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Register a new userFollowing the Cosmic Evolution of Pristine Gas III: The Observational Consequences of the Unknown Properties of Population III Stars
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We study the observational consequences of several unknown properties of Population III (Pop III) stars using large-scale cosmological simulations that include a subgrid model to track the unresolved mixing of pollutants. Varying the value of the critical metallicity that marks the boundary between Pop III and Population II (Pop II) star formation across 2 dex has a negligible effect on the fraction of Pop III stars formed and the subsequent fraction of Pop III flux from high-redshift galaxies. However, adopting a log normal initial mass function (IMF) for Pop III stars, in place of a baseline Salpeter IMF, results in a Pop III star formation rate density (SFRD) that is 1/4 of the baseline rate. The flux from high-redshift galaxies modeled with this IMF is highly bimodal, resulting in a tiny fraction of $z leq 8$ galaxies with more than 75% of their flux coming from Pop III stars. However, at $z=9$, right before reionization in our simulations, $approx$ 20% of galaxies are Pop III-bright with $m_{rm UV} le 31.4$ mag and at least 75% of their flux generated by Pop III stars . Additionally, the log normal Pop III IMF results in a population of carbon enhanced, metal poor stars in reasonable agreement with MW halo observations. Our analysis supports the conclusion that the Pop III IMF was dominated by stars in the 20-120$M_{odot}$ range that generate SN with carbon-enhanced ejecta.
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Supermassive primordial stars forming in atomically-cooled halos at $z sim15-20$ are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star evolves under accretion rates of $0.1 - 1$ $M_odot$ yr$^{-1}$ until the general relativistic instability triggers its collapse to a black hole at masses of $sim10^5$ $M_odot$. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionising radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates $0.001 - 10$ $M_odot$ yr$^{-1}$, computed with the GENEVA stellar evolution code including general relativistic corrections to the internal structure. 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 $0.001$ $M_odot$ yr$^{-1}$ the stars evolve as red, cool supergiants with surface temperatures below $10^4$ K towards masses $>10^5$ $M_odot$, and become blue and hot, with surface temperatures above $10^5$ K, only for rates $lesssim0.001$ $M_odot$ yr$^{-1}$. Compared to previous studies, our results extend the range of masses and accretion rates at which the ionising feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars.
The most metal-deficient stars hold important clues about the early build-up and chemical evolution of the Milky Way, and carbon-enhanced metal-poor (CEMP) stars are of special interest. However, little is known about CEMP stars in the Galactic bulge. In this paper, we use the large spectroscopic sample of metal-poor stars from the Pristine Inner Galaxy Survey (PIGS) to identify CEMP stars ([C/Fe] > +0.7) in the bulge region and to derive a CEMP fraction. We identify 96 new CEMP stars in the inner Galaxy, of which 62 are very metal-poor ([Fe/H] < -2.0); this is more than a ten-fold increase compared to the seven previously known bulge CEMP stars. The cumulative fraction of CEMP stars in PIGS is $42^{,+14,}_{,-13} %$ for stars with [Fe/H] < -3.0, and decreases to $16^{,+3,}_{,-3} %$ for [Fe/H] < -2.5 and $5.7^{,+0.6,}_{,-0.5} %$ for [Fe/H] < -2.0. The PIGS inner Galaxy CEMP fraction for [Fe/H] < -3.0 is consistent with the halo fraction found in the literature, but at higher metallicities the PIGS fraction is substantially lower. While this can partly be attributed to a photometric selection bias, such bias is unlikely to fully explain the low CEMP fraction at higher metallicities. Considering the typical carbon excesses and metallicity ranges for halo CEMP-s and CEMP-no stars, our results point to a possible deficiency of both CEMP-s and CEMP-no stars (especially the more metal-rich) in the inner Galaxy. The former is potentially related to a difference in the binary fraction, whereas the latter may be the result of a fast chemical enrichment in the early building blocks of the inner Galaxy.
The most metal-poor, high redshift damped Lyman-alpha systems (DLAs) provide a window to study some of the first few generations of stars. In this paper, we present a novel model to investigate the chemical enrichment of the near-pristine DLA population. This model accounts for the mass distribution of the enriching stellar population, the typical explosion energy of their supernovae, and the average number of stars that contribute to the enrichment of these DLAs. We conduct a maximum likelihood analysis of these model parameters using the observed relative element abundances ([C/O], [Si/O], and [Fe/O]) of the 11 most metal-poor DLAs currently known. We find that the mass distribution of the stars that have enriched this sample of metal-poor DLAs can be well-described by a Salpeter-like IMF slope at M > 10 M_sun and that a typical metal-poor DLA has been enriched by < 72 massive stars (95 per cent confidence), with masses < 40 M_sun. The inferred typical explosion energy (E_exp = 1.8^{+0.3}_{-0.2}x10^51 erg) is somewhat lower than that found by recent works that model the enrichment of metal-poor halo stars. These constraints suggest that some of the metal-poor DLAs in our sample may have been enriched by Population II stars. Using our enrichment model, we also infer some of the typical physical properties of the most metal-poor DLAs. We estimate that the total stellar mass content is log10(M_*/M_sun) = 3.5^{+0.3}_{-0.4} and the total gas mass is log10(M_gas/M_sun) = 7.0^{+0.3}_{-0.4} for systems with a relative oxygen abundance [O/H] ~ -3.0.
Extremely metal-poor stars are uniquely informative on the nature of massive Population III stars. Modulo a few elements that vary with stellar evolution, the present-day photospheric abundances observed in extremely metal-poor stars are representative of their natal gas cloud composition. For this reason, the chemistry of extremely metal-poor stars closely reflects the nucleosynthetic yields of supernovae from massive Population III stars. Here we collate detailed abundances of 53 extremely metal-poor stars from the literature and infer the masses of their Population III progenitors. We fit a simple initial mass function to a subset of 29 of theinferred Population III star masses, and find that the mass distribution is well-represented by a power law IMF with exponent $alpha = 2.35^{+0.29}_{-0.24}$. The inferred maximum progenitor mass for supernovae from massive Population III stars is $M_{rm{max}} = 87^{+13}_{-33}$ M$_odot$, and we find no evidence in our sample for a contribution from stars with masses above $sim$120 M$_odot$. The minimum mass is strongly consistent with the theoretical lower mass limit for Population III supernovae. We conclude that the IMF for massive Population III stars is consistent with the initial mass function of present-day massive stars and there may well have formed stars much below the supernova mass limit that could have survived to the present day.
We present the evolutionary models of metal-free stars in the mass range from 0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to the onset of hydrogen mixing into a convection, driven by the helium flash at red giant or asymptotic giant branch phase. The models of mass M >= 0.9 Msun undergo the central hydrogen flash, triggered by the carbon production due to the 3-alpha reactions. We find that the border of the off-center and central ignition of helium core flash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msun experience the hydrogen mixing at the tip of red giant branch while the models of M = 1.2 Msun during the helium shell flashes on the asymptotic giant branch. The equation of state for the Coulomb liquid region, where electron conduction and radiation compete, is shown to be important since it affects the thermal state in the helium core and influences the red giant branch evolution. It is also found that the non-resonant term of 3-alpha reactios plays an important role, although it has negligible effect in the evolution of stars of younger populations. We compare our models with the computations by several other sets of authors, to confirm the good agreement except for one study which finds the helium ignition much closer to the center with consequences important for subsequent evolution.
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