Massive black hole factories: Supermassive and quasi-star formation in primordial halos (1305.5923v1)

Abstract: Supermassive stars and quasi-stars (massive stars with a central black hole) are both considered as potential progenitors for the formation of supermassive black holes. They are expected to form from rapidly accreting protostars in massive primordial halos. We explore how long rapidly accreting protostars remain on the Hayashi track, implying large protostellar radii and weak accretion luminosity feedback. We assess the potential role of energy production in the nuclear core, and determine what regulates the evolution of such protostars into quasi-stars or supermassive stars. We follow the contraction of characteristic mass scales in rapidly accreting protostars, and infer the timescales for them to reach nuclear densities. We compare the characteristic timescales for nuclear burning with those for which the extended protostellar envelope can be maintained. We find that the extended envelope can be maintained up to protostellar masses of 3.6x10^8 \dot{m}^3 solar, where \dot{m} denotes the accretion rate in solar masses per year. We expect the nuclear core to exhaust its hydrogen content in 7x10^6 yrs. If accretion rates \dot{m}>>0.14 can still be maintained at this point, a black hole may form within the accreting envelope, leading to a quasi-star. Alternatively, the accreting object will gravitationally contract to become a main-sequence supermassive star. Due to the limited gas reservoir in dark matter halos with 10^7 solar masses, the accretion rate onto the central object may drop at late times, implying the formation of supermassive stars as the typical outcome of direct collapse. However, if high accretion rates are maintained, a quasi-star with an interior black hole may form.

Massive Black Hole Factories: Supermassive and Quasi-Star Formation in Primordial Halos

This paper presents an in-depth exploration of the formation mechanisms of supermassive black holes (SMBHs) in the early universe, focusing on supermassive stars and quasi-stars as potential progenitors. The authors investigate the conditions under which these massive stellar objects form in primordial halos, characterized by rapid gas accretion.

Key Objectives and Methodology

The primary objective of the paper is to understand the evolutionary pathways from rapidly accreting protostars to either supermassive stars or quasi-stars. The authors specifically aim to determine the duration these protostars remain on the Hayashi track—indicating large radii and minimal feedback from accretion luminosity—before transitioning to main-sequence stars or collapsing into black holes. The paper examines critical timescales for nuclear burning and the sustainability of extended protostellar envelopes.

The methodological approach entails an analytical model to trace the contraction of accreting protostars, calculate the timescale for shells to reach nuclear densities, and subsequently evaluate the implications for core evolution and potential black hole formation.

Findings

A fundamental conclusion of the paper is that protostars can maintain extended envelopes until reaching substantial masses of $3.6 \times 10^8 \dot{m}^3 \, M_\odot$, where $\dot{m}$ denotes the accretion rate in solar masses per year. This threshold suggests that sustained high accretion rates are crucial for forming quasi-stars. The paper highlights that if accretion rates remain significantly greater than 0.14 solar masses per year at this juncture, the protostar may form a black hole within its envelope, transitioning into a quasi-star. Alternatively, a gravitational collapse leads to a supermassive star.

The authors also explore the role of nuclear burning, showing the transition from the pp-chain to the more energy-efficient CNO-cycle as the core evolves. They posit that exhaustion of hydrogen in the core, anticipated within $7 \times 10^6$ years, marks a critical point leading potentially to an interior black hole if accretion conditions are favorable.

Theoretical and Practical Implications

The findings imply that the prevalent absence of strong UV feedback allows for the formation of supermassive stars or quasi-stars, given conducive conditions such as adequate mass supply and accretion rates. This informs the theoretical modeling of SMBH seeds, impacting cosmological simulations and understanding of early SMBH growth.

From a practical standpoint, these insights shed light on the parameter space—specifically accretion rates and halo gas reservoir—required for forming different stellar endpoints. The requirement of specific conditions might explain the rarity of observed SMBHs with masses exceeding $10^9 \, M_\odot$ at high redshift.

Future Directions

The paper suggests avenues for further research, including simulations with variable accretion rates, and a more comprehensive inclusion of physical processes like rotation and magnetic fields in stellar evolution models. Additionally, observational strategies might be undertaken by upcoming telescopes like the James Webb Space Telescope to detect signatures of early SMBH progenitors.

Overall, the paper presents a robust framework for understanding the complex processes involved in massive black hole formation within primordial halos, providing both clarity and impetus for ongoing research in astrophysical phenomena of the early universe.