Precision radial velocities of 15 M5–M9 dwarfs

Abstract

We present radial velocity measurements of a sample of M5V–M9V stars from our Red-Optical Planet Survey, operating at 0.652–1.025 µm. Radial velocities for 15 stars, with rms precision down to 2.5 m s−1 over a week-long time-scale, are achieved using thorium–argon reference spectra. We are sensitive to planets with mp sin i ≥ 1.5 M⊕ (3 M⊕ at 2σ) in the classical habitable zone, and our observations currently rule out planets with mp sin i > 0.5 MJ at 0.03 au for all our targets. A total of 9 of the 15 targets exhibit rms < 16 m s−1, which enables us to rule out the presence of planets with mp sin i > 10 M⊕ in 0.03 au orbits. Since the mean rotation velocity is of the order of 8 km s−1 for an M6V star and 15 km s−1 for M9V, we avoid observing only slow rotators that would introduce a bias towards low axial inclination (i 90◦) systems, which are unfavourable for planet detection. Our targets with the highest v sin i values exhibit radial velocities significantly above the photon-noise-limited precision, even after accounting for v sin i. We have therefore monitored stellar activity via chromospheric emission from the Hα and Ca II infrared triplet lines. A clear trend of log10(LHα/Lbol) with radial velocity rms is seen, implying that significant starspot activity is responsible for the observed radial velocity precision floor. The implication that most late M dwarfs are significantly spotted, and hence exhibit time varying line distortions, indicates that observations to detect orbiting planets need strategies to reliably mitigate against the effects of activity-induced radial velocity variations