2014MNRAS.439.3094B

We present radial velocity measurements of a sample of M5V-M9V stars from our Red-Optical Planet Survey, operating at 0.652-1.025µm. Radial velocities for 15 stars, with rms precision down to 2.5m/s over a week-long time-scale, are achieved using thorium-argon reference spectra. We are sensitive to planets with mpsini ≥ 1.5M⊕ (3M⊕ at 2σ) in the classical habitable zone, and our observations currently rule out planets with mpsini > 0.5 MJ at 0.03au for all our targets. A total of 9 of the 15 targets exhibit rms < 16m/s, which enables us to rule out the presence of planets with mpsini > 10M⊕ in 0.03au orbits. Since the mean rotation velocity is of the order of 8 km/s for an M6V star and 15 km/s 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 vsin i values exhibit radial velocities significantly above the photon-noise-limited precision, even after accounting for vsin i. We have therefore monitored stellar activity via chromospheric emission from the Hα and Caii 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.