Tidal breakup of binary stars at the Galactic Center. II., Hydrodynamic simulations

Fabio Antonini, James C. Lombardi Jr., David Merritt


(1) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA
(2) Department of Physics, Allegheny College, 520 North Main Street, Meadville, PA 16335, USA
(3) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA

Paper: ApJ, submitted

Weblink: http://arxiv.org/abs/1008.5369


Abstract:

In Paper I, we followed the evolution of binary stars as they orbited near the supermassive black hole (SMBH) at the Galactic center, noting the cases in which the two stars would come close enough together to collide. In this paper we replace the point-mass stars by fluid realizations, and use a smoothed-particle hydrodynamics (SPH) code to follow the close interactions. We model the binary components as main-sequence stars with initial masses of 1, 3 and 6 Solar masses, and with chemical composition profiles taken from stellar evolution codes. Outcomes of the close interactions include mergers, collisions that leave both stars intact, and ejection of one star at high velocity accompanied by capture of the other star into a tight orbit around the SMBH. For the first time, we follow the evolution of the collision products for many (> 100) orbits around the SMBH. Stars that are initially too small to be tidally disrupted by the SMBH can be puffed up by close encounters or collisions, with the result that tidal stripping occurs in subsequent periapse passages. In these cases, mass loss occurs episodically, sometimes for hundreds of orbits before the star is completely disrupted. Repeated tidal flares, of either increasing or decreasing intensity, are a predicted consequence. In collisions involving a low-mass and a high-mass star, the merger product acquires a high core hydrogen abundance from the smaller star, effectively resetting the nuclear evolution ``clock'' to a younger age. Elements like Li, Be and B that can exist only in the outermost envelope of a star are severely depleted due to envelope ejection during collisions and due to tidal forces from the SMBH. Tidal spin-up due to either a collision or tidal torque by the SMBH at periapsis can explain the observed high rotational velocity of HVS 8. However, in the absence of collisions, tidal spin-up of stars is only important in a narrow range of periapse distances, r_t/2<=sssim r_per <=sssim r_t with r_t the tidal disruption radius. We discuss the implications of these results for the formation of the S-stars and the hypervelocity stars


Preprints available from the authors at antonini@astro.rit.edu , or the raw TeX (no figures) if you click here.

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