two decades the astrophysics community has depended on opacity tables produced at Los Alamos. In the present work we offer new radiative Rosseland mean opacity tables calculated with the OPAL code developed independently at LLNL. We give extensive results for the recent Anders-Grevesse mixture which allow accurate interpolation in temperature, density, hydrogen mass fraction, as well as metal mass fraction. The tables are organized differently from previous work. Instead of rows and columns of constant temperature and density, we use temperature and follow tracks of constant R, where R = density/(temperature)3. The range of R and temperature are such as to cover typical stellar conditions from the interior through the envelope and the hotter atmospheres. Cool atmospheres are not considered since photoabsorption by molecules is neglected. Only radiative processes are taken into account so that electron conduction is not included. For comparison purposes we present some opacity tables for the Ross-Aller and Cox-Tabor metal abundances. Although in many regions the OPAL opacities are similar to previous work, large differences are reported. For example, factors of 2-3 opacity enhancements are found in stellar envelop conditions.

Low-temperature Rosseland opacities

Article

Full-text available

Dec 1994

A new, comprehensive set of low-temperature opacity data has been assembled. From this basic data set, Rosseland and Planck mean opacities have been computed for temperatures between 12,500 and 700 K. In addition to the usual continuous absorbers, atomic line absorption (with more than 8 million lines), molecular line absorption (with nearly 60 million lines), and grain absorption and scattering (by silicates, iron, carbon, and SiC) have been accounted for. The absorption due to lines is computed monochromatically and included in the mean with the opacity sampling technique. Grains are assumed to form in chemical equilibrium with the gas and to form into a continuous distribution of ellipsoids. Agreement of these opacities with other recent tabulations of opacities for temperatures above 5000 K is excellent. It is shown that opacities which neglect molecules become unreliable for temperatures below 5000 K. Triatomic molecules become important absorbers at 3200 K. Similarly, grains must be included in the computation for temperatures below 1700 K.

The Formation of the First Star in the Universe

Article

Full-text available

Feb 2002

We describe results from a fully self-consistent three-dimensional hydrodynamical simulation of the formation of one of the first stars in the Universe. In current models of structure formation, dark matter initially dominates, and pregalactic objects form because of gravitational instability from small initial density perturbations. As they assemble via hierarchical merging, primordial gas cools through ro-vibrational lines of hydrogen molecules and sinks to the center of the dark matter potential well. The high-redshift analog of a molecular cloud is formed. As the dense, central parts of the cold gas cloud become self-gravitating, a dense core of approximately 100 M (where M is the mass of the Sun) undergoes rapid contraction. At particle number densities greater than 10(9) per cubic centimeter, a 1 M protostellar core becomes fully molecular as a result of three-body H2 formation. Contrary to analytical expectations, this process does not lead to renewed fragmentation and only one star is formed. The calculation is stopped when optical depth effects become important, leaving the final mass of the fully formed star somewhat uncertain. At this stage the protostar is accreting material very rapidly (approximately 10(-2) M year-1). Radiative feedback from the star will not only halt its growth but also inhibit the formation of other stars in the same pregalactic object (at least until the first star ends its life, presumably as a supernova). We conclude that at most one massive (M 1 M) metal-free star forms per pregalactic halo, consistent with recent abundance measurements of metal-poor galactic halo stars.

Zero-metallicity stars I. Evolution at constant mass

Article

Full-text available

Feb 2001

We present extensive evolutionary models of stars with initial zero-metallicity, covering a large range of initial masses (i.e. 0.7 <= M <= 100 Msun). Calculations are carried out at constant mass, with updated input physics, and applying an overshooting scheme to convective boundaries. The nuclear network includes all the important reactions of the p-p chain, CNO-cycle and alpha-captures, and is solved by means of a suitable semi-implicit method. The evolution is followed up to the thermally pulsing AGB in the case of low- and intermediate-mass stars, or to the onset of carbon burning in massive stars. The main evolutionary features of these models are discussed, also in comparison with models of non-zero metallicity. Among several interesting aspects, particular attention has been paid to describe: i) the first synthesis of 12C inside the stars, that may suddenly trigger the CNO-cycle causing particular evolutionary features; ii) the pollution of the stellar surface by the dredge-up events, that are effective only within particular mass ranges; iii) the mass limits which conventionally define the classes of low-, intermediate-, and high-mass stars on the basis of common evolutionary properties, including the upper mass limit for the achievement of super-Eddington luminosities before C-ignition in the high-mass regime; and iv) the expected pulsational properties of zero-metallicity stars. All relevant information referring to the evolutionary tracks and isochrones is made available in computer-readable format at http://pleiadi.pd.astro.it . Comment: 22 pages, 17 figures, to appear in Astronomy & Astrophysics, electronic data tables are available at http://pleiadi.pd.astro.it

The Power Spectrum Dependence of Dark Matter Halo Concentrations

Article

Full-text available

Dec 2000

High-resolution N-body simulations are used to examine the power spectrum dependence of the concentration of galaxy-sized dark matter halos. It is found that dark halo concentrations depend on the amplitude of mass fluctuations as well as on the ratio of power between small and virial mass scales. This finding is consistent with the original results of Navarro, Frenk & White (NFW), and allows their model to be extended to include power spectra substantially different from Cold Dark Matter (CDM). In particular, the single-parameter model presented here fits the concentration dependence on halo mass for truncated power spectra, such as those expected in the warm dark matter scenario, and predicts a stronger redshift dependence for the concentration of CDM halos than proposed by NFW. The latter conclusion confirms recent suggestions by Bullock et al., although this new modeling differs from theirs in detail. These findings imply that observational limits on the concentration, such as those provided by estimates of the dark matter content within individual galaxies, may be used to constrain the amplitude of mass fluctuations on galactic and subgalactic scales. The constraints on

