Enrico Ramirez-Ruiz

University of California, Santa Cruz, Santa Cruz, California, United States

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Publications (278)1407.64 Total impact

  • Justyn R. Maund · Enrico Ramirez-Ruiz
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    ABSTRACT: We present an analysis of late-time Hubble Space Telescope Wide Field Camera 3 (WFC3) and Wide Field Planetary Camera 2 (WFPC2) observations of the site of the Type Ic supernova (SN) 2007gr in NGC 1058. The SN is barely recovered in the late-time WFPC2 observations, while a possible detection in the later WFC3 data is debatable. These observations were used to conduct a multiwavelength study of the surrounding stellar population. We fit spatial profiles to a nearby bright source that was previously proposed to be a host cluster. We find that, rather than being an extended cluster, it is consistent with a single point-like object. Fitting stellar models to the observed spectral energy distribution of this source, we conclude it is A1-A3 Yellow Supergiant, possibly corresponding to a star with MZAMS = 40 M⊙. SN 2007gr is situated in a massive star association, with diameter of ≈300 pc. We present a Bayesian scheme to determine the properties of the surrounding massive star population, in conjunction with the Padova isochrones. We find that the stellar population, as observed in either the WFC3 and WFPC2 observations, can be well fit by two age distributions with mean ages: ∼6.3 Myr and ∼50 Myr. The stellar population is clearly dominated by the younger age solution (by factors of 3.5 and 5.7 from the WFPC2 and WFC3 observations, respectively), which corresponds to the lifetime of a star with MZAMS ∼ 30 M⊙. This is strong evidence in favour of the hypothesis that SN 2007gr arose from a massive progenitor star, possibly capable of becoming a Wolf–Rayet star.
    No preview · Article · Mar 2016 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{\mathrm{UV}} = 3.5 \times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a High-Velocity Cloud), and narrow absorption and broad (FWHM $\approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $\Delta v = -(250$-400) km s$^{-1}$. Together with the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars.
    No preview · Article · Jan 2016
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    ABSTRACT: We have observed a sample of 10 white dwarf candidates in the rich open cluster NGC 2323 (M50) with the Keck Low-Resolution Imaging Spectrometer. The spectroscopy shows eight to be DA white dwarfs, with six of these having high S/N appropriate for our analysis. Two of these white dwarfs are consistent with singly evolved cluster membership, and both are high mass $\sim$1.07 M$_\odot$, and give equivalent progenitor masses of 4.69 M$_\odot$. To supplement these new high-mass white dwarfs and analyze the initial-final mass relation (IFMR), we have also looked at 30 white dwarfs from publicly available data that are mostly all high-mass ($\gtrsim$0.9 M$_\odot$). These original published data exhibited significant scatter, and to test if this scatter is true or simply the result of systematics, we have uniformly analyzed the white dwarf spectra and have adopted thorough photometric techniques to derive uniform cluster parameters for their parent clusters. The resulting IFMR scatter is significantly reduced, arguing that mass-loss rates are not stochastic in nature and that within the ranges of metallicity and mass analyzed in this work mass loss is not highly sensitive to variations in metallicity. Lastly, when adopting cluster ages based on Y$^2$ isochrones, the slope of the high-mass IFMR remains steep and consistent with that found from intermediate-mass white dwarfs, giving a linear IFMR from progenitor masses between 3 to 6.5 M$_\odot$. In contrast, when adopting the slightly younger cluster ages based on PARSEC isochrones, the high-mass IFMR has a moderate turnover near an initial mass of 4 M$_\odot$.
