On monolithic supermassive stars

Abstract

Supermassive stars have been proposed as the progenitors of the massive (∼ 10 9 M ⊙ ) 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 1D stellar evolution code Kepler to explore the theoretical limiting case of zero-metallicity, non-rotating stars, formed monolithically with initial masses between 10 kM ⊙ and 190 kM ⊙ . We find that stars born with masses between ∼ 60 kM ⊙ and ∼ 150 kM ⊙ collapse at the end of the main sequence, burning stably for ∼ 1.5 Myr. More massive stars collapse directly through the general relativistic instability after only a thermal timescale of ∼ 3 kyr-4 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.

General-relativistic instability in hylotropic supermassive stars

“…We see that the changes in central temperature over the narrow range of relevant values does not change significantly the stability limit. For polytropic structures, the maximum mass consistant with stability is 1 − 1.5 × 10 5 M , in agreement with Woods et al (2020). But when the mass fraction of the convective core decreases, we see that the limit of stability moves towards larger masses.…”

On the Formation and Interaction of Multiple Supermassive Stars in Cosmological Flows

“…As the primary disk continues to fragment, 3-body interactions fling the companion disk into a highly elliptical orbit ∼ 300 kyr after its formation. The initial burst of accretion is followed by a long quiescent phase (∼ 0.5 Myr) during which the second SMS, which is now ∼ 40 kM , becomes almost completely thermally relaxed and almost fully convective (Woods et al 2020). The rapid accretion beginning at ∼ 750 kyr builds up a massive radiative envelope on top of this convective core and the mass of the star grows to 186 kM by the end of the simulation.…”

“…The stars all initially exhibit a deep radiative envelope corresponding to the surge in accretion associated with the formation of their natal disks, which proceeds on much shorter times than the star's thermal timescale and leads to the buildup of a steep entropy gradient (e.g., Begelman 2010;Hosokawa et al 2013;Woods et al 2017;Haemmerlé et al 2018a). In several cases, such as Halos 01 and 16, however, almost all of mass of the star eventually lies within its convective core because there is a long ( 100 kyr) quiescent phase ( Kelvin-Helmholtz time-scale) in accretion in which the star can thermally relax and its structure can approach that of an n = 3 polytrope (Chandrasekhar 1964; Woods et al 2020). In other models, we see the formation of both transient and long-lived convective cells in the otherwise deep, high-entropy radiative envelopes of some rapidly-accreting stars, similar to those in some constant-accretion rate models (e.g., Woods et al 2017;Haemmerlé et al 2018a).…”

The Formation of Very Massive Stars in Early Galaxies and Implications for Intermediate Mass Black Holes

“…For each snapshot, we divide the stars formed between those which are accreting > 5 × 10 −3 M yr −1 , which will evolve on the Hayashi track due to H − opacity in their inflated envelopes (e.g., , and those accreting below this threshold, which will evolve blue-ward as they contract to become hot ionising sources on the ZAMS (Haemmerlé et al 2018). We assume that all rapidly-accreting ("red") stars remain negligible ionising sources, and that all slowly-or non-accreting ("blue") stars have thermally-relaxed to a main sequence temperature of ≈ 10 5 K (Schaerer 2002;Woods, Heger & Haemmerlé 2020). We further take their luminosities to be approximately Eddington (L ≈ 1.3 × 10 38 × (M/M ) erg s −1 ), and their spectra to be well-approximated as blackbodies.…”