\Lambda

CDM models posed by the dark mass within the solar circle in the Milky Way and by the zero-point of the Tully-Fisher relation are revisited, with the result that neither dataset is clearly incompatible with the `concordance' (

\Omega_0=0.3

\Lambda_0=0.7

\sigma_8=0.9

)

\Lambda

CDM cosmogony. This conclusion differs from that reached recently by Navarro & Steinmetz, a disagreement that can be traced to inconsistencies in the normalization of the

\Lambda

CDM power spectrum used in that work. Comment: 12 pages including 7 figures using emulateapj, submitted to ApJ

Low mass stellar evolution with WIMP capture and annihilation

Article

Full-text available

Dec 2007

Recent work has indicated that WIMP annihilation in stellar cores has the potential to contribute significantly to a star's total energy production. We report on progress in simulating the effects of WIMP capture and annihilation upon stellar structure and evolution near supermassive black holes, using the new DarkStars code. Preliminary results indicate that low-mass stars are the most influenced by WIMP annihilation, which could have consequences for upcoming observational programs.

The Formation of the First Stars I. Mass Infall Rates, Accretion Disk Structure and Protostellar Evolution

Article

Full-text available

Jul 2003

We present a theoretical model for primordial star formation. First we describe the structure of the initial gas cores as virialized, quasi-hydrostatic objects in accord with recent high resolution numerical studies. The accretion rate can then be related to characteristic densities and temperatures that are set by the cooling properties of molecular hydrogen. We allow for rotation of the gas core, assuming angular momentum conservation inside the sonic point of the flow. In the typical case, most mass then reaches the star via an accretion disk. The structure of the inner region of this disk is described with the standard theory of viscous disks, but with allowance for the substantial energies absorbed in ionizing and dissociating the gas. The size of the protostar and its luminosity depend upon the accretion rate, the energetics of the accreting gas, and the ability of the radiation to escape from the stellar accretion shock. We combine these models for the infall rate, inner disk structure, and protostellar evolution to predict the radiation field that is the basis for radiative feedback processes acting against infall (Paper II). For realistic initial angular momenta, the photosphere of the protostar is much smaller and hotter than in the spherical case, leading to stronger radiative feedback at earlier stages in the evolution. In particular, once the star is older than its Kelvin-Helmholtz time, contraction towards the main sequence causes a rapid increase in ionizing and far-ultraviolet luminosity at masses ~30Msun in the fiducial case. Since the cores out of which the first stars formed were much more massive than 30Msun and since feedback is dynamically unimportant at lower masses, we conclude that the first stars should have had masses >~30Msun. Comment: 20 pages, Accepted to ApJ, some re-arrangement of text for improved clarity

Zero-metallicity stars II. Evolution of very massive objects with mass loss

Article

Full-text available

Dec 2002

We present evolutionary models of zero-metallicity very massive objects, with initial masses in the range 120 Msun -- 1000 Msun, covering their quiescent evolution up to central carbon ignition. In the attempt of exploring the possible occurrence of mass loss by stellar winds, calculations are carried out with recently-developed formalisms for the mass-loss rates driven by radiation pressure (Kudritzki 2002) and stellar rotation (Maeder & Meynet 2000).The study completes the previous analysis by Marigo et al. (2001) on the constant-mass evolution of primordial stars. Our results indicate that radiation pressure (assuming a minimum metallicity Z = 10^{-4} Zsun)is not an efficient driving force of mass loss, except for very massive stars with M >= 750 Msun. On the other hand, stellar rotation might play a crucial role in triggering powerful stellar winds, once the (Omega-Gamma)-limit is approached. However, this critical condition of intense mass loss can be maintained just for short, as the loss of angular momentum due to mass ejection quickly leads to the spinning down of the star. As by-product to the present work, the wind chemical yields from massive zero-metallicity stars are presented. The helium and metal enrichments, and the resulting Delta(Y)/Delta(Z) ratio are briefly discussed. Comment: 14 pages, 11 postscript figures, accepted for publication in A&A

High‐Energy Neutrino Signals from the Epoch of Reionization

Article

Full-text available

Aug 2007

We perform a new estimate of the high energy neutrinos expected from GRBs associated with the first generation of stars in light of new models and constraints on the epoch of reionization and a more detailed evaluation of the neutrino emission yields. We also compare the diffuse high energy neutrino background from Population III stars with the one from "ordinary stars" (Population II), as estimated consistently within the same cosmological and astrophysical assumptions. In disagreement with previous literature, we find that high energy neutrinos from Population III stars will not be observable with current or near future neutrino telescopes, falling below both IceCube sensitivity and atmospheric neutrino background under the most extreme assumptions for the GRB rate. This rules them out as a viable diagnostic tool for these still elusive metal-free stars.

Towards Forming a Primordial Protostar in a Cosmological AMR Simulation

Article

Mar 2008

Modeling the formation of the first stars in the universe is a well-posed problem and ideally suited for computational investigation.We have conducted high-resolution numerical studies of the formation of primordial stars. Beginning with primordial initial conditions appropriate for a ΛCDM model, we used the Eulerian adaptive mesh refinement code (Enzo) to achieve unprecedented numerical resolution, resolving cosmological scales as well as sub-stellar scales simultaneously. Building on the work of Abel, Bryan and Norman (2002), we followed the evolution of the first collapsing cloud until molecular hydrogen is optically thick to cooling radiation. In addition, the calculations account for the process of collision-induced emission (CIE) and add approximations to the optical depth in both molecular hydrogen roto-vibrational cooling and CIE. Also considered are the effects of chemical heating∕cooling from the formation∕destruction of molecular hydrogen. We present the results of these simulations, showing the formation of a 10 Jupiter-mass protostellar core bounded by a strongly aspherical accretion shock. Accretion rates are found to be as high as one solar mass per year.

