J band Variability of M Dwarfs in the WFCAM Transit Survey

TL;DR

This study investigates the variability of M dwarfs in the J band using the WFCAM Transit Survey, revealing periodic variability in a small fraction of stars and highlighting the advantages of J band observations for detecting stellar activity and transits.

Contribution

It provides the first extensive analysis of M dwarf variability in the J band, including a large sample, periodicity detection, and simulation-based estimates of active star fractions.

Findings

68 periodic variable M dwarfs identified

Detected variability amplitudes up to 0.115 mag in J band

Simulations suggest 1-3% of stars with >10% spot coverage are periodic

Abstract

We present an analysis of the photometric variability of M dwarfs in the WFCAM Transit Survey. Although periodic lightcurve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the J band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200,000 objects are made using spectral type-colour relations, and over 9600 dwarfs (J<17) with spectral types later than K7 were found. The light curves of the late-type sample are searched for periodicity using a Lomb-Scargle periodogram analysis. A total of 68 periodic variable M dwarfs are found in the sample with periods ranging from 0.16 days to 90.33 days, with amplitudes in the range of ~0.009 to ~0.115 in the J band. We simulate active M dwarfs with a range of latitude-independent spot coverages and estimate a periodically variable fractions of 1-3 per cent for stars where spots cover more than 10 per cent of the star's surface. Our simulated spot distributions indicate that operating in the J band, where spot contrast ratios are minimised, enables variability in only the most active of stars to be detected. These findings affirm the benefits of using the $J$ band for planetary transit searches compared to visible bands. We also serendipitously find a \Delta J>0.2 mag flaring event from an M4V star in our sample.