HD 63021: Chromospheric Activity and Mass Transfer in a Close Binary

TL;DR

HD 63021 is a close binary system with active chromospheric secondary star, undergoing episodic mass transfer, characterized by rapid orbital and rotational dynamics, and detected through spectroscopic, photometric, and X-ray observations.

Contribution

This study reveals the detailed properties of HD 63021 as a non-eclipsing Algol-like binary with active mass transfer and chromospheric activity, based on multi-epoch spectroscopic and photometric data.

Findings

Short orbital period of 2.9 days.

Secondary star nearly fills its Roche lobe.

Evidence of episodic mass transfer and chromospheric activity.

Abstract

Prompted by X-ray detections from multiple surveys, we investigated the A-type star HD 63021 and found that it is a double-lined spectroscopic binary with highly variable emission associated with the primary star. Analysis of our multi-epoch spectroscopic observations, the majority of which were carried out on small aperture telescopes, indicates a very short orbital period of just $2.9$ days, and a mass ratio M$_2$/M$_1$ of $0.23$. The A1 V star is a slow rotator, with a rotational speed of $\sim34$ km/s. Assuming its mass is $2.3$ M$_{\odot}$, the present-day secondary is an evolved star of $\sim0.5$ M$_{\odot}$ that nearly fills its Roche lobe. This secondary star rotates comparatively rapidly at $\sim44$ km/s, and we see evidence that it is chromospherically active. Analysis of a photometric lightcurve from TESS reveals two strong periods, one at the orbital period for the system and another at half the orbital period. These findings suggest that HD 63021 is a close binary system undergoing mass transfer from the secondary star onto the primary star -- in all ways like an Algol eclipsing binary system, except without the eclipse. We discuss the system's mass transfer, which is not steady but seems to run in fits and bursts, and infer the system's basic physical properties from an orbital parameter study, the Roche lobe geometry, and its extant X-ray emission.