Numerical study of the relation between photometric variability and the parameters of an FS CMa-type binary system

Abstract

FS CMa-type objects are emission-line B[e] stars with hot dust whose evolutionary status remains under debate. Growing evidence indicates that a significant fraction of them are binaries that have passed through, or are currently undergoing, a stage of mass exchange. We construct a numerical model of the photometric variability of a generic FS CMa-type binary that includes an ellipsoidal component, the reflection effect, periastron brightening associated with tidally modulated mass transfer, as well as an aperiodic “red” component and white photometric noise. Light curves were synthesised over a grid of parameters (mass ratio q, eccentricity e, inclination i, orbital period P) using realistic time sampling of ground-based surveys and TESS-like space photometry. The detectability of the orbital signal was studied by means of the GLS periodogram, PDM and windowed time–frequency analysis. Detectability maps were built by Monte Carlo simulations and posterior parameter distributions by MCMC. Periastron brightening with an amplitude ≳(4–6)·10^–3 of the flux is shown to be confidently detected in ground-based time series about 4 years long almost independently of eccentricity, whereas ellipsoidal variability is accessible only to space photometry and only for short-period systems. The period and the eccentricity are recovered from photometry with high accuracy (median period error 0.08%, rms eccentricity error 0.05), while q, i and the argument of periastron remain essentially unconstrained by photometry.