Standard and nonstandard plasma neutrino emission revisited

Article

Apr 1994

On the basis of Braaten & Segel's representation of the electromagnetic dispersion relations in a Quantum Electro-Dynamic (QED) plasma, we check the numerical accuracy of several published analytic approximations to the plasma neutrino emission rates. As we find none of them satisfactory, we derive a new analytic approximation which is accurate to within 4% where the plasma process dominates. The correct emission rates in the parameter regime relevant for the red giant branch in globular clusters are larger by about 10%-20% than those of previous stellar evolution calculations. Therefore, the core mass of red giants at the He flash is larger by about 0.005 solar mass, or 1%, than previously thought. Our bounds on neutrino magnetic dipole moments remain virtually unchanged.

Screening Factors for Nuclear Reactions. II. Intermediate Screen-Ing and Astrophysical Applications

Article

Apr 1973

Printing Options Send high resolution image to Level 2 Postscript Printer Send low resolution image to Level 2 Postscript Printer Send low resolution image to Level 1 Postscript Printer Get high resolution PDF image Get low resolution PDF Send 300 dpi image to PCL Printer Send 150 dpi image to PCL Printer More Article Retrieval Options HELP for Article Retrieval Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints

A stellar probe of dark matter annihilation in galactic nuclei

Article

Feb 1989

Stellar evolution within dense clouds of slowly annihilating cold dark matter (CDM) is studied. It is shown that stars catalyze the annihilation rate, leading to observable consequences for the evolution of low-mass stars as viewed in the cumulative light from distant galactic nuclei. The evolution of a single star in the presence of an extreme concentration of CDM is examined. A dynamical scenario for the evolution of a galactic nucleus containing stars and CDM is described, and the effects of dynamical friction due to the CDM on stellar orbits is estimated.

Radiative opacities for carbon- and oxygen-rich mixtures

Article

Jul 1993

Recent OPAL calculations have obtained significant differences in the Rosseland mean opacities compared with earlier Los Alamos work. These new opacities have had a favorable impact on several astrophysical problems, but the efforts have concentrated on hydrogen main-sequence stars or stellar envelopes. The present calculations consider carbon- and oxygen-rich mixtures. It is shown that, for such mixtures, the Coulomb corrections beyond the weak-coupling approximation are not negligible in the ionization-balance calculations when Rosseland mean opacities are computed. As for hydrogen-rich compositions, the hydrogen-depleted mixtures can show factors of 2-3 enhancements in the opacity compared with the Los Alamos results at temperatures of a few hundred thousand degrees. For temperatures above a million degrees there are opacity increases as well as decreases of approximately 20 percent, depending on density. Tables of Rosseland mean opacities are provided that allow accurate interpolation for compositions of arbitrary amounts of hydrogen, carbon, and oxygen.

Resonant enhancements in weakly interacting massive particle capture by the earth

Article

Sep 1987

Andrew Gould

The exact formulas for the capture of weakly interacting massive particles (WIMPs) by a massive body are derived. Capture by the earth is found to be significantly enhanced whenever the WIMP mass is roughly equal to the nuclear mass of an element present in the earth in large quantities. For Dirac neutrino WIMPs of mass 10-90 GeV, the capture rate is 10-300 times that previously believed. Capture rates for the sun are also recalculated and found to be from 1.5 times higher to 3 times lower than previously believed, depending on the mass and type of WIMP. The earth alone or the earth in combination with the sun is found to give a much stronger annihilation signal from Dirac neutrino WIMPs than the sun alone over a very large mass range. This is particularly important in the neighborhood of the mass of iron where previous analyses could not set any significant limits.

Publisher's Note: Search for dark matter WIMPs using upward through-going muons in Super-Kamiokande [ Phys. Rev. D 70, 083523 (2004)]

Article

Oct 2004

DOI:https://doi.org/10.1103/PhysRevD.70.109901

Electrical and thermal conductivities of dense matter in the liquid metal phase. I - High-temperature results

Article

Sep 1983

Electrical and thermal conductivities are calculated for the dense matter in the liquid metal phase for various elemental compositions of astrophysical importance. The calculation takes account of the best knowledge available on the structure factor of the ions in the high-temperature, classical limit and the dielectric screening due to degenerate electrons. The numerical results are parameterized in analytic formulae that would facilitate practical uses of the results. An overall accuracy of about 1 percent has been retained for most of the analytic formulae. Compared with the results of the present calculation, it is found that Yakovlev and Urpin's (1980) results overestimate the resistivity (underestimate the conductivity) by 30-60 percent at relatively low densities.

Numerical models of star clusters with a central black hole. I - Adiabatic models

Article

Nov 1980

Young P. J.

Numerical models of star clusters containing a massive black hole are computed for the case of a black hole which grows adiabatically in the cluster center. The growth of the hole is assumed to be at a rate longer than the cluster dynamical time scale but shorter than the relaxation time scale. The angular momentum and radial action of each star in the cluster are conserved during the adiabatic variations. This leads to the invariance of the distribution function in (E, J) space which is used to facilitate the numerical calculations. A power-law density cusp forms near the black hole. When the hole has grown to exceed the core mass of the cluster, this cusp joins smoothly onto the isothermal density law.

