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Astron. Astrophys. 333, 379-384 (1998)

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1. Introduction

One likely outcome of the tidal disruption of a star by a massive black hole is a bright flare with duration of a few months to years (e.g. Rees 1988, Ulmer 1998a; hereafter U98). The flares are generally thought to be quite hot. For example, the temperature associated with an Eddington luminosity emitted from a spherical photosphere at the tidal radius is

[EQUATION]

where [FORMULA] is the black hole mass in units of a million solar masses and [FORMULA] and [FORMULA] are the mass and radius of the star. Consequently, the discussions of the spectra and observability of flares have focused on hard emission (e.g. Sembay & West, 1993) or the extreme, [FORMULA]  magnitude, bolometric corrections to optical (e.g. U98). Loeb & Ulmer (1997, hereafter LU97) make the interesting suggestion that if part of the tidal debris forms an extended envelope around the black hole, then the light may be largely reprocessed down to optical with effective temperatures of

[EQUATION]

In the model, an accretion disk forms close to the black hole, and the energy released from the disk is reprocessed by the extended envelope which connects in an unknown way to the disk.

In Sect.  2, we show that without knowledge of the base luminosity which is provided by disk accretion onto the black hole, it is impossible to specify uniquely the resulting envelope structure. In Sect.  3, we discuss the small, but important, redshift effects from the Schwarzschild geometry and show that the envelopes are convective. The system's time scales are investigated in Sect.  4, and using simple energy arguments, we make predictions regarding the evolution of the envelope. A discussion of these results is presented in Sect.  5.

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© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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