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Maximally accreting supermassive stars: a fundamental limit imposed by hydrostatic equilibrium


Haemmerlé, L; Meynet, G; Mayer, L; Klessen, R S; Woods, T E; Heger, A (2019). Maximally accreting supermassive stars: a fundamental limit imposed by hydrostatic equilibrium. Astronomy and Astrophysics, 632:L2.

Abstract

Context. Major mergers of gas-rich galaxies provide promising conditions for the formation of supermassive black holes (SMBHs; ≳105 M⊙) by direct collapse because they can trigger mass inflows as high as 104 − 105 M⊙ yr−1 on sub-parsec scales. However, the channel of SMBH formation in this case, either dark collapse (direct collapse without prior stellar phase) or supermassive star (SMS; ≳104 M⊙), remains unknown.
Aims. Here, we investigate the limit in accretion rate up to which stars can maintain hydrostatic equilibrium.
Methods. We compute hydrostatic models of SMSs accreting at 1–1000 M⊙ yr−1, and estimate the departures from equilibrium a posteriori by taking into account the finite speed of sound.
Results. We find that stars accreting above the atomic cooling limit (≳10 M⊙ yr−1) can only maintain hydrostatic equilibrium once they are supermassive. In this case, they evolve adiabatically with a hylotropic structure, that is, entropy is locally conserved and scales with the square root of the mass coordinate.
Conclusions. Our results imply that stars can only become supermassive by accretion at the rates of atomically cooled haloes (∼0.1 − 10 M⊙ yr−1). Once they are supermassive, larger rates are possible.

Abstract

Context. Major mergers of gas-rich galaxies provide promising conditions for the formation of supermassive black holes (SMBHs; ≳105 M⊙) by direct collapse because they can trigger mass inflows as high as 104 − 105 M⊙ yr−1 on sub-parsec scales. However, the channel of SMBH formation in this case, either dark collapse (direct collapse without prior stellar phase) or supermassive star (SMS; ≳104 M⊙), remains unknown.
Aims. Here, we investigate the limit in accretion rate up to which stars can maintain hydrostatic equilibrium.
Methods. We compute hydrostatic models of SMSs accreting at 1–1000 M⊙ yr−1, and estimate the departures from equilibrium a posteriori by taking into account the finite speed of sound.
Results. We find that stars accreting above the atomic cooling limit (≳10 M⊙ yr−1) can only maintain hydrostatic equilibrium once they are supermassive. In this case, they evolve adiabatically with a hylotropic structure, that is, entropy is locally conserved and scales with the square root of the mass coordinate.
Conclusions. Our results imply that stars can only become supermassive by accretion at the rates of atomically cooled haloes (∼0.1 − 10 M⊙ yr−1). Once they are supermassive, larger rates are possible.

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Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Institute for Computational Science
Dewey Decimal Classification:530 Physics
Uncontrolled Keywords:Space and Planetary Science, Astronomy and Astrophysics
Language:English
Date:1 December 2019
Deposited On:14 Feb 2020 08:18
Last Modified:29 Jul 2020 13:42
Publisher:EDP Sciences
ISSN:0004-6361
OA Status:Hybrid
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1051/0004-6361/201936716

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