CoRoT photometry and high-resolution spectroscopy of the interacting eclipsing binary AU Mon

TL;DR

This study combines space photometry and spectroscopy to refine the fundamental properties of the AU Mon binary system, revealing variable circumbinary material and complex stellar interactions.

Contribution

It provides the first detailed model of AU Mon using combined photometric and spectroscopic data, including new ephemerides and insights into the system's long-term variability.

Findings

Derived new orbital and brightness variation periods

Estimated stellar masses, radii, and temperatures with improved accuracy

Identified variable circumbinary material affecting light attenuation

Abstract

Analyses of very accurate CoRoT space photometry, past Johnson V photoelectric photometry and high-resolution \'echelle spectra led to the determination of improved and consistent fundamental stellar properties of both components of AU Mon. We derived new, accurate ephemerides for both the orbital motion (with a period of 11.113d) and the long-term, overall brightness variation (with a period of 416.9d) of this strongly interacting Be + G semi-detached binary. It is shown that this long-term variation must be due to attenuation of the total light by some variable circumbinary material. We derived the binary mass ratio $M_{\rm G}/M_{\rm B}$ = 0.17\p0.03 based on the assumption that the G-type secondary fills its Roche lobe and rotates synchronously. Using this value of the mass ratio as well as the radial velocities of the G-star, we obtained a consistent light curve model and improved estimates of the stellar masses, radii, luminosities and effective temperatures. We demonstrate that the observed lines of the B-type primary may not be of photospheric origin. We also discover rapid and periodic light changes visible in the high-quality residual CoRoT light curves. AU Mon is put into perspective by a comparison with known binaries exhibiting long-term cyclic light changes.