Following the Cosmic Evolution of Pristine Gas. III. The Observational Consequences of the Unknown Properties of Population III Stars

We study the observational consequences of several unknown properties of Population 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 Population III and Population II star formation across 2 dex has a negligible effect on the fraction of Population III stars formed and the subsequent fraction of Population III flux from high-redshift galaxies. However, adopting a lognormal initial mass function (IMF) for Population III stars, in place of a baseline Salpeter IMF, results in a Population III star formation rate density 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 ≤ 8 galaxies with more than 75% of their flux coming from Population III stars. However, at z = 9, right before reionization in our simulations, ≈20% of galaxies are Population III-bright with {m}UV}≤slant 31.4 mag, and at least 75% of their flux is generated by Population III stars. Additionally, the lognormal Population 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 Population III IMF was dominated by stars in the 20–120 {M}ȯ range that generate supernovae with carbon-enhanced ejecta

Could Failed Supernovae Explain the High r-process Abundances in Some Low Metallicity Stars?

Rapid neutron capture process (r-process) elements have been detected in a large number of metal-poor halo stars. The observed large abundance scatter in these stars suggests that r-process elements have been produced in a site that is rare compared to core-collapse supernovae (CCSNe). Although being rare, neutron star mergers (NSM) alone have difficulties explaining the observations, especially at low metallicities. In this paper, we present a complementary scenario: Using black hole - neutron star mergers (BHNSMs) as additional r-process site. We show that both sites together are able to explain the observed r-process abundances in the Galaxy

Stochastic Chemical Evolution of Radioactive Isotopes with a Monte Carlo Approach

Short-lived radionuclides (SLRs) with mean lives τ of a few to hundreds of Myr provide unique opportunities to probe recent nucleosynthesi events in the interstellar medium and the physical conditions in whic the Sun formed. Here we quantify the uncertainty in the predicte evolution of SLRs within a parcel of interstellar gas given th stochastic nature of stellar enrichment events. We assume that a enrichment progenitor is formed at every time interval γ. For eac progenitor, we randomly sample the delay time between its formation an its enrichment event, based on several delay-time distribution (DTD functions that cover a wide range of astrophysical sites. For each se of τ, γ, and DTD functions, we follow the abundances of SLRs for 15 Gy and repeat this process thousands of times to derive their probabilit distributions. For τ/γ ≳ 2, the distributions depend on the DT function, and we provide tabulated values and analytical expressions t quantify the spread. The relative abundance uncertainty reaches maximum of ∼60% for τ/γ = 1. For τ/γ ≲ 1, we provide the probability fo the SLR abundance to carry the signature of only one enrichment event which is greater than 50% when τ/γ ≲ 0.3. For 0.3 ≲ τ/γ ≲ 2, a smal number of events contributed to the SLR abundance. This case needs to b investigated with a separate statistical method. We find that a isolation time for the birth of the Sun of roughly 9─13 Myr i consistent with the observed abundances of 60Fe 107Pd, and 182Hf in the early solar system whe assuming τ/γ ∼ 3 for these isotope

Enrichment of the Galactic disc with neutron-capture elements: Gd, Dy, and Th

The study of the origin of heavy elements is one of the main goals of nuclear astrophysics. In this paper, we present new observational data for the heavy r-process elements gadolinium (Gd, Z= 64), dysprosium (Dy, Z= 66), and thorium (Th, Z= 90) in a sample of 276 Galactic disc stars (-1.0 < [Fe/H] < + 0.3). The stellar spectra have a high resolution of 42 000 and 75 000, and the signal-to-noise ratio higher than 100. The LTE abundances of Gd, Dy, and Th have been determined by comparing the observed and synthetic spectra for three Gd lines (149 stars), four Dy lines (152 stars), and the Th line at 4019.13 angstrom (170 stars). For about 70 per cent of the stars in our sample, Gd and Dy are measured for the first time, and Th for 95 per cent of the stars. Typical errors vary from 0.07 to 0.16 dex. This paper provides the first extended set of Th observations in the Milky Way disc. Together with europium (Eu, Z= 63) data from our previous studies, we have compared these new observations with nucleosynthesis predictions and Galactic Chemical Evolution simulations. We confirm that [Gd/Fe] and [Dy/Fe] show the same behaviour of Eu. We study with GCE simulations the evolution of [Th/Fe] in comparison with [Eu/Fe], showing that unlike Eu, either the Th production is metallicity dependent in case of a unique source of the r-process in the Galaxy, or the frequency of the Th-rich r-process source is decreasing with the increase in [Fe/H]