Formation of Primordial Stars in a ΛCDM Universe

Article

Dec 2008

We study the formation of the first generation of stars in the standard cold dark matter model. We use a very high resolution cosmological hydrodynamic simulation that achieves a dynamic range of ~1010 in length scale. With accurate treatment of atomic and molecular physics, including the effect of molecular line opacities and cooling by collision-induced continuum emission, it allows us to study the chemothermal evolution of primordial gas clouds to densities up to ρ ~ 2 × 10-8 g cm-3 (nH ~ 1016 cm-3) without assuming any a priori equation of state, an improvement of 6 orders of magnitude over previous three-dimensional calculations. We study the evolution of a primordial star-forming gas cloud in the cosmological simulation in detail. The cloud core becomes marginally unstable against chemothermal instability when the gas cooling rate is increased owing to three-body molecule formation. However, since the core is already compact at that point, runaway cooling simply leads to fast condensation to form a single protostellar seed. During the final dynamical collapse, small angular momentum material collapses faster than the rest of the gas and selectively sinks inward. Consequently, the central regions have little specific angular momentum, and rotation does not halt collapse. We, for the first time, obtain an accurate gas mass accretion rate within a 10 M☉ innermost region around the protostar. We carry out protostellar evolution calculations using the obtained accretion rate. The resulting mass of the first star when it reaches the zero-age main sequence is MZAMS ~ 100 M☉, and less (60 M☉) for substantially reduced accretion rates.

Contraction of dark matter galactic halos due to baryonic infall

Article

Feb 1986

Varied evidence suggests that galaxies consist of roughly 10 percent baryonic matter by mass and that baryons sink dissipatively by about a factor of 10 in. radius during galaxy formation. It is shown that such infall strongly perturbs the underlying dark matter distribution, pulling it inward and creating cores that are considerably smaller and denser than would have evolved without dissipation. Any discontinuity between the baryonic and dark matter mass distributions is smoothed out by the coupled motions of the two components. If dark halos have large core radii in the absence of dissipation, the above infall scenario yields rotation curves that are flat over large distances, in agreement with observations of spiral galaxies. Such large dissipationless cores may plausibly result from large internal kinetic energy in protogalaxies at maximum expansion, perhaps as a result of subclustering, tidal effects, or anisotropic collapse.

Constraints on the initial mass function of the first stars

Article

Jun 2006

Motivated by theoretical predictions that the first stars were predominantly very massive, we investigate the physics of the transition from an early epoch dominated by massive Pop III stars to a later epoch dominated by familiar low-mass Pop II/I stars by means of a numerically generated catalogue of dark matter haloes coupled with a self-consistent treatment of chemical and radiative feedback. Depending on the strength of the chemical feedback, Pop III stars can contribute a substantial fraction (several per cent) of the cosmic star formation activity even at moderate redshifts, z≈ 5. We find that the three z≈ 10 sources tentatively detected in Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Ultra Deep Fields (UDFs) should be powered by Pop III stars, if these are massive; however, this scenario fails to reproduce the derived Wilkinson Microwave Anisotropy Probe (WMAP) electron scattering optical depth. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson initial mass function.

Cosmic Stellar Relics in the Galactic Halo

Article

Oct 2007

We study the stellar population history and chemical evolution of the Milky Way (MW) in a hierarchical Λ cold dark matter model for structure formation. Using a Monte Carlo method based on the semi-analytical extended Press & Schechter formalism, we develop a new code GAlaxy MErger Treeand Evolution (gamete) to reconstruct the merger tree of the Galaxy and follow the evolution of gas and stars along the hierarchical tree. Our approach allows us to compare the observational properties of the MW with model results, exploring different properties of primordial stars, such as their initial mass function and the critical metallicity for low-mass star formation, Zcr. In particular, by matching our predictions to the metallicity distribution function (MDF) of metal-poor stars in the Galactic halo we find that: (i) a strong supernova (SN) feedback is required to reproduce the observed properties of the MW; (ii) stars with [Fe/H] < −2.5 form in haloes accreting Galactic medium (GM) enriched by earlier SN explosions; (iii) the fiducial model (Zcr= 10−4 Z⊙, mPopIII= 200 M⊙) provides an overall good fit to the MDF, but cannot account for the two hyper-metal-poor (HMP) stars with [Fe/H] < −5; the latter can be accommodated if Zcr≤ 10−6 Z⊙ but such model overpopulates the ‘metallicity desert’, that is, the range −5.3 < [Fe/H] < −4 in which no stars have been detected; (iv) the current non-detection of metal-free stars robustly constrains either Zcr > 0 or the masses of the first stars mPopIII > 0.9 M⊙; (v) the statistical impact of truly second-generation stars, that is, stars forming out of gas polluted only by metal-free stars, is negligible in current samples; and (vi) independent of Zcr, 60 per cent of metals in the GM are ejected through winds by haloes with masses M < 6 × 109 M⊙, thus showing that low-mass haloes are the dominant population contributing to cosmic metal enrichment. We discuss the limitations of our study and comparison with previous work.