    No preview · Article · Jan 2016 · The Astrophysical Journal
  • Nathaniel Roth · Daniel Kasen · James Guillochon · Enrico Ramirez-Ruiz
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    ABSTRACT: Observations of luminous flares resulting from the possible tidal disruption of stars by supermassive black holes have raised a number of puzzles. Outstanding questions include the origin of the optical and ultraviolet (UV) flux, the weakness of hydrogen lines in the spectrum, and the occasional simultaneous observation of x-rays. Here we study the emission from tidal disruption events (TDEs) produced as radiation from black hole accretion propagates through an extended, optically thick envelope formed from stellar debris. We analytically describe key physics controlling spectrum formation, and present detailed radiative transfer calculations that model the spectral energy distribution (SED) and optical line strengths of TDEs near peak brightness. The steady-state transfer is coupled to a non local thermodynamic equilibrium treatment of the excitation and ionization states of hydrogen, helium and oxygen (as a representative metal). Our calculations show how an extended envelope can reprocess a fraction of soft x-rays and produce the observed optical fluxes of order 10^43 ergs per second. Variations in the mass or size of the envelope may help explain how the optical flux changes over time with roughly constant color. For high enough accretion luminosities, x-rays can highly ionize the reprocessing region and escape to be observed simultaneously with the optical flux, producing an SED not described by a single blackbody. Due to optical depth effects, hydrogen Balmer line emission is often strongly suppressed relative to helium line emission (with HeII-to-H line ratios of at least 5:1 in some cases) even in the disruption of a solar-composition star. We discuss the implications of our results to understanding the type of stars destroyed in TDEs and the physical processes responsible for producing the observed flares.
    No preview · Article · Oct 2015
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    ABSTRACT: Tidal forces close to massive black holes can violently disrupt stars that make a close approach. These extreme events are discovered via bright X-ray and optical/UV flares in galactic centers. Prior studies based on modeling decaying flux trends have been able to estimate broad properties, such as the mass accretion rate. Here we report the detection of flows of highly ionized X-ray gas in high-resolution X-ray spectra of a nearby tidal disruption event. Variability within the absorption-dominated spectra indicates that the gas is relatively close to the black hole. Narrow line widths indicate that the gas does not stretch over a large range of radii, giving a low volume filling factor. Modest outflow speeds of a few hundred kilometers per second are observed, significantly below the escape speed from the radius set by variability. The gas flow is consistent with a rotating wind from the inner, super-Eddington region of a nascent accretion disk, or with a filament of disrupted stellar gas near to the apocenter of an elliptical orbit. Flows of this sort are predicted by fundamental analytical theory and more recent numerical simulations.
    Preview · Article · Oct 2015 · Nature
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    ABSTRACT: We present our extensive observational campaign on the Swift-discovered GRB141121A, al- most ten years after its launch. Our observations covers radio through X-rays, and extends for more than 30 days after discovery. The prompt phase of GRB 141121A lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is E{\gamma},iso = 8.0x10^52 erg. Due to the long prompt duration, GRB141121A falls into the recently discovered class of UL-GRBs. Peculiar features of this burst are a flat early-time optical light curve and a radio-to-X-ray rebrightening around 3 days after the burst. The latter is followed by a steep optical-to-X-ray decay and a much shallower radio fading. We analyze GRB 141121A in the context of the standard forward-reverse shock (FS,RS) scenario and we disentangle the FS and RS contributions. Finally, we comment on the puzzling early-time (t ~3 d) behavior of GRB 141121A, and suggest that its interpretation may require a two-component jet model. Overall, our analysis confirms that the class of UL-GRBs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful GRB explosions, and future missions (like SVOM, XTiDE, or ISS-Lobster) will provide many more of such objects.
    Full-text · Article · Oct 2015 · The Astrophysical Journal
  • Morgan MacLeod · Michele Trenti · Enrico Ramirez-Ruiz
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    ABSTRACT: When embedded in dense cluster cores, intermediate mass black holes (IMBHs) acquire close stellar or stellar-remnant companions. These companions are not only gravitationally bound, they tend to hierarchically isolate from other cluster stars through series of multibody encounters. In this paper, we study the demographics of IMBH companions in compact star clusters through direct $N$-body simulation. We study clusters initially composed of $10^5$ or $2\times 10^5$ stars with IMBHs of 75 and 150 solar masses, and follow their evolution for 6-10 Gyr. A tight innermost binary pair of IMBH and stellar object rapidly forms. The IMBH has a companion with orbital semi-major axis at least three times tighter than the second-most bound object over 90% of the time. These companionships have typical periods of order years and are subject to cycles of exchange and destruction. The most frequently observed, long-lived pairings persist for $\sim 10^7$ yr. The demographics of IMBH companions in clusters are diverse; they include both main sequence, giant stars, and stellar remnants. Companion objects may reveal the presence of an IMBH in a cluster in one of several ways. Most-bound companion stars routinely suffer grazing tidal interactions with the IMBH, offering a dynamical mechanism to produce repeated flaring episodes like those seen in the IMBH candidate HLX-1. Stellar winds of companion stars provide a minimum quiescent accretion rate for IMBHs, with implications for radio searches for IMBH accretion in globular clusters. Finally, gravitational wave inspirals of compact objects are found to occur with promising frequency.
