The Effects of Close Companions (and Rotation) on the Magnetic Activity of M Dwarfs

TL;DR

This study investigates how close companions influence magnetic activity in M dwarfs, revealing that early types are more active in binaries, but late types show similar activity to single stars, highlighting the role of stellar rotation and convection.

Contribution

It provides a large, high-quality sample analysis of WD+dM binaries to understand the impact of close companions on magnetic activity across spectral types.

Findings

Early-type M dwarfs in binaries are more active and have longer activity lifetimes.

Activity fractions in late-type M dwarfs in binaries are similar to single stars.

Close binary effects diminish for fully convective M dwarfs around M5.

Abstract

We present a study of close white dwarf and M dwarf (WD+dM) binary systems and examine the effect that a close companion has on the magnetic field generation in M dwarfs. We use a base sample of 1602 white dwarf -- main sequence binaries from Rebassa et al. to develop a set of color cuts in GALEX, SDSS, UKIDSS, and 2MASS color space to construct a sample of 1756 WD+dM high-quality pairs from the SDSS DR8 spectroscopic database. We separate the individual WD and dM from each spectrum using an iterative technique that compares the WD and dM components to best-fit templates. Using the absolute height above the Galactic plane as a proxy for age, and the H{\alpha} emission line as an indicator for magnetic activity, we investigate the age-activity relation for our sample for spectral types \leqM7. Our results show that early-type M dwarfs (\leqM4) in close binary systems are more likely to be active and have longer activity lifetimes compared to their field counterparts. However, at a spectral type of M5 (just past the onset of full convection in M dwarfs), the activity fraction and lifetimes of WD+dM binary systems becomes more comparable to that of the field M dwarfs. One of the implications of having a close binary companion is presumed to be increased stellar rotation through disk-disruption, tidal effects, or angular momentum exchange. Thus, we interpret the similarity in activity behavior between late-type dMs in WD+dM pairs and late-type field dMs to be due to a decrease in sensitivity in close binary companions (or stellar rotation), which has implications for the nature of magnetic activity in fully-convective stars. (Abridged)