The first generation of stars in the Λ cold dark matter cosmology

Article

Jun 2007

We have performed a large set of high-resolution cosmological simulations using smoothed particle hydrodynamics to study the formation of the first luminous objects in the Lambda cold dark matter cosmology. We follow the collapse of primordial gas clouds in eight early structures and document the scatter in the properties of the first star-forming clouds. Our first objects span formation redshifts from z∼ 10 to ∼50 and cover an order of magnitude in halo mass. We find that the physical properties of the central star-forming clouds are very similar in all of the simulated objects despite significant differences in formation redshift and environment. This suggests that the formation path of the first stars is largely independent of the collapse redshift; the physical properties of the clouds have little correlation with spin, mass or assembly history of the host halo. The collapse of protostellar objects at higher redshifts progresses much more rapidly due to the higher densities, which accelerates the formation of molecular hydrogen, enhances initial cooling and shortens the dynamical time-scales. The mass of the star-forming clouds cover a broad range, from a few hundred to a few thousand solar masses, and exhibit various morphologies: some have disc-like structures which are nearly rotational supported; others form flattened spheroids; still others form bars. All of them develop a single protostellar ‘seed’ which does not fragment into multiple objects up to the moment that the central gas becomes optically thick to H2 cooling lines. At this time, the instantaneous mass accretion rate on to the centre varies significantly from object to object, with disc-like structures having the smallest mass accretion rates. The formation epoch and properties of the star-forming clouds are sensitive to the values of cosmological parameters.

Population III stars: Hidden or disappeared?

Article

Dec 2007

ABSTRACTA Population III/Population II transition from massive to normal stars is predicted to occur when the metallicity of the star-forming gas crosses the critical range Zcr= 10−5±1 Z⊙. To investigate the cosmic implications of such a process, we use numerical simulations which follow the evolution, metal enrichment and energy deposition of both Population II and Population III stars. We find that: (i) due to inefficient heavy element transport by outflows and slow ‘genetic’ transmission during hierarchical growth, large fluctuations around the average metallicity arise; as a result, Population III star formation continues down to z= 2.5, but at a low peak rate of 10−5 M⊙ yr−1 Mpc−3 occurring at z≈ 6 (about 10−4 of the Population II one); and (ii) Population III star formation proceeds in an ‘inside–out’ mode in which formation sites are progressively confined to the periphery of collapsed structures, where the low gas density and correspondingly long free-fall time-scales result in a very inefficient astration. These conclusions strongly encourage deep searches for pristine star formation sites at moderate (2 < z < 5) redshifts where metal-free stars are likely to be hidden.

Radiation from the first forming stars

Article

Aug 2002

The evolution of radiation emitted during the dynamical collapse of metal-free protostellar clouds is investigated within a spherically symmetric hydrodynamical scheme that includes the transfer of radiation and the chemistry of the primordial gas. The cloud centre collapses on a time-scale of ∼105–6 yr, thanks to line cooling from molecular hydrogen (H2). For most of the collapse time, when the evolution proceeds self-similarly, the luminosity slowly rises up to ∼1036 erg and is essentially a result of H2 infrared (IR) line emission. Later, continuum IR radiation provides an additional contribution, which is mostly a result of the accretion of an infalling envelope upon a small hydrostatic protostellar core that develops in the centre. We follow the beginning of the accretion phase, when the enormous accretion rate (∼0.1 M⊙ yr−1) produces a very high continuum luminosity of ∼1036 erg. Despite the high luminosities, the radiation field is unable to affect the gas dynamics during the collapse and the first phases of accretion, because the opacity of the infalling gas is too small; this is quite different from present-day star formation. We also find that the protostellar evolution is similar among clouds with different initial configurations, including those resulting from three-dimensional cosmological simulations of primordial objects; in particular, the shape of the molecular spectra is quite universal. Finally, we briefly discuss the detectability of this initial cosmic star formation activity.

A semi-implicit mid-point rule for stiff systems of ordinary differential equations

Article

Jan 1983

The paper introduces a new semi-implicit extrapolation method especially designed for the numerical solution of stiff systems of ordinary differential equations. The existence of a quadratic asymptotic expansion in terms of the stepsize is shown. Moreover, the new discretization is analyzed in the light of well-known stability models. The efficiency of the new integrator is clearly demonstrated by solving a series of challenging test problems including real life examples.

Diffuse cosmic neutrino background from population III stars

Article

Nov 2004
ASTROPART PHYS

We study the expected diffuse cosmic neutrino flux produced by population III (PopIII) stars during their nuclear burning phases as well as from their final stages of evolution (core collapse). Assuming a fraction fIII = 10−3 of all baryons forms PopIII stars, our flux estimate is comparable to the diffuse neutrino flux produced by the ordinary stars and by the ordinary core-collapse supernovae in the universe, i.e. of order 1–10 cm−2 s−1. Due to the large cosmic redshift, however, the typical energies are in the MeV and sub-MeV range where the solar and geophysical neutrino fluxes are much larger. A direct detection of the diffuse cosmic flux is out of the question with presently known experimental techniques.

Thermonuclear Reaction Rates V

Article

Nov 1988

Analytic expressions are given for the reaction rates of astrophysically important thermonuclear reactions involving low-mass nuclei (1 ⩽ Z ⩽ 14). Numerical values of the rates are tabulated for the temperature range 106 ⩽ T ⩽ 1010 K. This provides a comprehensive update of our previous publications.

Cosmic asymmetry, neutrinos and the sun

Article

Dec 1987
NUCL PHYS B

We consider the effects a cosmological asymmetry would have on various consequences of cold dark matter. To be specific, we suppose that stable Dirac neutrinos exist with masses of a few GeV. We then consider the contribution these neutrinos make towards a closure density for the universe and the possibility of capturing neutrinos in the sun and observing their annihilation products. We concentrate on the role asymmetry plays in altering previous discussions. The arguments concerning the sun are only relevant if the neutrino mass is greater than the “evaporation” mass, mev. We evaluate mev = 3.3 GeV using a detailed balance technique.

