SYNTHETIC SPECTRA OF COOL, ROCHE LOBE–FILLING STARS IN CLOSE BINARY SYSTEMS

We present the details of LinBrod, a program that calculates synthetic spectra of cool Roche lobe–filling stars in close binary systems. The program has two modes of operation. In the primary mode it calculates the spectra by adding wavelength-dependent, velocity-shifted specific intensities from the distorted and gravity-darkened surface of the lobe-filling star. The wavelength-dependent specific intensities are calculated externally using the ATLAS9 stellar atmosphere program and a modified version of the MOOG spectrum synthesis program. In its secondary mode LinBrod calculates phase-dependent line-broadening functions that can be convolved with the spectra of nonrotating single stars to yield approximate synthetic spectra. We use the line-broadening functions to isolate and assess the effects of the physical processes that broaden the absorption lines in the spectra of lobe-filling stars. The synthetic spectra can be used to extract radial velocities, mass ratios, and chemical abundances from observed spectra of lobe-filling stars. Originally written to analyze observations of the secondary stars in X-ray binaries containing black holes, the program can also be used to analyze observations of the secondary stars in Algol systems, cataclysmic variables, and low-mass X-ray binaries containing neutron stars.