Starspot distributions on fully convective M dwarfs: implications for radial velocity planet searches

TL;DR

This study presents high-resolution Doppler imaging of two fully convective M dwarfs, revealing starspot distributions that impact radial velocity measurements and informing methods to mitigate stellar jitter in planet searches.

Contribution

First high-resolution Doppler images of M4.5 and M9 dwarfs showing starspot distributions and their effects on radial velocity variability.

Findings

Starspots are predominantly high latitude with low contrast.

Starspot-induced velocity variability can be reduced by surface brightness modeling.

Low amplitude photometric variability is linked to high-latitude starspots.

Abstract

Since M4.5 - M9 dwarfs exhibit equatorial rotation velocities of order 10 km/s on average, radial velocity surveys targeting this stellar population will likely need to find methods to effectively remove starspot jitter. We present the first high resolution Doppler images of the M4.5 dwarf, GJ 791.2A, and the M9 dwarf, LP 944-20. The time series spectra of both objects reveal numerous line profile distortions over the rotation period of each star which we interpret as starspots. The transient distortions are modelled with spot/photosphere contrast ratios that correspond to model atmosphere temperature differences of Tphot-Tspot = 200 and 300 K. GJ 791.2A is a fully convective star with vsini = 35.1 km/s. Although we find more starspot structure at high latitudes, we reconstruct spots at a range of phases and latitudes with a mean spot filling of ~3%. LP 944-20 is one of the brightest known late-M dwarfs, with spectral type M9V and vsini = 30.8 km/s. Its spectral time series exhibits two dominant transient line distortions that are reconstructed as high latitude spots, while a mean spot filling factor of only 1.5% is found. The occurrence of low-contrast spots at predominantly high latitudes, which we see in both targets here, is in general likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. For GJ 791.2A, the radial velocities induced by the starspot features yield an rms velocity variability of 138 m/s, which can be reduced by a factor of 1.9 using our reconstructed surface brightness distributions.