Baryonic Pinching of Galactic Dark Matter Halos

Article

Dec 2006

High resolution cosmological N-body simulations of four galaxy-scale dark matter halos are compared to corresponding N-body/hydrodynamical simulations containing dark matter, stars and gas. The simulations without baryons share features with others described in the literature in that the dark matter density slope continuously decreases towards the center, with a density {rho}{sub DM}{proportional_to}r{sup -1.3{+-}}{sup 0.2}, at about 1% of the virial radius for our Milky Way sized galaxies. The central cusps in the simulations which also contain baryons steepen significantly, to {rho}{sub DM}{proportional_to}r{sup -1.9{+-}}{sup 0.2}, with an indication of the inner logarithmic slope converging. Models of adiabatic contraction of dark matter halos due to the central buildup of stellar/gaseous galaxies are examined. The simplest and most commonly used model, by Blumenthal et al., is shown to overestimate the central dark matter density considerably. A modified model proposed by Gnedin et al. is tested and it is shown that, while it is a considerable improvement, it is not perfect. Moreover, it is found that the contraction parameters in their model not only depend on the orbital structure of the dark-matter-only halos but also on the stellar feedback prescription which is most relevant for the baryonic distribution. Implications for dark matter annihilation at the galactic center are discussed and it is found that, although our simulations show a considerable reduced dark matter halo contraction as compared to the Blumenthal et al. model, the fluxes from dark matter annihilation are still expected to be enhanced by at least a factor of a hundred, as compared to dark-matter-only halos. Finally, it is shown that, while dark-matter-only halos are typically prolate, the dark matter halos containing baryons are mildly oblate with minor-to-major axis ratios of c/a=0.73{+-}0.11, with their flattening aligned with the central baryonic disks.

Colour distributions in E-S0 galaxies.

Article

Jan 2000

Colours distributions have been studied in 12 S0 galaxies, most of these nearly edge-on, from high resolution frames obtained at the CFHT and the Pic du Midi 2 m telescope, supplemented for some purposes by low resolution frames from the Observatoire de Haute Provence 120 cm telescope.  Great attention has been given to reduce the effects of “differential seeing", resulting from different PSF's in the two frames needed for a colour measurement. Errors from various sources, and their effect upon measured quantities, have been evaluated from the study of pseudo-colours (obtained by applying our measuring techniques to pair of frames taken in the same passband), and from the comparison of colours data from our various series.  Data are given about the following topics: – Radial isophotal colour profiles, leading to reference colours at

r_{\rm e}

and gradients in 4 colour indices (except for missing data); – Reference colours and gradients in possibly dustless regions of nearly pure bulge or disk, allowing a comparison between the two components; – Dust patterns, seen in 7 objects; – Systematic differences between major and minor axis colours upon isophotal contours (outside dust patterns). Both a redder or bluer major axis are encountered depending upon the object, or the central distance in a given object. The observations may sometimes be explained by local dust concentration in disks. In other cases population differences between spheroidal and disk components should be invoked; – Asymmetries in light and colour distributions along the minor axis in 3 objects, due to dust concentrated in the disk; – Correlations between ring structure and major axis colour profiles. 

Forming the First Stars in the Universe: The Fragmentation of Primordial Gas

Article

Dec 1999

In order to constrain the initial mass function of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution of an isolated 3 sigma peak of mass 2x106 M middle dot in circle that collapses at zcoll approximately 30 using smoothed particle hydrodynamics. We find that the gas dissipatively settles into a rotationally supported disk that has a very filamentary morphology. The gas in these filaments is Jeans unstable with MJ approximately 103 M middle dot in circle. Fragmentation leads to the formation of high-density (n>108 cm-3) clumps that subsequently grow in mass by accreting the surrounding gas and by merging with other clumps up to masses of approximately 104 M middle dot in circle. This suggests that the very first stars were rather massive. We explore the complex dynamics of the merging and tidal disruption of these clumps by following their evolution over a few dynamical times.

Dark Matter and the First Stars: A New Phase of Stellar Evolution

Article

Mar 2008

A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. A "dark star" may result: a giant ( greater, similar 1 AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion. Observational consequences are discussed.

The Nucleosynthetic Signature of Population III

Article

Jul 2001

Growing evidence suggests that the first generation of stars may have been quite massive (~100-300 M_sun). Could these stars have left a distinct nucleosynthetic signature? We explore the nucleosynthesis of helium cores in the mass range M_He=64 to 133 Msun, corresponding to main-sequence star masses of approximately 140 to 260 M_sun. Above M_He=133 M_sun, without rotation and using current reaction rates, a black hole is formed and no nucleosynthesis is ejected. For lighter helium core masses, ~40 to 63 M_sun, violent pulsations occur, induced by the pair instability and accompanied by supernova-like mass ejection, but the star eventually produces a large iron core in hydrostatic equilibrium. It is likely that this core, too, collapses to a black hole, thus cleanly separating the heavy element nucleosynthesis of pair instability supernovae from those of other masses, both above and below. Indeed, black hole formation is a likely outcome for all Population III stars with main sequence masses between about 25 M_sun and 140 M_sun (M_He = 9 to 63 M_sun) as well as those above 260 M_sun. Nucleosynthesis in pair-instability supernovae varies greatly with the mass of the helium core which determines the maximum temperature reached during the bounce. At the upper range of exploding core masses, a maximum of 57 M_sun of Ni56 is produced making these the most energetic and the brightest thermonuclear explosions in the universe. Integrating over a distribution of masses, we find that pair instability supernovae produce a roughly solar distribution of nuclei having even nuclear charge, but are remarkably deficient in producing elements with odd nuclear charge. Also, essentially no elements heavier than zinc are produced due to a lack of s- and r-processes. Comment: 20 pages, including 5 figures; accepted by ApJ

