Formation of Primordial Supermassive Stars by Rapid Mass Accretion
The paper by Hosokawa and colleagues investigates the theoretical formation and evolution of primordial supermassive stars (SMSs) through rapid mass accretion, potentially seeding supermassive black holes (SMBHs) in the early universe. The study extends earlier work and underscores the significant role that SMSs might play in the context of cosmological SMBH formation scenarios, especially considering the observations of bright quasars at redshifts beyond 6. This research seeks to offer detailed stellar evolution calculations for primordial stars with masses reaching well into (10{4-5}) (M_\odot).
The SMS formation under rapid mass accretion involves passages through the supergiant protostar stage, characterized by an extended envelope and a contracting core. During this stage, the stellar radius grows with mass, reaching mondes of approximately 100 AU at masses beyond (104) (M_\odot). Accompanying this stage is a lower effective temperature, below (104) K, contributing to the transparency of the accreting material to the star's radiation. Only at masses (M_* \gtrsim 105) (M_\odot) does the stellar UV feedback become pronounced enough to potentially disrupt further accretion.
Estimations of SMS observational prospects are promising, with signals expected to be detectable by upcoming observational platforms like the James Webb Space Telescope. These findings highlight the comparability of SMS and supermassive dark stars (SMDSs), adding to the narrative of SMS as potent prospective origins for SMBHs due to their highly luminous but low-temperature spectrums.
In examining the pulsational stability of accreting SMS, the authors assert that the pulsation-driven mass loss remains minor compared to accretion rates, thus posing no significant barrier to SMS growth. Such SMSs are fueled by gravitational energetics and demonstrate subdued UV feedback due to their expansive radii and reduced effective temperatures. Consequently, their accretion regimes can proceed largely unimpeded, advocating for the consideration of SMSs as feasible precursors to the most massive and ancient black holes.
The paper advances to contemplate the ultimate fate of SMSs under continued rapid accretion. A general relativistic (GR) stability analysis predicts that only the inner core of an SMS becomes unstable before collapse ensues, potentially forming a massive black hole—an event hypothesized to happen at masses nearing a few (105) (M_\odot). This scenario aligns with the hypothesis that the formation of SMBHs in the early universe may often begin with SMS a critical antecedent.
The authors emphasize further theoretical and numerical simulations to thoroughly verify these processes. Such efforts should encapsulate the dynamic interactions between SMS evolution and its environmental factors, addressing aspects such as effective temperature rises and feedback mechanisms that could lead to termination of accretion, leaving stars vulnerable to ZAMS contraction and eventual collapse. Given the intricacies of stellar metallicity, rotation, and mass accretion history, these variables warrant detailed exploration to ascertain their influences on the stars' final evolutionary paths.
In summary, by presenting calculated models of SMS formation and elucidating plausible pathways to SMBHs, this paper lays vital groundwork for comprehending the early universe's cosmological landscape, galvanizing further inquiries into SMS dynamics and their observational signatures. The implications span both theoretical models and practical astronomical endeavors, bridging gaps in understanding the formation of massive cosmic entities.