LSPM J1112+7626: detection of a 41-day M-dwarf eclipsing binary from the MEarth transit survey

TL;DR

This paper reports the discovery and detailed characterization of a 41-day period M-dwarf eclipsing binary, providing precise mass measurements and revealing radius inflation consistent with theoretical models, and highlighting activity-related radius inflation.

Contribution

It presents the first detailed analysis of a long-period M-dwarf eclipsing binary with precise mass and radius measurements, expanding understanding beyond short-period systems.

Findings

Component masses determined to better than 1% accuracy.

The sum of radii is inflated by about 3.8% compared to models.

Detected 2% out-of-eclipse modulation likely due to stellar spots.

Abstract

We report the detection of eclipses in LSPM J1112+7626, which we find to be a moderately bright (I_C = 12.14 +/- 0.05) very low-mass binary system with an orbital period of 41.03236 +/- 0.00002 days, and component masses M_1 = 0.395 +/- 0.002 Msol and M_2 = 0.275 +/- 0.001 Msol in an eccentric (e = 0.239 +/- 0.002) orbit. A 65 day out of eclipse modulation of approximately 2% peak-to-peak amplitude is seen in I-band, which is probably due to rotational modulation of photospheric spots on one of the binary components. This paper presents the discovery and characterization of the object, including radial velocities sufficient to determine both component masses to better than 1% precision, and a photometric solution. We find that the sum of the component radii, which is much better-determined than the individual radii, is inflated by 3.8 +0.9 -0.5 % compared to the theoretical model predictions, depending on the age and metallicity assumed. These results demonstrate that the difficulties in reproducing observed M-dwarf eclipsing binary radii with theoretical models are not confined to systems with very short orbital periods. This object promises to be a fruitful testing ground for the hypothesized link between inflated radii in M-dwarfs and activity.