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Understanding the nature of the first stars is key to understanding the early Universe. With new facilities such as James Webb Space Telescope (JWST) we may soon have the first observations of the earliest stellar populations, but to understand these observations we require detailed theoretical models. Here we compute a grid of stellar evolution models using the Geneva code with the aim to improve our understanding of the evolution of zero-metallicity stars, with particular interest in how rotation affects surface properties, interior structure, and metal enrichment. We produce a range of models of initial masses (M ini) from |$1.7$| to |$120\, \mathrm{M}_{\odot }$|⁠ , focusing on massive models of |$9 \le M_{\rm ini}\le 120\, \mathrm{M}_{\odot }$|⁠. Our grid includes models with and without rotation, with rotating models having an initial velocity of 40 per cent of the critical velocity. We find that rotation strongly impacts the evolution of the first stars, mainly through increased core size and stronger H-burning shells during core He-burning. Without radiative mass loss, angular momentum builds at the surface in rotating models, thus models of initial masses |$M_{\rm ini}\ge 60 \, \mathrm{M}_{\odot }$| reach critical rotation on the main sequence and experience mass loss. We find that rotational mixing strongly affects metal enrichment, but does not always increase metal production as we see at higher metallicities. This is because rotation leads to an earlier CNO boost to the H shell during He-burning, which may hinder metal enrichment depending on initial mass and rotational velocity. Electronic tables of this new grid of Population III models are publicly available. [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]

Supermassive primordial stars forming in atomically cooled haloes at ɀ ~ 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 0.1-1M ⨀ 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 > 105M ⨀. 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 M ⨀ yr-1 < Ṁ < 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. [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]