    No preview · Article · Aug 2015
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    ABSTRACT: The merger of two white dwarfs (WDs) has for many years not been considered as the favoured model for the progenitor system of Type Ia supernovae (SNe Ia). But recent years have seen a change of opinion as a number of studies, both observational and theoretical, have concluded that they should contribute significantly to the observed SN Ia rate. In this paper, we study the ignition and propagation of detonation through post-merger remnants and we follow the resulting nucleosynthesis up to the point where a homologous expansion is reached. In our study we cover the entire range of WD masses and compositions. For the emergence of a detonation we study three different setups. The first two are guided by the merger remnants from our earlier simulations, while for the third one the ignitions were set by placing hotspots with properties determined by spatially resolved calculations taken from the literature. There are some caveats to our approach which we investigate. We carefully compare the nucleosynthetic yields of successful explosions with SN Ia observations. Only three of our models are consistent with all the imposed constraints and potentially lead to a standard Type Ia event. The first one, a 0.45 M⊙ helium (He) + 0.9 M⊙ carbon–oxygen (CO) WD system produces a sub-luminous, SN 1991bg-like event while the other two, a 0.45 M⊙ He+1.1 M⊙ oxygen–neon WD system and a 1.05 + 1.05 M⊙ system with two CO WDs, are good candidates for common SNe Ia.
    Preview · Article · Aug 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: In this paper, we model the observable signatures of tidal disruptions of white dwarf (WD) stars by massive black holes (MBHs) of moderate mass, $\approx 10^3 - 10^5 M_\odot$. When the WD passes deep enough within the MBH's tidal field, these signatures include thermonuclear transients from burning during maximum compression. We combine a hydrodynamic simulation that includes nuclear burning of the disruption of a $0.6 M_\odot$ C/O WD with a Monte Carlo radiative transfer calculation to synthesize the properties of a representative transient. The transient's emission emerges in the optical, with lightcurves and spectra reminiscent of type I SNe. The properties are strongly viewing-angle dependent, and key spectral signatures are $\approx 10,000$ km s$^{-1}$ Doppler shifts due to the orbital motion of the unbound ejecta. Disruptions of He WDs likely produce large quantities of intermediate-mass elements, offering a possible production mechanism for Ca-rich transients. Accompanying multiwavelength transients are fueled by accretion and arise from the nascent accretion disk and relativistic jet. If MBHs of moderate mass exist with number densities similar to those of supermassive MBHs, both high energy wide-field monitors and upcoming optical surveys should detect tens to hundreds of WD tidal disruptions per year. The current best strategy for their detection may therefore be deep optical follow up of high-energy transients of unusually-long duration. The detection rate or the non-detection of these transients by current and upcoming surveys can thus be used to place meaningful constraints on the extrapolation of the MBH mass function to moderate masses.
    Preview · Article · Aug 2015
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    ABSTRACT: We present optical and near-infrared light curves and optical spectra of SN 2013dx, associated with the nearby (redshift 0.145) gamma-ray burst GRB130702A. The prompt isotropic gamma-ray energy released from GRB130702A is measured to be E ;iso = 6:4+1:3
    Full-text · Article · Aug 2015 · The Astrophysical Journal
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    ABSTRACT: We have expanded the sample of observed white dwarfs in the rich open cluster NGC 2099 (M37) with the Keck Low-Resolution Imaging Spectrometer. Of 20 white dwarf candidates, the spectroscopy shows 19 to be true white dwarfs with 14 of these having high S/N. We find 11 of these 14 to be consistent with singly evolved cluster members. They span a mass range of $\sim$0.7 to 0.95 M$_\odot$, excluding a low-mass outlier, corresponding to progenitor masses of $\sim$3 to 4 M$_\odot$. This region of the initial final mass relation (IFMR) has large scatter and a slope that remains to be precisely determined. With this large sample of white dwarfs that belong to a single age and metallicity population, we find an initial-final mass relation of (0.171$\pm$0.057)M$_{\rm initial}$+0.219$\pm$0.187 M$_\odot$, significantly steeper than the linear relation adopted over the full observed white dwarf mass range in many previous studies. Comparison of this new relation from the solar metallicity NGC 2099 to 18 white dwarfs in the metal-rich Hyades and Praesepe shows that their IFMR also has a consistently steep slope. This strong consistency also suggests that there is no significant metallicity dependence of the IFMR at this mass and metallicity range. As a result, the IFMR can be more reliably determined with this broad sample of 29 total white dwarfs giving M$_{\rm final}$=(0.163$\pm$0.022)M$_{\rm initial}$+0.238$\pm$0.071 M$_\odot$ from M$_{\rm initial}$ of 3 to 4 M$_\odot$. A steep IFMR in this mass range indicates that the full IFMR is nonlinear.
