HD 5501: a rapidly evolving interacting eclipsing binary with a variable light curve and Hα emission

ABSTRACT HD 5501, a hitherto little-studied eclipsing binary with an early A-type primary, has been caught in a short-lived, astrophysically interesting phase of its binary evolution. Recent photometric and spectroscopic observations, including photometric data from TESS, show it has a highly variable light curve as well as complex spectral variability, particularly in both the absorption and emission components at H$\alpha$. Our current campaign, including both professional and amateur observers, has determined that the primary is evolving rapidly across the Hertzsprung gap and that, unusually in the case of mass transfer, the orbital period is declining with a characteristic time-scale $P/\dot{P} \approx$ 170 000 yr. Significantly, the orbit is eccentric and it appears that mass transfer from the primary to the secondary occurs only near periastron. Modelling indicates the presumed B7 V secondary to be surrounded by an accretion torus, which likely has dynamically chaotic variations in size and shape. Our analysis further implies the presence of a circumbinary disc or shell supplied by mass loss through the Lagrange $L_3$ point. That mass loss appears to account for most of the emission at H$\alpha$. We describe how this astrophysically interesting system may yield valuable information about binary star evolution at the onset of Roche lobe overflow, as well as insights into eccentricity-modifying mechanisms such as the Soker mechanism.