UX Monocerotis as a W Serpentis binary

TL;DR

This paper presents a new interpretation of UX Monocerotis as a W Serpentis binary in a rare phase of rapid mass transfer, with detailed modeling and evidence of a decreasing orbital period and a mass ratio greater than one.

Contribution

It introduces a novel model of UX Mon as a W Ser binary undergoing fast mass transfer, supported by new photometric, spectroscopic data and advanced modeling techniques.

Findings

Orbital period is decreasing at 0.260 seconds per year.

Mass ratio q is greater than 1, most probable value 1.15±0.1.

Model aligns reasonably with observations, explaining the system's properties.

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 $\dot{P}=-0.260$ seconds 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 $q=1.15\pm0.1$. 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.