    Full-text · Article · May 2015 · The Astrophysical Journal
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    ABSTRACT: Neutron star (binary neutron star and neutron star - black hole) mergers are believed to produce short-duration gamma-ray bursts. They are also believed to be the dominant source of gravitational waves to be detected by the advanced LIGO and the dominant source of the heavy r-process elements in the universe. Whether or not these mergers produce short-duration GRBs depends sensitively on the fate of the core of the remnant (whether, and how quickly, it forms a black hole). In this paper, we combine the results of merger calculations and equation of state studies to determine the fate of the cores of neutron star mergers. Using population studies, we can determine the distribution of these fates to compare to observations. We find that black hole cores form quickly only for equations of state that predict maximum non-rotating neutron star masses below 2.3-2.4 solar masses. If quick black hole formation is essential in producing gamma-ray bursts, LIGO observed rates compared to GRB rates could be used to constrain the equation of state for dense nuclear matter.
    Preview · Article · Apr 2015 · The Astrophysical Journal
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    ABSTRACT: We report here the discovery by the Intermediate Palomar Transient Factory (iPTF) of iPTF14yb, a luminous ($M_{r}\approx-27.8$ mag), cosmological (redshift 1.9733), rapidly fading optical transient. We demonstrate, based on probabilistic arguments and a comparison with the broader population, that iPTF14yb is the optical afterglow of the long-duration gamma-ray burst GRB 140226A. This marks the first unambiguous discovery of a GRB afterglow prior to (and thus entirely independent of) an associated high-energy trigger. We estimate the rate of iPTF14yb-like sources (i.e., cosmologically distant relativistic explosions) based on iPTF observations, inferring an all-sky value of $\Re_{\mathrm{rel}}=610$ yr$^{-1}$ (68% confidence interval of 110-2000 yr$^{-1}$). Our derived rate is consistent (within the large uncertainty) with the all-sky rate of on-axis GRBs derived by the Swift satellite. Finally, we briefly discuss the implications of the nondetection to date of bona fide "orphan" afterglows (i.e., those lacking detectable high-energy emission) on GRB beaming and the degree of baryon loading in these relativistic jets.
    Full-text · Article · Apr 2015
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    James Guillochon · Enrico Ramirez-Ruiz
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    ABSTRACT: The disruption of a main-sequence star by a supermassive black hole results in the initial production of an extended debris stream that winds repeatedly around the black hole, producing a complex three-dimensional figure that may self-intersect. Both analytical work and simulations have shown that typical encounters generate streams that are extremely thin. In this paper we show that this implies that even small relativistic precessions attributed to black hole spin can induce deflections that prevent the stream from self-intersecting even after many windings. Additionally, hydrodynamical simulations have demonstrated that energy is deposited very slowly via hydrodynamic processes alone, resulting in the liberation of very little gravitational binding energy in the absence of stream-stream collisions. This naturally leads to a "dark period" in which the flare is not observable for some time, persisting for up to a dozen orbital periods of the most bound material, which translates to years for disruptions around black holes with mass $\sim 10^{7} M_{\odot}$. We find that more-massive black holes tend to have more violent stream self-intersections, resulting in short viscous times that lead to prompt accretion onto the black hole. For these tidal disruption events (TDEs), the accretion rate onto the black hole should still closely follow the original fallback rate after a fixed delay time $t_{\rm delay}$. For lower black hole masses ($M_{\rm h} \lesssim 10^{6}$), we find that flares are typically slowed down by about an order of magnitude, and because the accretion rates for TDEs about higher-mass black hole are already sub-Eddington, this results in the majority of TDEs being sub-Eddington at peak. This also implies that current searches for TDEs are biased towards prompt flares, with slowed flares likely having been unidentified. [abridged]
    Preview · Article · Jan 2015 · The Astrophysical Journal
