The Benchmark M Dwarf Eclipsing Binary CM Draconis With TESS: Spots, Flares and Ultra-Precise Parameters

TL;DR

This study uses TESS data to precisely measure the fundamental parameters of the M dwarf binary CM Draconis, uncover stellar activity phenomena like spots and flares, and explore their relationships, with implications for exoplanet habitability.

Contribution

The paper provides ultra-precise measurements of masses and radii, and reports new findings on stellar activity, including spot modulation, flares, and their correlations, using extensive TESS observations.

Findings

Masses and radii measured to 0.12% and 0.06% precision

Detected 163 flares and variable spot activity

Flares more likely when stars are brighter, possibly polar

Abstract

A gold standard for the study of M dwarfs is the eclipsing binary CM Draconis. It is rare because it is bright ($J_{\rm mag}=8.5$) and contains twin fully convective stars on an almost perfectly edge-on orbit. Both masses and radii were previously measured to better than $1\%$ precision, amongst the best known. We use 15 sectors of data from the Transiting Exoplanet Survey Satellite (TESS) to show that CM Draconis is the gift that keeps on giving. Our paper has three main components. First, we present updated parameters, with radii and masses constrained to previously unheard of precisions of $\approx 0.06\%$ and $\approx 0.12\%$, respectively. Second, we discover strong and variable spot modulation, suggestive of spot clustering and an activity cycle on the order of $\approx 4$ years. Third, we discover 163 flares. We find a relationship between the spot modulation and flare rate, with flares more likely to occur when the stars appear brighter. This may be due to a positive correlation between flares and the occurrence of bright spots (plages). The flare rate is surprisingly not reduced during eclipse, but one flare may show evidence of being occulted. We suggest the flares may be preferentially polar, which has positive implications for the habitability of planets orbiting M dwarfs.