The evolution of supermassive Population III stars Haemmerlé, Lionel; Woods, T E; Klessen, Ralf S ...
Monthly notices of the Royal Astronomical Society,
02/2018, Letnik:
474, Številka:
2
Journal Article
Recenzirano
Abstract
Supermassive primordial stars forming in atomically cooled haloes at z ∼ 15–20 are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star ...evolves under accretion rates of 0.1–1 M⊙ yr−1 until the general relativistic instability triggers its collapse to a black hole at masses of ∼105 M⊙. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionizing radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates 0.001–10 M⊙ yr−1, computed with the geneva stellar evolution code including general relativistic corrections to the internal structure. We compute for the first time evolutionary tracks in the mass range M > 105 M⊙. We use the polytropic stability criterion to estimate the mass at which the collapse occurs, which has been shown to give a lower limit of the actual mass at collapse in recent hydrodynamic simulations. We find that at accretion rates higher than 0.01 M⊙ yr−1, the stars evolve as red, cool supergiants with surface temperatures below 104 K towards masses >105 M⊙. Moreover, even with the lower rates 0.001 ${\mathrm{M}_{{\odot }}}\, {\rm yr}{^{-1}}<\dot{M}<$ 0.01 M⊙ yr−1, the surface temperature is substantially reduced from 105 to 104 K for M ≳ 600 M⊙. Compared to previous studies, our results extend the range of masses and accretion rates at which the ionizing feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars. We provide numerical tables for the surface properties of our models.
To date, the question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, with the single and double degenerate scenarios remaining the ...leading contenders. The former implies a large population of hot accreting white dwarfs with photospheric temperatures of T ∼ 105-106 K during some part of their accretion history. We show that in early-type galaxies, a population of accreting white dwarfs large enough to reproduce the SN Ia rate would contribute significantly to the ionizing ultraviolet (UV) radiation expected from the stellar population. For mean stellar ages 5 Gyr, single degenerate progenitors would dominate the ionizing background produced by stars, increasing the continuum beyond the He ii-ionizing limit more than 10-fold. This opens a new avenue for constraining the progenitors of SNe Ia, through consideration of the spatially extended low-ionization emission-line regions now found in many early-type galaxies. Modelling the expected emission, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in E/S0 galaxies from upper limits on the luminosity of He ii recombination lines in the optical and far-ultraviolet (FUV). We discuss future directions, as well as possible implications for the evolution of SNe Ia in old stellar populations.
Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z ∼ 6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or ...considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution, and collapse of accreting, non-rotating supermassive stars under accretion rates of 0.01-10 M yr−1 using the stellar evolution code Kepler. Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000-330,000 M for accretion rates of 0.1-10 M yr−1, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.
Accreting, steadily nuclear-burning white dwarfs are associated with so-called close-binary supersoft X-ray sources (SSSs), observed to have temperatures of a few × 105 K and luminosities on the ...order of 1038 erg s−1. These and other types of SSSs are expected to be capable of ionizing their surrounding circumstellar medium; however, to date only one such nebula was detected in the Large Magellanic Cloud (of its six known close-binary SSSs), surrounding the accreting, nuclear-burning WD CAL 83. This has led to the conclusion that most SSSs cannot have been both luminous (≳1037 erg s−1) and hot (≳few × 104 K) for the majority of their past accretion history, unless the density of the interstellar medium (ISM) surrounding most sources is much less than that inferred for the CAL 83 nebula (4–10 cm− 3). Here, we demonstrate that most SSSs must lie in much lower density media than CAL 83. Past efforts to detect such nebulae have not accounted for the structure of the ISM in star-forming galaxies and, in particular, for the fact that most of the volume is occupied by low density warm and hot ISM. CAL 83 appears to lie in a region of ISM which is at least ∼40-fold overdense. We compute the probability of such an event to be ≈18 per cent, in good agreement with observed statistics. We provide a revised model for the ‘typical’ SSS nebula, and outline the requirements of a survey of the Magellanic Clouds which could detect the majority of such objects. We then briefly discuss some of the possible implications, should there prove to be a large population of previously undiscovered ionizing sources.
In interacting binaries, comparison of a donor star's radial response to mass loss (ML) with the response of its Roche radius determines whether ML persists and, if so, determines the timescale and ...stability of the ensuing evolutionary phase. For giants with deep convective envelopes, the canonical description holds that once mass transfer begins it typically proceeds catastrophically on the dynamical timescale, as the star cannot lose sufficient heat in order to avoid expansion. However, we demonstrate that the local thermal timescale of the envelope's superadiabatic outer surface layer remains comparable to that of ML in most cases of 'dynamical' ML. We argue therefore that if ML proceeds on a timescale longer than this, then even a deep convective envelope will not dramatically expand, as the surface layer will have time to relax thermally and reconstitute itself. We demonstrate that in general the polytropic approximation gives much too strict a criterion for stability and discuss the dependence of the donor's response on its radius in addition to its core mass. In general, we find that the effective response of the donor on rapid timescales cannot be determined accurately without detailed evolutionary calculations.
