The Formation Rates of Population III Stars and Chemical Enrichment of Halos during the Reionization Era
📝 Abstract
[abridged] The First Stars in the Universe form out of pristine primordial gas clouds that have been radiatively cooled to a few hundreds of degrees Kelvin either via molecular or atomic (Lyman-Alpha) hydrogen lines. This primordial mode of star formation is eventually quenched once radiative and/or chemical (metal enrichment) feedbacks mark the transition to Population II stars. In this paper we present a model for the formation rate of Population III stars based on Press-Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model we derive the star formation history of Population III stars, their contribution to the re-ionization of the Universe and the time of the transition from Population III star formation in minihalos to that in more massive halos where atomic hydrogen cooling is also possible. We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal free stars contribute only to a minor fraction of the total number of photons required to re-ionize the universe. In addition, metal free star formation is primarily located in minihalos and chemically enriched halos become the dominant locus of star formation very early in the life of the Universe, at redshift z~25. If instead multiple metal free stars are allowed to form out of a single halo, then there is an overall boost of Population III star formation, with a consequent significant contribution to the re-ionizing radiation budget.
💡 Analysis
[abridged] The First Stars in the Universe form out of pristine primordial gas clouds that have been radiatively cooled to a few hundreds of degrees Kelvin either via molecular or atomic (Lyman-Alpha) hydrogen lines. This primordial mode of star formation is eventually quenched once radiative and/or chemical (metal enrichment) feedbacks mark the transition to Population II stars. In this paper we present a model for the formation rate of Population III stars based on Press-Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model we derive the star formation history of Population III stars, their contribution to the re-ionization of the Universe and the time of the transition from Population III star formation in minihalos to that in more massive halos where atomic hydrogen cooling is also possible. We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal free stars contribute only to a minor fraction of the total number of photons required to re-ionize the universe. In addition, metal free star formation is primarily located in minihalos and chemically enriched halos become the dominant locus of star formation very early in the life of the Universe, at redshift z~25. If instead multiple metal free stars are allowed to form out of a single halo, then there is an overall boost of Population III star formation, with a consequent significant contribution to the re-ionizing radiation budget.
📄 Content
arXiv:0901.0711v1 [astro-ph.CO] 6 Jan 2009 The formation rates of Population III stars and chemical enrichment of halos during the Reionization Era Michele Trenti University of Colorado, Center for Astrophysics and Space Astronomy, 389-UCB, Boulder, CO 80309 USA; Space Telescope Science Institute, 3700 San Martin Drive Baltimore MD 21218 USA trenti@colorado.edu Massimo Stiavelli Space Telescope Science Institute, 3700 San Martin Drive Baltimore MD 21218 USA ABSTRACT The First Stars in the Universe form out of pristine primordial gas clouds that have been radiatively cooled to a few hundreds of degrees Kelvin either via molecular or atomic (Lyman-α) hydrogen lines. This primordial mode of star formation is eventually quenched once radiative and/or chemical (metal enrich- ment) feedbacks mark the transition to Population II stars. In this paper we present a model for the formation rate of Population III stars based on Press- Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model we derive the star formation history of Popula- tion III stars, their contribution to the re-ionization of the Universe and the time of the transition from Population III star formation in minihalos (M ≈106M⊙, cooled via molecular hydrogen) to that in more massive halos (M ≳2 × 107M⊙, where atomic hydrogen cooling is also possible). We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal free stars contribute only to a minor fraction of the total number of photons required to re-ionize the universe. In addition, metal free star formation is primarily lo- cated in minihalos and chemically enriched halos become the dominant locus of star formation very early in the life of the Universe — at redshift z ≈25 — even assuming a modest fraction (0.5%) of enriched gas converted in stars. If instead – 2 – multiple metal free stars are allowed to form out of a single halo, then there is an overall boost of Population III star formation, with a consequent significant contribution to the re-ionizing radiation budget. In addition, the bulk of metal free stars are produced in halos with M ≳2 × 107M⊙. Subject headings: cosmology: theory - galaxies: high-redshift - early universe - ISM: evolution - stars: formation 1. Introduction Population III stars are considered to be the first luminous objects formed during the Dark Ages of the Universe, when the hydrogen is in a neutral state (e.g. see Bromm & Larson 2004). The first generation of stars, formed out of pristine primordial gas, had a top-heavy initial mass function, with a typical mass scale of order of ≈100M⊙and most probably just one star per halo (e.g. see Abel et al. 2002; O’Shea & Norman 2007). These stars start forming after about 30 −40 million years from the big-bang at redshift z ≈55 −65 (Naoz et al. 2006; Trenti & Stiavelli 2007; see also Gao et al. 2005) and, given their high mass, they live only a few million years ending with either a pair instability supernova phase or a direct collapse to a black hole (Heger et al. 2003). Population III stars thus initiate the chemical enrichment of the Universe and open the way to more normal modes of star formation, namely Population II (e.g. see Ostriker & Gnedin 1996; Furlanetto & Loeb 2003). In fact, the metals released into the IGM after a pair in- stability supernova explosion can travel outside the parent dark matter halo that hosts the Population III star. Calculations by Bromm et al. (2001) found that a region containing up to about 108M⊙can be enriched to a critical metallicity Zcrit ≳10−4Z⊙by the most massive pair instability supernovae. More typical explosions may instead enrich significantly less gas (≈106M⊙) although at a correspondingly higher metallicity (see Bromm et al. 2003; Kitayama & Yoshida 2005; Greif et al. 2007; Whalen et al. 2008). Even in the latter case, a halo of mass ≲108M⊙that had one of its progenitors hosting a Pair Instability Supernova is still likely to be enriched to an average metallicity of ≳10−4Z⊙thanks to violent relaxation mixing (Lynden-Bell 1967) during its hierarchical build-up. Population III stars are also the sources that start to re-ionize the Universe, creating ionized islands within the neutral hydrogen inter-stellar and inter-galactic medium. Ionizing photons are emitted with an enhanced efficiency compared to Population II stars due to the high effective temperatures of massive metal-free stars (Tumlinson & Shull 2000; Schaerer 2002), and these sources could be responsible for a significant fraction of the Thompson – 3 – optical depth to reionization deriving from z > 7 (Shu
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