AAS 202nd Meeting, May 2003
Session 13 Stars and Supernovae
Oral, Monday, May 26, 2003, 10:00-11:30am, 204

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[13.08] Models of Rotational Line-Broadening in Black Hole Binaries

E. L. Robinson (UT)

Soft X-ray Transients (SXTs) are interacting binary stars in which a compact star -- a black hole in ~/3 of the SXTs, a neutron star in the others -- is stripping mass from a relatively normal companion star (the ``secondary'' star). One step in the usual method for measuring the mass of the black holes in these systems is to determine the mass ratio from the rotational broadening of the absorption lines in the spectrum of the secondary star. With a few outstanding exceptions (eg, Shahbaz 2003), the observations have typically been analyzed assuming that the observed spectrum is the convolution of the spectrum of a non-rotating star with a line-broadening kernel appropriate for a rotating, spherical star.

We describe a new program for calculating the profiles of the absorption lines in the spectra of stars that fill their Roche lobes in close binary stars. The program calculates the observed spectrum by summing specific intensities over the visible surface of the distorted, lobe-filling star. We use ATLAS to calculate the stellar atmospheres and MOOG to calculate the specific intensities. We show that 1) the absorption-line profiles of a lobe-filling star depart greatly from the profiles of a rotating spherical star, 2) the widths of the profiles vary strongly with orbital phase, and 3) convolving a line-broadening kernel with the spectrum of a non-rotating star does not yield a good model for the profiles because the limb darkening differs from line to line and also differs from the continuum limb darkening.

This work is supported by the NSF under grant AST-0206029.

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