Lightcurve Analysis of Eclipsing Binaries Containing Cepheids

Abstract

<p>A method for solving the lightcurve of an eclipsing binary system which contains a Cepheid variable as one of its components is presented. We construct a geometric model where the component stars are assumed to be spherical and on circular orbits. The emergent system flux is computed as a function of time, with the intrinsic variations in temperature and radius of the Cepheid treated self-consistently. Fitting the adopted model to photometric observations, incorporating data from multiple bandpasses, yields a single parameter set best describing the system.</p> <p>This method is then applied to three eclipsing Cepheid systems from the MACHO Project Large Magellenic Cloud database: MACHO ID's 6.6454.5, 78.6338.24 and 81.8997.87. Best-fit values are obtained for each system's orbital period and inclination and for the relative radii, relative surface brightnesses and limb-darkening coefficients of each star. Pulsation periods, radial amplitudes and parameterizations of the intrinsic surface brightness variations of the Cepheids are also obtained.</p> <p>The system 6.6454.5 is found to contain a 4.97-day Cepheid with an unexpectedly brighter companion. The system 78.6338.24 consists of a 17.7-day, W Vir Class Type II Cepheid with a smaller, dimmer companion. And the system 81.8997.87 contains an intermediate-mass, 2.03-day overtone Cepheid with a dimmer, red giant secondary.</p> <p>For each star, an LMC distance modulus of 18.5 mag was used to convert mean magnitudes into luminosities, and effective temperature values were found from a Teff - (V-R) calibratation. Comparison with theoretical tracks and isochrones.</p> <p>A complete inventory of observations of all three systems is listed, including data not utilized in this work.</p>