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    ABSTRACT: We present optical and near-infrared (NIR) photometry of 28 gamma-ray bursts (GRBs) detected by the Swift satellite and rapidly observed by the Reionization and Transients Infrared/Optical (RATIR) camera. We compare the optical flux at fiducial times of 5.5 and 11 h after the high-energy trigger to that in the X-ray regime to quantify optical darkness. 46 ± 9 per cent (13/28) of all bursts in our sample and 55 ± 10 per cent (13/26) of long GRBs are optically dark, which is statistically consistently with previous studies. Fitting RATIR optical and NIR spectral energy distributions of 19 GRBs, most (6/7) optically dark GRBs either occur at high redshift (z > 4.5) or have a high dust content in their host galaxies (AV > 0.3). Performing Kolmogorov–Smirnov tests, we compare the RATIR sample to those previously presented in the literature, finding our distributions of redshift, optical darkness, host dust extinction and X-ray-derived column density to be consistent. The one reported discrepancy is with host galaxy dust content in the BAT6 sample, which appears inconsistent with our sample and other previous literature. Comparing X-ray-derived host galaxy hydrogen column densities to host galaxy dust extinction, we find that GRBs tend to occur in host galaxies with a higher metal-to-dust ratio than our own Galaxy, more akin to the Large and Small Magellanic Clouds. Finally, to mitigate time evolution of optical darkness, we measure βOX, rest at a fixed rest-frame time, trest = 1.5 h and fixed rest-frame energies in the X-ray and optical regimes. Choosing to evaluate optical flux at λrest = 0.25 μm, we remove high redshift as a source of optical darkness, demonstrating that optical darkness must result from either high redshift, dust content in the host galaxy along the GRB sight line, or a combination of the two.
    Full-text · Article · Dec 2014 · Monthly Notices of the Royal Astronomical Society
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    J. P. Naiman · E. Ramirez-Ruiz · J. Debuhr · C. -P. Ma
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    ABSTRACT: During galaxy mergers the gas falls to the center, triggers star formation, and feeds the rapid growth of supermassive black holes (SMBHs). SMBHs respond to this fueling by supplying energy back to the ambient gas. Numerical studies suggest that this feedback is necessary to explain why the properties of SMBHs and the formation of bulges are closely related. This intimate link between the SMBH's mass and the large scale dynamics and luminosity of the host has proven to be a difficult issue to tackle with simulations due to the inability to resolve all the relevant length scales simultaneously. In this paper we simulate SMBH growth at high-resolution with {\it FLASH}, accounting for the gravitational focusing effects of nuclear star clusters (NSCs), which appear to be ubiquitous in galactic nuclei. In the simulations, the NSC core is resolved by a minimum cell size of about 0.001 pc or approximately $10^{-3}$ of the cluster's radius. We discuss the conditions required for effective gas funneling to occur, which are mainly dominated by a relationship between NSC velocity dispersion and the local sound speed, and provide a sub-grid prescription for the augmentation of central SMBH accretion rates in the presence of NSCs. For the conditions expected to persist in the centers of merging galaxies, the resultant large central gas densities in NSCs should produce drastically enhanced embedded SMBH accretion rates - up to an order of magnitude increase can be achieved for gas properties resembling those in large-scale galaxy merger simulations. This will naturally result in faster black hole growth rates and higher luminosities than predicted by the commonly used Bondi-Hoyle-Lyttleton accretion formalism.
    Preview · Article · Oct 2014 · The Astrophysical Journal
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    ABSTRACT: We present follow-up observations of an optical transient (OT) discovered by ROTSE on Jan. 21, 2009. Photometric monitoring was carried out with ROTSE-IIIb in the optical and Swift in the UV up to +70 days after discovery. The light curve showed a fast rise time of ~10 days followed by a steep decline over the next 60 days, which was much faster than that implied by 56Ni - 56Co radioactive decay. The SDSS DR10 database contains a faint, red object at the position of the OT, which appears slightly extended. This and other lines of evidence suggest that the OT is of extragalactic origin, and this faint object is likely the host galaxy. A sequence of optical spectra obtained with the 9.2-m Hobby-Eberly Telescope (HET) between +8 and +45 days after discovery revealed a hot, blue continuum with no visible spectral features. A few weak features that appeared after +30 days probably originated from the underlying host. Fitting synthetic templates to the observed spectrum of the host galaxy revealed a redshift of z = 0.19. At this redshift the peak magnitude of the OT is close to -22.5, similar to the brightest super-luminous supernovae; however, the lack of identifiable spectral features makes the massive stellar death hypothesis less likely. A more plausible explanation appears to be the tidal disruption of a sun-like star by the central super-massive black hole. We argue that this transient likely belongs to a class of super-Eddington tidal disruption events.
