The effects of close binaries on the magnetic activity of M dwarfs as probed using close white dwarf companions
Morgan, Dylan Parker
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I present a study of close white dwarf (WD) and M dwarf (dM) binary systems (WD+dM) to examine the effects that close companions have on the magnetic field generation in dMs. Using the Sloan Digital Sky Survey (SDSS) Data Release 8 spectroscopic database, I construct a sample of 1756 WD+dM high-quality pairs. I show that high-mass dMs (≤M4) in close binary systems are more likely to be magnetically active (as measured by Hα emission) and are able to remain active longer than field dMs. At lower masses (≥M5), where dMs become fully convective, the activity fraction and activity lifetimes of WD+dM binary systems become more comparable to those of the field dMs. The implications of having a close binary companion may include, increased stellar rotation through disk disruption, tidal effects, and/or angular momentum exchange. Thus, the similarity in activity between late-type field dMs and late-type dMs with close companions is likely due to the mechanism generating magnetic fields being less sensitive to the effects caused by a close companion; namely, increased stellar rotation. Using a subset of 181 close WD+dM pairs, matched to the time-domain SDSS Stripe 82 catalog, I show that enhanced magnetic activity extends to the flaring behavior of dMs in close binaries. Specifically, early spectral type dMs (M0-M1), in close WD+dM pairs, are two orders of magnitude more likely to flare than field dMs, whereas mid-type dMs (M2-M3) and late-type dMs (M4-M6) flare as frequently or less than the mid- to late-type field dM sample. To test whether the presence of a close companion leads to star-star interactions, I search for correlations between the WD occultations and flares from the dM member in KOI-256, an eclipsing WD+dM system from Kepler I find no correlations between the flaring activity of the dM and the WD occultations, indicating the there are no obvious signs of star-star interactions at work. In addition, the dM member of KOI-256 flares more than any other dM observed by Kepler and shows evidence for solar-like magnetic activity cycles, a feature not seen in many dMs to date.