UX Monocerotis as a W Serpentis binary

¹ Hvar Observatory, Faculty of Geodesy, Kačićeva 26, University of Zagreb, 10000 Zagreb, Croatia
e-mail: dsudar@geof.hr
² Astronomical Institute of the Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
³ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
⁴ Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria BC V8W 3P6, Canada

Received: 3 May 2010
Accepted: 15 January 2011

Abstract

Using our new photometric and spectroscopic observations as well as all available published data, we present a new interpretation of the properties of the peculiar emission-line binary UX Mon. We conclude that this binary is in a rare phase of fast mass transfer between the binary components prior to the mass ratio reversal. We firmly establish that the orbital period is secularly decreasing at a rate of Ṗ = −0.260 s per year. From several lines of reasoning, we show that the mass ratio of the component losing mass to the mass-gaining component q must be larger than 1 and find our most probable value to be q = 1.15 ± 0.1. The BINSYN suite of programs and the steepest descent method were used to perform the final modeling. We modeled the star as a W Ser star with a thick disk around its primary. Although the remaining uncertainties in some of the basic physical elements describing the system in our model are not negligible, the model is in fair agreement with available observations. Only the nature of the light variations outside the primary eclipse remains unexplained.