The Fragmentation of Pre-enriched Primordial Objects

Article

Apr 2001
MON NOT R ASTRON SOC

Recent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. In this paper, we study the effect of metallicity on the evolution of the gas in a collapsing dark matter mini-halo. We model such a system as an isolated 3\sigma peak of mass 2x10^6 M_sun that collapses at z_coll=30, using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either 10^-4 Z_sun or 10^-3 Z_sun. We find that the evolution proceeds very differently for the two cases. The gas in the lower metallicity simulation fails to undergo continued collapse and fragmentation, whereas the gas in the higher metallicity case dissipatively settles into the center of the dark matter halo. The central gas, characterized by densities n > 10^4 cm^-3, and a temperature, T \sim 90 K, which closely follows that of the CMB, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity, Z_crit \sim 5x10^-4 Z_sun, and its possible dependence on redshift. Compared to the pure H/He case, the fragmentation of the 10^-3 Z_sun gas leads to a larger relative number of low-mass clumps. Comment: Minor revisions, 7 pages, 6 figures, MNRAS in press

In the Beginning: The First Sources of Light and the Reionization of the Universe

Article

Oct 2000
PHYS REP

The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at redshift 30 and reionized most of the hydrogen in the universe by redshift 7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

On the Initial Mass Function of Population III Stars

Article

Oct 2000

The collapse and fragmentation of filamentary primordial gas clouds are explored using 1D and 2D hydrodynamical simulations coupled with the nonequilibrium processes of H2 formation. The simulations show that depending upon the initial density,there are two occasions for the fragmentation of primordial filaments. If a filament has relatively low initial density, the radial contraction is slow due to less effective H2 cooling. This filament tends to fragment into dense clumps before the central density reaches

10^{8-9}

^{-3}

, where H2 cooling by three-body reactions is effective and the fragment mass is more massive than some tens

M_\odot

. In contrast, if a filament is initially dense, the more effective H2 cooling with the help of three-body reactions allows the filament to contract up to

n\sim 10^{12}

^{-3}

. After the density reaches

n\sim 10^{12}

^{-3}

, the filament becomes optically thick to H2 lines and the radial contraction subsequently almost stops. At this final hydrostatic stage, the fragment mass is lowered down to

\approx 1M_\odot

because of the high density of the filament. The dependence of the fragment mass upon the initial density could be translated into the dependence on the local amplitude of random Gaussian density fields or the epoch of the collapse of a parent cloud. Hence, it is predicted that the initial mass function of Population III stars is likely to be bimodal with peaks of

\approx 10^2 M_\odot

and

\approx 1M_\odot

, where the relative heights could be a function of the collapse epoch. Comment: Accepted by ApJ

Protostellar Collapse with Various Metallicities

Article

Mar 2000

Kazuyuki Omukai

The thermal and chemical evolution of gravitationally collapsing protostellar clouds is investigated, focusing attention on their dependence on metallicity. Calculations are carried out for a range of metallicities spanning the local interstellar value to zero. During the time when clouds are transparent to continuous radiation, the temperatures are higher for those with lower metallicity, reflecting lower radiative ability. However, once the clouds become opaque, in the course of the adiabatic contraction of the transient cores, their evolutionary trajectories in the density-temperature plane converge to a unique curve that is determined by only physical constants. The trajectories coincide with each other thereafter. Consequently, the size of the stellar core at the formation is the same regardless of the gas composition of the parent cloud. Comment: 30 pages. The Astrophysical Journal, 533, in press

Particle Dark Matter: Evidence, Candidates and Constraints

Article

May 2004
PHYS REP

In this review article, we discuss the current status of particle dark matter, including experimental evidence and theoretical motivations. We discuss a wide array of candidates for particle dark matter, but focus on neutralinos in models of supersymmetry and Kaluza-Klein dark matter in models of universal extra dimensions. We devote much of our attention to direct and indirect detection techniques, the constraints placed by these experiments and the reach of future experimental efforts. (C) 2004 Published by Elsevier B.V.

Dark Matter Densities during the Formation of the First Stars and in Dark Stars

Article

May 2008

The first stars in the universe form inside

\sim 10^6 M_\odot

dark matter (DM) haloes whose initial density profiles are laid down by gravitational collapse in hierarchical structure formation scenarios. During the formation of the first stars in the universe, the baryonic infall compresses the dark matter further. The resultant dark matter density is presented here, using an algorithm originally developed by Young to calculate changes to the profile as the result of adiabatic infall in a spherical halo model; the Young prescription takes into account the non-circular motions of halo particles. The density profiles obtained in this way are found to be within a factor of two of those obtained using the simple adiabatic contraction prescription of Blumenthal et al. Our results hold regardless of the nature of the dark matter or its interactions and rely merely on gravity. If the dark matter consists of weakly interacting massive particles, which are their own antiparticles, their densities are high enough that their annihilation in the first protostars can indeed provide an important heat source and prevent the collapse all the way to fusion. In short, a ``Dark Star'' phase of stellar evolution, powered by DM annihilation, may indeed describe the first stars in the universe.

Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass

Article

Feb 2008
J COSMOL ASTROPART P

The annihilation of weakly interacting massive particles can provide an important heat source for the first (Pop III, 'Pop' standing for 'population') stars, potentially leading to a new phase of stellar evolution known as a 'dark star'. When dark matter (DM) capture via scattering off baryons is included, the luminosity from DM annihilation may dominate over the luminosity due to fusion, depending on the DM density and scattering cross section. The influx of DM due to capture may thus prolong the dark star phase of stellar evolution as long as the ambient DM density is high enough. Comparison of DM luminosity with the Eddington luminosity for the star may constrain the stellar mass of zero-metallicity stars. Alternatively, if sufficiently massive Pop III stars are found, they might be used to bound dark matter properties.

