405
2,812.59
6.94
532

Publication History View all

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The existence of water in extrasolar planetary systems is of great interest because it constrains the potential for habitable planets and life. We have identified a circumstellar disk that resulted from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs.
    Science 10/2013; 342(6155):218-220.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides an instructive exception, and we present a close-up view of its quiescent x-ray emission based on 3 megaseconds of Chandra observations. Although the x-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission on the basis of the lack of predicted iron (Fe) Kα emission. The extremely weak hydrogen (H)-like Fe Kα line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.
    Science 08/2013; 341(6149):981-3.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Gaia is a cornerstone European Space Agency astrometry space mission and a successor to the Hipparcos mission. Gaia will observe the whole sky for 5 years, providing a serendipitous opportunity for the discovery of large numbers of transient and anomalous events, e.g. supernovae, novae and microlensing events, gamma-ray burst afterglows, fallback supernovae, as well as theoretical or unexpected phenomena. In this paper, we discuss our preparations to use Gaia to search for transients at optical wavelengths, and briefly describe the early detection, classification and prompt publication of anomalous sources.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 06/2013; 371(1992):20120239.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The core collapse supernova (CC SN) rate provides a strong lower limit for the star formation rate (SFR). Progress in using it as a cosmic SFR tracer requires some confidence that it is consistent with more conventional SFR diagnostics. We compare standard SFR measurements based on Halpha , Far Ultraviolet (FUV) and Total Infrared (TIR) galaxy luminosities with the observed CC SN rate in the same galaxy sample. The comparison can be viewed from two perspectives. Firstly, by adopting an estimate of the minimum stellar mass to produce a CC SN one can determine a SFR from SN numbers. Secondly, the radiative SFRs can be assumed to be robust and then the SN statistics provides a constraint on the minimum stellar mass for CC SN progenitors. We exploit the multi-wavelength data set from 11HUGS, a volume-limited survey designed to provide a census of SFR in the local Volume. There are 14 SNe discovered in this sample of galaxies within the last 13 years. Assuming a lower limit for CC SN progenitor of 8 M⊙, the CC SN rate matches the SFR from the FUV luminosity. However, the SFR based on mbox {M_{B}}ox{H\alpha} luminosity is lower than these two estimates by a factor of about 2. If we assume that the FUV or mbox {M_{B}}ox{H\alpha} based luminosities are a true reflection of the SFR, we find that the minimum mass for CC SN progenitors is 8 ±1 M⊙, and 6± 1 M⊙, respectively.
    Memorie della Societa Astronomica Italiana Supplementi. 01/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in galaxy merger events involving supermassive black holes, and in the formation of stellar mass binaries in star clusters. We work out the gravitational torque between the binary and disc, and show that their angular momenta stably counteralign if their initial orientation is sufficiently retrograde, specifically if the angle θ between them obeys cos θ < −Jd/2Jb, on a time short compared with the mass gain time of the central accretor(s). The magnitude Jb remains unchanged in this process. Counteralignment can promote the rapid merger of supermassive black hole binaries, and possibly the formation of coplanar but retrograde planets around stars in binary systems.
    Monthly Notices of the Royal Astronomical Society Letters 09/2011; 417(1):L66 - L69.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a model for the satellites of the Milky Way in which galaxy formation is followed using semi-analytic techniques applied to the six high-resolution N-body simulations of galactic haloes of the Aquarius project. The model, calculated using the galform code, incorporates improved treatments of the relevant physics in the Λ cold dark matter cosmogony, particularly a self-consistent calculation of reionization by ultraviolet (UV) photons emitted by the forming galaxy population, including the progenitors of the central galaxy. Along the merger tree of each halo, the model calculates gas cooling (by Compton scattering off cosmic microwave background photons, molecular hydrogen and atomic processes), gas heating (from hydrogen photoionization and supernova energy), star formation and evolution. The evolution of the intergalactic medium is followed simultaneously with that of the galaxies. Star formation in the more massive progenitor subhaloes is suppressed primarily by supernova feedback, while for smaller subhaloes, it is suppressed primarily by photoionization due to external and internal sources. The model is constrained to match a wide range of properties of the present-day galaxy population as a whole, but at high redshift it requires an escape fraction of UV photons near unity in order to completely reionize the universe by redshift z≳ 8. In the most successful model, the local sources photoionize the pre-galactic region completely by z≃ 10. In addition to the luminosity function of Milky Way satellites, the model matches their observed luminosity–metallicity relation, their radial distribution and the inferred values of the mass within 300 pc, which in the models increase slowly but significantly with luminosity. There is a large variation in satellite properties from halo to halo, with the luminosity function, for example, varying by a factor of ∼2 among the six simulations.
    Monthly Notices of the Royal Astronomical Society 09/2011; 417(2):1260 - 1279.
  • [Show abstract] [Hide abstract]
