Early Cosmological Hii/HeiiiRegions and Their Impact on Second‐Generation Star Formation

Abstract

We present the results of three-dimensional radiation hydrodynamics simulations of the formation and evolution of early H ii/He iii regions around the first stars. Cooling and recollapse of the gas in the relic H ii region is also followed in a full cosmological context, until second-generation stars are formed. We first carry out ray-tracing simulations of ionizing radiation transfer from the first star. Hydrodynamics is directly coupled with photoionization heating as well as radiative and chemical cooling. The photoionized hot gas is evacuated out of the host halo at a velocity of $30 km s À1. This radiative feedback effect quenches further star formation within the halo for over tens to a hundred million years. We show that the thermal and chemical evolution of the photoionized gas in the relic H ii region is remarkably different from that of a neutral primordial gas. Efficient molecular hydrogen production in the recombining gas enables it to cool to $100 K, where fractionation of HD/H 2 occurs. The gas further cools by HD line cooling down to a few tens of kelvins. Interestingly, at high redshifts (z > 10), the minimum gas temperature is limited by that of the cosmic microwave background with T CMB ¼ 2:728(1þ z). The gas cloud experiences runaway collapse when its mass is $40 M , which is significantly smaller than a typical clump mass of $200Y300 M for early primordial gas clouds. We argue that massive, rather than very massive, primordial stars may form in the relic H ii region. Such stars might be responsible for early metal enrichment of the interstellar medium from which recently discovered hyperYmetal-poor stars were born.

Primordial star formation under the influence of far ultraviolet radiation: 1540 cosmological haloes and the stellar mass distribution

“…Here, we describe each step of our procedure, emphasizing newly developed techniques and the improvements over previous studies. Cosmological simulations are performed using the parallel N -body / smoothed particle hydrodynamics (SPH) solver gadget-2 (Springel 2005) Yoshida et al (2006Yoshida et al ( , 2007. We have updated the cooling rates for H2 and HD (see Galli & Palla 2013) as well as the three-body H2 formation rates (Forrey 2013a,b).…”

“…We have indeed found potential sites of Pop III.2I star formation, i.e., stars forming in gas clouds that were previously ionized by radiation from nearby stars. In principle, one can simulate the evolution of such clouds and nearby H II regions by solving the transfer of EUV (hν > 13.6 eV) radiation (e.g., Yoshida et al 2007). This will likely add an additional population of lower mass primordial stars (e.g., M * = 17 M⊙; , which can be verified in future studies.…”

“…In the latter case (Pop III.2I), radiation fields affect the thermal evolution in the opposite direction; photoionization first enhances the abundance of free electrons, which are the catalyst for generating H2 molecules, and then a collapsing cloud evolves at low temperature due to the enhanced H2 and HD formation and cooling (e.g., Yoshida et al 2007;). Overall, these theoretical studies suggest that the mass of a primordial star depends critically on the local strength of the radiation emitted by other stars.…”

SPH Radiative Hydrodynamics Methods

“…As for the method to calculate the optical depth, the codes developed so far are basically divided into two categories. the first contains the "neighbor connecting" schemes (Kessel-Deynet & Burkert 2000, Susa & Umemura 2004, Susa 2006, Miao et al 2006, Dale et al 2007, Yoshida et al 2007, Gritschneder et al 2009). In these schemes the optical depth from the point source is assessed utilizing the neighbor lists of SPH particles.…”

“…Another approach to overcome the difficulty of the ionizing photon transfer is called "Ifront tracking method" or "Strömgren volume approximation" (Alvarez et al 2006, Dale et al 2007, Yoshida et al 2007, Bisbas et al 2009). This method basically tries to follow the position of ionization front (I-front) for every direction (Fig.1).…”