Supermassive star formation via super competitive accretion in slightly metal-enriched clouds

Abstract

Direct collapse black hole (DCBH) formation with mass 10 5 M is a promising scenario for the origin of high-redshift supermassive black holes. It has usually been supposed that the DCBH can only form in the primordial gas since the metal enrichment enhances the cooling ability and causes the fragmentation into smaller pieces. What actually happens in such an environment, however, has not been explored in detail. Here, we study the impact of the metal enrichment on the clouds, conducting hydrodynamical simulations to follow the cloud evolution in cases with different degree of the metal enrichment Z/Z = 10 −6 − 10 −3 . Below Z/Z = 10 −6 , metallicity has no effect and supermassive stars form along with a small number of low-mass stars. With more metallicity Z/Z > ∼ 5 × 10 −6 , although the dust cooling indeed promotes fragmentation of the cloud core and produces about a few thousand low-mass stars, the accreting flow preferentially feeds the gas to the central massive stars, which grows supermassive as in the primordial case. We term this formation mode as the super competitive accretion, where only the central few stars grow supermassive while a large number of other stars are competing for the gas reservoir. Once the metallicity exceeds 10 −3 Z and metal-line cooling becomes operative, the central star cannot grow supermassive due to lowered accretion rate. Supermassive star formation by the super competitive accretion opens up a new window for seed BHs, which relaxes the condition on metallicity and enhances the seed BH abundance.

Light, medium-weight, or heavy? The nature of the first supermassive black hole seeds

“…As anticipated in section 3.2, we have also investigated the effects of the recently proposed SCA model (Chon & Omukai 2020). In this model, SMSs leading to Red dashed lines are the critical dust-to-gas ratio and metallicity thresholds adopted in the SCA model (see Table 2).…”

“…In a recent work Chon & Omukai (2020) (see also Tagawa et al 2020) used a suite of hydrodynamical simulations to investigate the outcome of star formation in slightly metal-enriched gas clouds illuminated by a very strong UV field. The initial conditions were set by extracting -from a cosmological simulation -one halo that was shown to lead to the formation of a heavy seed when the gas was metal-free (Chon et al 2018).…”

“…Interestingly, a similar evolution is found for initial metallicities up to Z = 10 −4 Z , although the initial number of low-mass protostars formed as a result of dust-cooling increases with Z (about 4000 stars are formed when Z = 10 −4 Z ). When the metallicity becomes Z = 10 −3 Z (the maximum metallicity considered in their analysis), Chon & Omukai (2020) found that metal fine-structure line cooling starts to be effective on larger scales (when the central gas density is n ≥ 10 4 cm −3 ), and the associated decrease in the gas temperature leads to a suppression of the gas accretion rate by almost two orders of magnitude. As a result, the primary star mass grows only up to 350 M .…”

“…It is important to stress that this simple picture may be significantly affected by the dynamics of the infalling gas. Indeed, Chon & Omukai (2020) show that, in the absence of H 2 cooling, although dust cooling promotes the formation of a few thousand low-mass stars, the strong accreting flow preferentially feeds the central star which grows supermassive. Hence, in this super competitive accretion (SCA) mode, the formation of heavy seeds may continue even in moderately enriched halos, and medium-weight seeds form only when metal line cooling starts to decrease the gas temperature, at Z ∼ 10 −3 Z , lowering the accretion rate.…”

Stellar collisions in flattened and rotating Population III star clusters

“…Miyamoto and Nagai define the velocity dispersion as (σ r , σ ϕ , σ z ) (see also Ciotti & Pellegrini 1996), which correspond to the cylindrical coordinates (r, ϕ, z). Following Binney et al (1990), they assume a distribution function that depends only on the isolating integrals corresponding to total energy E and angular momentum L z .…”