    ABSTRACT: There exists a significant population of broad line, z∼ 2 quasi-stellar objects (QSOs) which have heavily absorbed X-ray spectra. Follow-up observations in the submillimetre show that these QSOs are embedded in ultraluminous starburst galaxies, unlike most unabsorbed QSOs at the same redshifts and luminosities. Here we present X-ray spectra from XMM–Newton for a sample of five such X-ray-absorbed QSOs that have been detected at submillimetre wavelengths. We also present spectra in the rest-frame ultraviolet from ground-based telescopes. All the five QSOs are found to exhibit strong C iv absorption lines in their ultraviolet spectra with equivalent width >5 Å. The X-ray spectra are inconsistent with the hypothesis that these objects show normal QSO continua absorbed by low-ionization gas. Instead, the spectra can be modelled successfully with ionized absorbers, or with cold absorbers if they possess unusually flat X-ray continuum shapes and unusual optical to X-ray spectral energy distributions. We show that the ionized absorber model provides the simplest, most self-consistent explanation for their observed properties. We estimate that the fraction of radiated power that is converted into kinetic luminosity of the outflowing winds is typically ∼4 per cent, in agreement with recent estimates for the kinetic feedback from QSOs required to produce the M–σ relation, and consistent with the hypothesis that the X-ray-absorbed QSOs represent the transition phase between obscured accretion and the luminous QSO phase in the evolution of massive galaxies.
    Monthly Notices of the Royal Astronomical Society 08/2011; 416(4):2792 - 2801.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: ABSTRACTA quarter of DA white dwarfs are metal polluted, yet elements heavier than helium sink down through the stellar atmosphere on time-scales of days. Hence, these white dwarfs must be the currently accreting material containing heavy elements. Here we consider whether the scattering of comets or asteroids from an outer planetary system, following stellar mass-loss on the asymptotic giant branch, can reproduce these observations. We use N-body simulations to investigate the effects of stellar mass-loss on a simple system consisting of a planetesimal belt whose inner edge is truncated by a planet. Our simulations find that, starting with a planetesimal belt population fitted to the observed main-sequence evolution, sufficient mass is scattered into the inner planetary system to explain the inferred heavy element accretion rates. This assumes that a fraction of the mass scattered into the inner planetary system ends up on star-grazing orbits, is tidally disrupted and is accreted on to the white dwarf. The simulations also reproduce the observed decrease in accretion rate with cooling age and predict accretion rates in old (>1 Gyr) white dwarfs, in line with observations. The efficiency we assumed for material scattered into the inner planetary system to end up on star-grazing orbits is based on a solar-like planetary system, since the simulations show that a single planet is not sufficient. Although the correct level of accretion is reproduced, the simulations predict a higher fraction of accreting white dwarfs than observed. This could indicate that the evolved planetary systems are less efficient in scattering bodies on to star-grazing orbits or that dynamical instabilities post-stellar mass-loss cause rapid planetesimal belt depletion for a significant fraction of systems.
    Monthly Notices of the Royal Astronomical Society 06/2011; 414(2):930 - 939.
  • Science 12/2010; 330(6012):1763.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper introduces new phase-space models of dwarf spheroidal galaxies (dSphs). The stellar component has an isotropic, lowered isothermal (or King) distribution function. A physical basis for the isotropization of stellar velocities is given by the theory of tidal stirring, whilst the isothermality of the distribution function guarantees the observed flatness of the velocity dispersion profile in the inner parts. For any analytic dark matter potential – whether of cusped or of cored form – the stellar density and velocity dispersion are analytic.The origin of the observational correlation between half-light radius Rh and line-of-sight central velocity dispersion σp, 0 is investigated. We prove that a power-law correlation Rh∝σDp,0 can exist if, and only if, the dark halo potential is a power law of the radius. Although a power law is a good approximation in the central parts (D= 2 for a Navarro–Frenk–White halo, D= 1 for cored haloes), the theoretical correlation curve between Rh and σp, 0 dramatically steepens at larger half-light radii. Using our phase-space models, we show that different dark halo profiles – whether cored or cusped – lead to very similar mass estimates within one particular radius, ≈1.7Rh. The formula for the enclosed mass M(<1.7Rh) is ≈5.8 σ2p,0Rh/G and extends out to larger radii than those in previous investigations. This is a tight result for models with a flattish projected velocity dispersion profile (out to several half-light radii). We show that deviations between mass measures due to different density profiles are substantially smaller than the uncertainties propagated by the observational errors on the half-light radius and central velocity dispersion. We produce a mass measure for each of the dSphs and find that the two most massive of the Milky Way dSphs are the most luminous, namely Sgr (M(<1.7Rh) ∼ 2.8 × 108 M⊙) and Fornax (∼1.3 × 108 M⊙). The least massive of the Milky Way satellites are Willman 1 (∼4 × 105 M⊙) and Segue 1 (∼6 × 105 M⊙).
    Monthly Notices of the Royal Astronomical Society 11/2010; 411(4):2118 - 2136.
Information provided on this web page is aggregated encyclopedic and bibliographical information relating to the named institution. Information provided is not approved by the institution itself. The institution’s logo (and/or other graphical identification, such as a coat of arms) is used only to identify the institution in a nominal way. Under certain jurisdictions it may be property of the institution.
View all

Top publications last week

 
Astronomy and Astrophysics 05/2012;
9 Downloads
 
Monthly Notices of the Royal Astronomical Society 11/2008; 391(1):136 - 163.
6 Downloads