On monolithic supermassive stars Woods, Tyrone E; Heger, Alexander; Haemmerlé, Lionel
Monthly notices of the Royal Astronomical Society,
05/2020, Letnik:
494, Številka:
2
Journal Article
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ABSTRACT
Supermassive stars have been proposed as the progenitors of the massive ($\sim \!10^{9}\, \mathrm{M}_{\odot }$) quasars observed at z ∼ 7. Prospects for directly detecting supermassive stars ...with next-generation facilities depend critically on their intrinsic lifetimes, as well as their formation rates. We use the one-dimensional stellar evolution code kepler to explore the theoretical limiting case of zero-metallicity non-rotating stars, formed monolithically with initial masses between $10$ and $190\, \mathrm{kM}_{\odot }$. We find that stars born with masses between $\sim\! 60$ and $\sim\! 150\, \mathrm{kM}_{\odot }$ collapse at the end of the main sequence, burning stably for $\sim\! 1.5\, \mathrm{Myr}$. More massive stars collapse directly through the general relativistic instability after only a thermal time-scale of $\sim\! 3$–$4\, \mathrm{kyr}$. The expected difficulty in producing such massive thermally relaxed objects, together with recent results for currently preferred rapidly accreting formation models, suggests that such ‘truly direct’ or ‘dark’ collapses may not be typical for supermassive objects in the early Universe. We close by discussing the evolution of supermassive stars in the broader context of massive primordial stellar evolution and the possibility of supermassive stellar explosions.
Abstract The ever-growing sample of observed supernovae (SNe) enhances our capacity for comprehensive SN population studies, providing a richer data set for understanding the diverse characteristics ...of Type Ia supernovae (SNe Ia) and possibly those of their progenitors. Here, we present a data-driven analysis of observed SN Ia photometric light curves collected in the Open Supernova Catalog. Where available, we add the environmental information from the host galaxy. We focus on identifying subclasses of SNe Ia without imposing the predefined subclasses found in the literature to date. To do so, we employ an implicit rank-minimizing autoencoder neural network for developing low-dimensional data representations, providing a compact representation of the SN light-curve diversity. When we analyze light curves alone, we find that one of our resulting latent variables is strongly correlated with redshift, allowing us to approximately “de-redshift” the other latent variables describing each event. After doing so, we find that three of our latent variables account for ∼95% of the variance in our sample, and provide a natural separation between 91T and 91bg thermonuclear SNe. Of note, the 02cx subclass is not unambiguously delineated from the 91bg sample in our results, nor do either the overluminous 91T or the underluminous 91bg/02cx samples form a clearly distinct population from the broader sample of “other” SN Ia events. We identify the physical characteristics of SN light curves that best distinguish SNe 91T from SNe 91bg and 02cx, and discuss prospects for future refinements and applications to other classes of SNe as well as other transients.
ABSTRACT
Type Ia supernovae are understood to arise from the thermonuclear explosion of a carbon–oxygen white dwarf, yet the evolutionary mechanisms leading to such events remain unknown. Many ...proposed channels, including the classical single-degenerate scenario, invoke a hot, luminous evolutionary phase for the progenitor, in which it is a prodigious source of photoionizing emission. Here, we examine the environment of SN 2014J for evidence of a photoionized nebula in pre- and post-explosion O iii λ5007 Å and H β images taken with the Hubble Space Telescope. From the absence of any extended emission, we exclude a stable nuclear-burning white dwarf at the location of SN 2014J in the last ∼100 000 years, assuming a typical warm interstellar medium (ISM) particle density of 1 cm−3. These limits greatly exceed existing X-ray constraints at temperatures typical of known supersoft sources. Significant extreme-UV/soft X-ray emission prior to explosion remains plausible for lower ISM densities (e.g. $n_{\rm ISM}\sim 0.1\,\rm {cm}^{-3}$). In this case, however, any putative nebula would be even more extended, allowing deeper follow-up observations to resolve this ambiguity in the near future.
ABSTRACT
A class of optical transients known as Luminous Red Novae (LRNe) have recently been associated with mass ejections from binary stars undergoing common-envelope evolution. We use the ...population synthesis code COMPAS to explore the impact of a range of assumptions about the physics of common-envelope evolution on the properties of LRNe. In particular, we investigate the influence of various models for the energetics of LRNe on the expected event rate and light curve characteristics, and compare with the existing sample. We find that the Galactic rate of LRNe is ∼0.2 yr−1, in agreement with the observed rate. In our models, the luminosity function of Galactic LRNe covers multiple decades in luminosity and is dominated by signals from stellar mergers, consistent with observational constraints from iPTF and the Galactic sample of LRNe. We discuss how observations of the brightest LRNe may provide indirect evidence for the existence of massive (>40 M⊙) red supergiants. Such LRNe could be markers along the evolutionary pathway leading to the formation of double compact objects. We make predictions for the population of LRNe observable in future transient surveys with the Large Synoptic Survey Telescope and the Zwicky Transient Facility. In all plausible circumstances, we predict a selection-limited observable population dominated by bright, long-duration events caused by common envelope ejections. We show that the Large Synoptic Survey Telescope will observe 20–750 LRNe per year, quickly constraining the luminosity function of LRNe and probing the physics of common-envelope events.
