Possible Bright Starspots on TRAPPIST-1

TL;DR

This study models bright starspots on TRAPPIST-1 to explain optical and infrared light curves, suggesting bright spots rather than dark ones, and finds a possible link between spots and flare activity, challenging the interpretation of the 3.3-day periodicity.

Contribution

The paper introduces a photometric model for bright spots on TRAPPIST-1 that explains observed light curves and proposes a new interpretation of the star's activity cycle.

Findings

Bright spots can explain Kepler and Spitzer light curves without dark spots.

Large flares are more frequent when bright spots face the observer.

The 3.3-day periodicity may be related to active regions, not stellar rotation.

Abstract

The M8V star TRAPPIST-1 hosts seven roughly Earth-sized planets and is a promising target for exoplanet characterization. Kepler/K2 Campaign 12 observations of TRAPPIST-1 in the optical show an apparent rotational modulation with a 3.3 day period, though that rotational signal is not readily detected in the Spitzer light curve at 4.5 $\mu$m. If the rotational modulation is due to starspots, persistent dark spots can be excluded from the lack of photometric variability in the Spitzer light curve. We construct a photometric model for rotational modulation due to photospheric bright spots on TRAPPIST-1 which is consistent with both the Kepler and Spitzer light curves. The maximum-likelihood model with three spots has typical spot sizes of $R_\mathrm{spot}/R_\star \approx 0.004$ at temperature $T_\mathrm{spot} \gtrsim 5300 \pm 200$ K. We also find that large flares are observed more often when the brightest spot is facing the observer, suggesting a correlation between the position of the bright spots and flare events. In addition, these flares may occur preferentially when the spots are increasing in brightness, which suggests that the 3.3 d periodicity may not be a rotational signal, but rather a characteristic timescale of active regions.