HD 5501: A Rapidly Evolving Interacting Eclipsing Binary with a Variable Light Curve and H$\alpha$ Emission

TL;DR

HD 5501 is a rapidly evolving eclipsing binary with complex spectral variability, eccentric orbit, and ongoing mass transfer, providing insights into early Roche-lobe overflow and eccentricity evolution in binary systems.

Contribution

This study presents detailed observations and modeling of HD 5501, revealing its rapid evolution, eccentric orbit, and mass transfer dynamics, which are novel for such systems.

Findings

Primary star is evolving rapidly across the Hertzsprung gap.

Orbital period is decreasing with a timescale of approximately 170,000 years.

Mass transfer occurs near periastron and involves a circumbinary disc or shell.

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 {\it 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 years. Significantly, the orbit is eccentric and it appears that mass transfer from the primary to the secondary occurs only near periastron. Modeling 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.