Theoretical models of stellar evolution predict that most of the lithium inside a star is destroyed as the star becomes a red giant. However, observations reveal that about 1% of red giants are ...peculiarly rich in lithium, often exceeding the amount in the interstellar medium or predicted from the big bang. With only about 150 lithium-rich giants discovered in the past four decades, and no distinguishing properties other than lithium enhancement, the origin of lithium-rich giant stars is one of the oldest problems in stellar astrophysics. Here we report the discovery of 2330 low-mass (1-3 M ) lithium-rich giant stars, which we argue are consistent with internal lithium production that is driven by tidal spin-up by a binary companion. Our sample reveals that most lithium-rich giants have helium-burning cores ( ), and that the frequency of lithium-rich giants rises with increasing stellar metallicity. We find that while planet accretion may explain some lithium-rich giants, it cannot account for the majority that have helium-burning cores. We rule out most other proposed explanations for the origin of lithium-rich giants. Our analysis shows that giants remain lithium-rich for only about two million years. A prediction from this lithium depletion timescale is that most lithium-rich giants with a helium-burning core have a binary companion.
Abstract
Supermassive primordial stars with masses exceeding ∼10
5
M
⊙
that form in atomically cooled halos are the leading candidates for the origin of high-redshift quasars at
z
> 6. Recent ...numerical simulations, however, find that multiple accretion disks can form within a halo, each of which can potentially host a supermassive star. We investigate the formation and evolution of secondary supermassive stars in atomically cooled halos, including strong variations in their accretion histories driven by gravitational interactions between their disks and those surrounding the primary supermassive stars in each halo. We find that all secondary disks produce long-lived supermassive stars under sustained rapid accretion. We also find, however, that the majority of secondary supermassive stars do undergo at least one protracted quiescent accretion phase, during which time they thermally relax and may become powerful sources of ionizing feedback. In many halos, the two satellite disks collide, suggesting that the two stars can come into close proximity. This may induce additional mass exchange between them, leading to a great diversity of possible outcomes. These range from coevolution as main-sequence stars to main sequence—black hole pairs and black hole—black hole mergers. We discuss the likely outcome for these binary interactions based on the evolutionary state of both supermassive stars at the end of our simulations, as well as prospects for their future detection by current and next-generation facilities.
On the Rotation of Supermassive Stars Haemmerlé, Lionel; Woods, Tyrone E.; Klessen, Ralf S. ...
Astrophysical journal. Letters,
01/2018, Letnik:
853, Številka:
1
Journal Article
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Supermassive stars (SMSs) born from pristine gas in atomically cooled halos are thought to be the progenitors of supermassive black holes at high redshifts. However, the way they accrete their mass ...is still an unsolved problem. In particular, for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here, we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution model of a supermassive Population III star simultaneously including accretion and rotation. We find that, for SMSs to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angular momentum. This tight constraint comes from the limit, at which the combination of radiation pressure and centrifugal force cancels gravity. It implies that SMSs are slow rotators, with a surface velocity less than 10%-20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, the deformation of the star due to rotation is negligible.
Modelling supermassive primordial stars with mesa Herrington, Nicholas P; Whalen, Daniel J; Woods, Tyrone E
Monthly notices of the Royal Astronomical Society,
03/2023, Letnik:
521, Številka:
1
Journal Article
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ABSTRACT
Supermassive stars forming at z ∼ 15–20 are one of the leading contenders for the origin of the first quasars, over 200 of which have now been discovered at z > 6. These stars likely form in ...pristine, atomically cooled haloes immersed in strong Lyman–Werner ultraviolet backgrounds or in highly supersonic baryon streaming flows. Atomic cooling triggers catastrophic baryon collapse capable of building up stars at rates of up to ∼1 M⊙ yr−1. Here, we examine the evolution of supermassive stars with a much larger and finer grid of accretion rates than in previous studies with the mesa stellar evolution code. We find that their final masses range from 3.5 × 103 to 3.7 × 105 M⊙ at accretion rates of 0.001–1 M⊙ yr−1, respectively. We also find that supermassive star evolution diverges at accretion rates of 0.01–0.02 M⊙ yr−1, above which they evolve as cool red hypergiants along the Hayashi track and collapse via the general relativistic instability during central hydrogen burning, and below which they evolve as hot blue supergiants and collapse at the end of their nuclear burning lifetimes after exiting the main sequence.
The population of Milky Way satellite galaxies is of great interest for cosmology, fundamental physics, and astrophysics. They represent the faint end of the galaxy luminosity function, are the most ...dark-matter-dominated objects in the local universe, and contain the oldest and most metal-poor stellar populations. Recent surveys have revealed around 60 satellites, but this could represent less than half of the total. Characterization of these systems remains a challenge due to their low luminosity. We consider the gravitational-wave observatory LISA as a potential tool for studying these satellites through observations of their short-period double white dwarf populations. LISA will observe the entire sky without selection effects due to dust extinction, complementing optical surveys, and could potentially discover massive satellites hidden behind the disk of the Galaxy.
Abstract
Primordial supermassive stars (SMSs) formed in atomic-cooling halos at
z
∼ 15–20 are leading candidates for the seeds of the first quasars. Past numerical studies of the evolution of SMSs ...have typically assumed constant accretion rates rather than the highly variable flows in which they form. We model the evolution of SMSs in the cosmological flows that create them using the Kepler stellar evolution and implicit hydrodynamics code. We find that they reach masses of 1 − 2 × 10
5
M
⊙
before undergoing direct collapse to black holes (DCBHs) during or at the end of their main-sequence hydrogen burning, at 1–1.5 Myr, regardless of halo mass, spin, or merger history. We also find that realistic, highly variable accretion histories allow for a much greater diversity of supermassive stellar structures, including in some cases largely thermally relaxed objects, which may provide a significant source of radiative feedback. Our models indicate that the accretion histories predicted for purely atomic-cooling halos may impose a narrow spectrum of masses on the seeds of the first massive quasars; however, further studies incorporating realistic feedback will be essential in order to confirm whether or not this holds true in all cases. Our results also indicate that multiple SMSs at disparate stages of evolution can form in these halos, raising the possibility of SMS binaries and supermassive X-ray binaries, as well as DCBH mergers that could be detected by LISA.
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