GJ 3236: a new bright, very low-mass eclipsing binary system discovered by the MEarth observatory

TL;DR

GJ 3236 is a newly discovered bright, very low-mass eclipsing binary with a short 0.77-day period, providing valuable data on stellar radii and activity near the transition to fully-convective stars.

Contribution

This paper reports the discovery and detailed analysis of GJ 3236, a low-mass eclipsing binary, including measurements of stellar masses, radii, and activity, expanding empirical data in a key stellar evolution region.

Findings

Component masses of 0.38 and 0.28 Msol.

Stellar radii larger than theoretical predictions.

High activity levels with strong Halpha emission.

Abstract

We report the detection of eclipses in GJ 3236, a bright (I = 11.6) very low mass binary system with an orbital period of 0.77 days. Analysis of light- and radial velocity curves of the system yielded component masses of 0.38 +/- 0.02 and 0.28 +/- 0.02 Msol. The central values for the stellar radii are larger than the theoretical models predict for these masses, in agreement with the results for existing eclipsing binaries, although the present 5% observational uncertainties limit the significance of the larger radii to approximately 1 sigma. Degeneracies in the light curve models resulting from the unknown configuration of surface spots on the components of GJ 3236 currently dominate the uncertainties in the radii, and could be reduced by obtaining precise, multi-band photometry covering the full orbital period. The system appears to be tidally synchronized and shows signs of high activity levels as expected for such a short orbital period, evidenced by strong Halpha emission lines in the spectra of both components. These observations probe an important region of mass-radius parameter space around the predicted transition to fully-convective stellar interiors, where there are a limited number of precise measurements available in the literature.