MOST photometry and DDO spectroscopy of the eclipsing (white dwarf + red dwarf) binary V471 Tau

TL;DR

This study combines photometry and spectroscopy of V471 Tau, revealing low starspot activity, high-energy flares, eclipse timing variations, and improved stellar parameter measurements, contributing to understanding the system's dynamics and activity cycle.

Contribution

It provides new nearly continuous photometric and spectroscopic data, detailed analysis of eclipse timing variations, and refined stellar parameters for the V471 Tau system.

Findings

Detected seven high-energy flare-like events.

Observed eclipse timing variations possibly due to a third body or orbital eccentricity.

Measured increased H_alpha velocity amplitude indicating changes in emission source distribution.

Abstract

The Hyades K2V+WD system 471 Tau is a prototype post-common envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability & Oscillations of STars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed in the past, suggesting that we monitored the star close to a minimum in its activity cycle. Despite the low spot activity, we still detected seven flare-like events whose estimated energies are among the highest ever observed in V471 Tau and whose times of occurrence do not correlate with the binary orbital phase. A detailed O-C analysis of the times of eclipse over the last ~35 years reveals timing variations which could be explained in several ways, including perturbations by an as-yet-undetected third body in the system or by a small orbital eccentricity inducing slow apsidal motion. The DDO spectra result in improved determinations of the K dwarf projected rotation velocity, V_K sin i = 92 km s^-1, and the orbital amplitude, K_K=150.5 km s^-1. The spectra also allow us to measure changes in H_alpha emission strength and radial velocity (RV) variations. We measure a larger H_alpha velocity amplitude than found previously suggesting that the source of the emission in V471 Tau was less concentrated around the sub-white-dwarf point on the K star than had been observed in previous studies.