Dark Matter Capture and Annihilation on the First Stars: Preliminary Estimates

Article

Feb 2008
ASTROPHYS J

Fabio Iocco

Assuming that Dark Matter is dominated by WIMPs, it accretes by gravitational attraction and scattering over baryonic material and annihilates inside celestial objects, giving rise to a "Dark Luminosity" which may potentially affect the evolution of stars. We estimate the Dark Luminosity achieved by different kinds of stars in a halo with DM properties characteristic of the ones where the first star formation episode occurs. We find that either massive, metal-free and small, galactic-like stars can achieve Dark Luminosities comparable or exceeding their nuclear ones. This might have dramatic effects over the evolution of the very first stars, known as Population III. Comment: One table (with data for actual ZAMS metal-free stars) added with respect to published version

Formation of Massive Primordial Stars in a Reionized Gas

Article

Jun 2007
ASTROPHYS J

We use cosmological hydrodynamic simulations with unprecedented resolution to study the formation of primordial stars in an ionized gas at high redshifts. Our approach includes all the relevant atomic and molecular physics to follow the thermal evolution of a prestellar gas cloud to very high densities of ~10^{18} cm^{-3}. We locate a star-forming gas cloud within a reionized region in our cosmological simulation. The first run-away collapse is triggered when the gas cloud's mass is ~40 Msun. We show that the cloud core remains stable against chemo-thermal instability and also against gravitational deformation throughout its evolution. Consequently, a single proto-stellar seed is formed, which accretes the surrounding hot gas at the rate ~10^{-3} Msun/year. We carry out proto-stellar evolution calculations using the inferred accretion rate. The resulting mass of the star when it reaches the zero-age main sequence is M_ZAMS ~40 Msun. We argue that, since the obtained M_ZAMS is as large as the mass of the collapsing parent cloud, the final stellar mass should be close to this value. Such massive, rather than exceptionally massive, primordial stars are expected to cause early chemical enrichment of the Universe by exploding as black hole-forming super/hypernovae, and may also be progenitors of high redshift gamma-ray bursts. The elemental abundance patterns of recently discovered hyper metal-poor stars suggest that they might have been born from the interstellar medium that was metal-enriched by supernovae of these massive primordial stars. Comment: Revised version. To appear in ApJL

Formation of Primordial Protostars

Article

Nov 1998

The evolution of collapsing metal free protostellar clouds is investigated for various masses and initial conditions. We perform hydrodynamical calculations for spherically symmetric clouds taking account of radiative transfer of the molecular hydrogen lines and the continuum, as well as of chemistry of the molecular hydrogen. The collapse is found to proceed almost self-similarly like Larson-Penston similarity solution. In the course of the collapse, efficient three-body processes transform atomic hydrogen in an inner region of \sim 1 M_{\sun} entirely into molecular form. However, hydrogen in the outer part remains totally atomic although there is an intervening transitional layer of several solar masses, where hydrogen is in partially molecular form. No opaque transient core is formed although clouds become optically thick to H

_{2}

collision-induced absorption continuum, since H

_{2}

dissociation follows successively. When the central part of the cloud reaches stellar densities (

\sim 10^{-2} {\rm g cm^{-3}}

), a very small hydrostatic core (\sim 5 \times 10^{-3} M_{\sun}) is formed and subsequently grows in mass as the ambient gas accretes onto it. The mass accretion rate is estimated to be 3.7 \times 10^{-2} M_{\sun} {\rm yr^{-1}} (M_{\ast}/M_{\sun})^{-0.37}, where

M_{\ast}

is instantaneous mass of the central core, by using a similarity solution which reproduces the evolution of the cloud before the core formation. Comment: 20 pages, 5 Postscript figures, uses AAS LaTeX

First Structure Formation. I. Primordial Star-forming Regions in Hierarchical Models

Article

May 1997

We investigate the possibility of very early formation of primordial star clusters from high-\sigma perturbations in cold dark matter dominated structure formation scenarios. For this we have developed a powerful 2-level hierarchical cosmological code with a realistic and robust treatment of multi-species primordial gas chemistry, paying special attention to the formation and destruction of hydrogen molecules, non-equilibrium ionization, and cooling processes. We performed 3-D simulations at small scales and at high redshifts and find that, analogous to simulations of large scale structure, a complex system of filaments, sheets, and spherical knots at the intersections of filaments form. On the mass scales covered by our simulations (5x10^5 - 1x10^9\Ms) that collapse at redshifts z>25, we find that only at the spherical knots can enough H2 be formed (n_{H_2}/n_H > 5x10^-4) to cool the gas appreciably. Quantities such as the time dependence of the formation of H2 molecules, the final H2 fraction, and central densities from the simulations are compared to the theoretical predictions of Abel (1995) and Tegmark et al. (1997) and found to agree remarkably well. Comparing the 3-D results to an isobaric collapse model we further discuss the possible implications of the extensive merging of small structure that is inherent in hierarchical models. Typically only 5-8% percent of the total baryonic mass in the collapsing structures is found to cool significanlty. Assuming the Padoan (1995) model for star formation our results would predict the first stellar systems to be as small as ~30\Ms. Some implications for primordial globular cluster formation scenarios are also discussed. Comment: 22 pages, 13 Figures. Submitted to ApJ. Laboratory for Computational Astrophysics at the National Center for Supercomputing Applications

Dark matter annihilation effects on the first stars

Abstract

No full-text available

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Dark Matter Densities during the Formation of the First Stars and in Dark Stars

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