Context. Major mergers of gas-rich galaxies provide promising conditions for the formation of supermassive black holes (SMBHs; ≳105 M⊙) by direct collapse because they can trigger mass inflows as ...high as 104 − 105 M⊙ yr−1 on sub-parsec scales. However, the channel of SMBH formation in this case, either dark collapse (direct collapse without prior stellar phase) or supermassive star (SMS; ≳104 M⊙), remains unknown. Aims. Here, we investigate the limit in accretion rate up to which stars can maintain hydrostatic equilibrium. Methods. We compute hydrostatic models of SMSs accreting at 1–1000 M⊙ yr−1, and estimate the departures from equilibrium a posteriori by taking into account the finite speed of sound. Results. We find that stars accreting above the atomic cooling limit (≳10 M⊙ yr−1) can only maintain hydrostatic equilibrium once they are supermassive. In this case, they evolve adiabatically with a hylotropic structure, that is, entropy is locally conserved and scales with the square root of the mass coordinate. Conclusions. Our results imply that stars can only become supermassive by accretion at the rates of atomically cooled haloes (∼0.1 − 10 M⊙ yr−1). Once they are supermassive, larger rates are possible.
Once thought to be devoid of warm and cold interstellar matter, elliptical galaxies are now commonly observed to host extended regions of neutral and ionized gas. Outside of the innermost nuclear ...regions of these galaxies, the favoured candidate ionizing source remains some component of the stellar population, with mounting evidence suggesting post-asymptotic-giant-branch stars (pAGBs). In a recent paper, we demonstrated that observations of recombination lines of He ii (or upper limits thereof) may provide a strong constraint on the presence of any other, higher temperature ionizing sources, in particular nuclear-burning white dwarfs (WDs) in the context of the single-degenerate (SD) scenario for Type Ia supernovae. The sensitivity of the He ii test is greatest for WD effective temperatures ∼2 × 105 K. Here, we extend our analysis to include predictions for all of the 'classical' strong optical lines, as well as UV, optical and infrared lines of neutral oxygen, nitrogen and singly ionized carbon. This allows us to extend the temperature range over which we can meaningfully constrain the collective luminosity of nuclear-burning WDs to 105 K T 106 K. We then demonstrate how observations of nearby early-type and post-starburst galaxies can place strong limits on the origin of Type Ia supernovae.
Accreting white dwarfs (WDs) with non-degenerate companions are expected to emit in soft X-rays and the UV, if accreted H-rich material burns stably. They are an important component of the unresolved ...emission of elliptical galaxies, and their combined ionizing luminosity may significantly influence the optical line emission from warm interstellar medium (ISM). In an earlier paper, we modelled populations of accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red giant companions with the population synthesis code bse, and then following their evolution with a grid of evolutionary tracks computed with mesa. Now we use these results to estimate the soft X-ray (0.3–0.7 keV), H- and He ii-ionizing luminosities of nuclear burning WDs and the number of supersoft X-ray sources for galaxies with different star formation histories. For the starburst case, these quantities peak at ∼1 Gyr and decline by ∼1–3 orders of magnitude by the age of 10 Gyr. For stellar ages of ∼10 Gyr, predictions of our model are consistent with soft X-ray luminosities observed by Chandra in nearby elliptical galaxies and He ii
4686 Å/H β line ratio measured in stacked Sloan Digital Sky Survey spectra of retired galaxies, the latter characterizing the strength and hardness of the UV radiation field. However, the soft X-ray luminosity and He ii 4686 Å/H β ratio are significantly overpredicted for stellar ages of ≲4–8 Gyr. We discuss various possibilities to resolve this discrepancy and tentatively conclude that it may be resolved by a modification of the typically used criteria of dynamically unstable mass-loss for giant stars.
The evolutionary mechanism underlying Type Ia supernova explosions remains unknown. However, recent efforts to constrain progenitor models based on the influence that their high energy emission would ...have on the interstellar medium within galaxies have proven successful. For individual remnants, Balmer-dominated shocks reveal the ionization state of hydrogen in the immediately surrounding gas. Here, we report deep upper limits on the temperature and luminosity of the progenitors of four Type Ia remnants with associated Balmer filaments: SN 1006, 0509-67.5, 0519-69.0, and DEM L71. For SN 1006, existing observations of helium-line emission in the diffuse emission ahead of the shock provide an additional constraint on the helium ionization state in the vicinity of the remnant. Using the photoionization code Cloudy, we show that these constraints exclude any hot, luminous progenitor for SN 1006, including stably hydrogen or helium nuclear-burning white dwarfs (WD), as well as any Chandrasekhar-mass WD accreting matter at 9.5 × 10−8 M yr−1 via a disk. For 0509-67.5, the Balmer emission alone rules out any such WD accreting 1.4 × 10−8 M yr−1. For 0519-69.0 and DEM L71, the inferred ambient ionization state of hydrogen is only weakly in tension with a recently hot, luminous progenitor, and cannot be distinguished from, e.g., a relatively higher local Lyman continuum background, without additional line measurements. Future deep spectroscopic observations will resolve this ambiguity, and can either detect the influence of any luminous progenitor or rule out the same for all resolved SN Ia remnants.
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