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Radiative feedback from massive Population III (Pop III) stars in the form of ionizing and photodissociating photons is widely believed to play a central role in shutting off accretion on to these stars. Understanding whether and how this occurs is vital for predicting the final masses reached by these stars and the form of the Pop III stellar initial mass function. To help us better understand the impact of UV radiation from massive Pop III stars on the gas surrounding them, we carry out high-resolution simulations of the formation and early evolution of these stars, using the arepo moving-mesh code coupled with the innovative radiative transfer module sprai. Contrary to most previous results, we find that the ionizing radiation from these stars is trapped in the dense accretion disc surrounding them. Consequently, the inclusion of radiative feedback has no significant impact on either the number or the total mass of protostars formed during the 20 kyr period that we simulate. We show that the reason that we obtain qualitatively different results from previous studies of Pop III stellar feedback lies in how the radiation is injected into the simulation. H  ii  region trapping only occurs if the photons are injected on scales smaller than the local scale height of the accretion disc, a criterion not fulfilled in previous 3D simulations of this process. Finally, we speculate as to whether outflows driven by the magnetic field or by Lyman- α radiation pressure may be able to clear enough gas away from the star to allow the H  ii  region to escape from the disc. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

Massive star formation requires the accretion of gas at high rate while the star is already bright. Its actual luminosity depends sensitively on the stellar structure. We compute premain- sequence tracks for massive and intermediate-mass stars with variable accretion rates and study the evolution of stellar radius, effective temperature and ionizing luminosity, starting at 2M⊙ with convective or radiative structures. The radiative case shows a much stronger swelling of the protostar for high accretion rates than the convective case. For radiative structures, the star is very sensitive to the accretion rate and reacts quickly to accretion bursts, leading to considerable changes in photospheric properties on time-scales as short as 100-1000 yr. The evolution for convective structures is much less influenced by the instantaneous accretion rate, and produces a monotonically increasing ionizing flux that can be many orders of magnitude smaller than in the radiative case. For massive stars, it results in a delay of the HII region expansion by up to 10 000 yr. In the radiative case, the HII region can potentially be engulfed by the star during the swelling, which never happens in the convective case. We conclude that the early stellar structure has a large impact on the radiative feedback during the pre-main-sequence evolution of massive protostars and introduces an important uncertainty that should be taken into account. Because of their lower effective temperatures, our convective models may hint at a solution to an observed discrepancy between the luminosity distribution functions of massive young stellar objects and compact HII regions. [ABSTRACT FROM AUTHOR]

Context. The Geneva Stellar Evolution Group has recently presented an extended database of rotating stellar models at three different metallicities for nine different initial rotation parameters and ten different masses corresponding to spectral types from early-F to late-O. With these grids we have contributed to the understanding of the evolution of single rotating stars, and we intend to use them to produce synthetic stellar populations that fully account for the effects of stellar rotation. However, up to now we still lacked stellar evolutionary tracks that rotate close to the critical limit during the whole main-sequence (MS) phase. This occurs because the flat internal profile of rotation imposed at the zero-age main sequence (ZAMS) is modified by the action of meridional currents immediately after the ZAMS, causing the surface rotational velocity to decrease abruptly until it reaches a quasi-stationary state. Aims. We compute stellar models with non-solid rotation at the ZAMS, which allows us to obtain stellar evolutionary tracks with a larger content of angular momentum that rotate close to the breakup limit throughout the whole MS. Methods. We produced stellar models by removing the assumption that stars rotate as solid bodies at the ZAMS. We obtained the stellar structure at the ZAMS with a differentially rotating profile for three different metallicities by performing pre-MS calculations and by proposing ad hoc initial rotational profiles. We then computed the MS evolution and later phases of stellar evolution of these models, which attain rotational equatorial velocities close to the critical limit throughout their whole MS phase. Results. Stellar models with solid rotation at the ZAMS adequately represent the overall characteristics and evolution of differentially rotating models of identical angular momentum content, but with a lower initial surface rotational velocity rate, at Z = 0:014, Z = 0:006, and Z = 0:002. For models with solid rotation at the ZAMS we therefore recommend to use as the initial rotational rate the values derived once the quasi-stationary state is reached, that is, after the abrupt decrease in surface velocity. By producing stellar structures at the ZAMS with differentially rotating profiles and larger angular momentum content than in our previous works, we obtain models that rotate close to the critical limit throughout the whole MS. These models have a longer MS lifetime and a higher surface chemical enrichment already at the end of the MS, particularly at Z = 0:002. Interestingly, the initial equatorial rotational velocities are virtually metallicity independent for all stellar models we computed in the B-type star range with the same mass and angular momentum content at the ZAMS. If, as some observational evidence indicates, B-type stars at Z = 0:002 rotate with a higher equatorial velocity at the ZAMS than stars with Z = 0:014, our finding would indicate that the angular momentum content of B-type stars in the SMC is higher than their Galactic counterparts. [ABSTRACT FROM AUTHOR]