UV flux variation study in contact binary VW Cephei

TL;DR

This study analyzes UV spectral variability in the contact binary VW Cephei, revealing significant flux and line width changes, with implications for understanding mass transfer and emission origins in such systems.

Contribution

It provides the first detailed UV spectroscopic analysis of VW Cephei, identifying emission lines and their variability, and estimating the mass transfer rate.

Findings

UV fluxes vary up to 45% over hours to years.

Emission lines are likely formed in Roche lobe overflow clouds.

Estimated mass transfer rate aligns with optical studies.

Abstract

Despite many attempts, the origin of UV emission line and continuum in contact binary stars remains unclear. We present a substantial UV spectroscopic analysis of VW Cephei, a late-type contact binary system, using 46 low-resolution spectra from the International Ultraviolet Explorer (IUE) in the wavelength range 1150-1978 \r{A}. By modelling continuum and emissions lines in individual spectra, we report the significant detection of OIII] (1660 and 1666 \r{A}) and SiIV (1393 and 1402 \r{A}) line complexes. We observe that UV fluxes for both continuum and emission lines like CIV, OIII], CII and SiIV vary significantly (fractional rms variability up to 45%) from hours to years. In addition, line widths also change by hundreds of kilometres/sec. The UV flux variabilities observed in the continuum bands and line emissions are uncorrelated. However, most of the flux values follow the binary orbital period observed from optical data. Our analysis indicates that, while the variation in continuum flux may be attributed to a heated photosphere, the line width measurements indicate that the emission lines are likely formed in the dynamical clouds associated with Roche lobe overflow. We estimate the mass transfer rate of $ \dot{M} = (0.82 \pm 0.01) \times 10^{-7} \ M_{\odot} {yr^{-1}}$ from UV line fluxes, which is in good agreement with optical studies.