    Preview · Article · Oct 2014 · The Astrophysical Journal
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    Morgan MacLeod · Enrico Ramirez-Ruiz
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    ABSTRACT: This paper models the orbital inspiral of a neutron star (NS) through the envelope of its giant-branch companion during a common envelope (CE) episode. These CE episodes are necessary to produce close pairs of NSs that can inspiral and merge due to gravitational wave losses in less than a Hubble time. Because cooling by neutrinos can be very efficient, NSs have been predicted to accumulate significant mass during CE events, perhaps enough to lead them to collapse to black holes. We revisit this conclusion with the additional consideration of CE structure, particularly density gradients across the embedded NS's accretion radius. This work is informed by our recent numerical simulations that find that the presence of a density gradient strongly limits accretion by imposing a net angular momentum to the flow around the NS. Our calculations suggest that NSs should survive CE encounters. They accrete only modest amounts of envelope material, $\lesssim 0.1M_\odot$, which is broadly consistent with mass determinations of double NS binaries. With less mass gain, NSs must spiral deeper to eject their CE, leading to a potential increase in mergers. The survival of NSs in CE events has implications for the formation mechanism of observed double NS binaries, as well as for predicted rates of NS binary gravitational wave inspirals and their electromagnetic counterparts.
    Preview · Article · Oct 2014
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    Morgan MacLeod · Enrico Ramirez-Ruiz
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    ABSTRACT: This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). These embedded objects spiral to tighter separations because of drag that is generated when gas collides and shocks as it is gravitationally focused. This flow convergence is expected to lead to gas accretion onto the inspiralling object. This process has been studied numerically and analytically in the context of Hoyle-Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of 3D HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by 1-2 orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically-derived rates of drag and accretion as a function of the density gradient, which can be applied to any CE system. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.
    Preview · Article · Oct 2014 · The Astrophysical Journal
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    ABSTRACT: Investigations of element abundances in the ancient and most metal deficient stars are extremely important because they serve as tests of variable nucleosynthesis pathways and can provide critical inferences of the type of stars that lived and died before them. The presence of r-process elements in a handful of carbon-enhanced metal-poor (CEMP) stars, which are assumed to be closely connected to the chemical yield from the first stars, is hard to reconcile with standard neutron star mergers. Here we show that the production rate of dynamically assembled compact binaries in high-z nuclear star clusters can attain a sufficient high value to be a potential viable source of heavy r-material in CEMP stars. The predicted frequency of such events in the early Galaxy, much lower than the frequency of Type II supernovae but with significantly higher mass ejected per event, can naturally lead to a high level of scatter of Eu as observed in CEMP stars.
    Preview · Article · Oct 2014

Publication Stats

8k Citations
1,407.64 Total Impact Points

Institutions

  • 2000-2015
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 2014
    • Harvard University
      Cambridge, Massachusetts, United States
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2011
    • University of Hertfordshire
      • Centre for Astrophysics Research (CAR)
      Hatfield, England, United Kingdom
  • 2009
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States
  • 2008
    • University of California Observatories
      Santa Cruz, California, United States
    • University of Washington Seattle
      • Institute for Nuclear Theory
      Seattle, WA, United States
  • 2001-2008
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, ENG, United Kingdom
  • 1999-2008
    • Los Alamos National Laboratory
      • Space Science and Applications Group
      Лос-Аламос, California, United States
  • 2004-2006
    • Institute for Advanced Study
      Princeton Junction, New Jersey, United States
  • 2005
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
    • Institute for Advanced Study
      Princeton, New Jersey, United States
    • University of Southampton
      • Department of Physics and Astronomy
      Southampton, England, United Kingdom
  • 1998
    • Universidad Nacional Autónoma de México
      • School of Science
      Ciudad de México, Mexico City, Mexico