Do stellar magnetic cycles influence the measurement of precise radial velocities?

Abstract

The ever increasing level of precision achieved by present and future radial-velocity instruments is opening the way to discovering very low-mass, long-period planets (e.g. solar-system analogs). These systems will be detectable as low-amplitude signals in radial-velocity (RV). However, an important obstacle to their detection may be the existence of stellar magnetic cycles on similar timescales. Here we present the results of a long-term program to simultaneously measure radial-velocities and stellar-activity indicators (CaII, H_alpha, HeI) for a sample of stars with known activity cycles. Our results suggest that all these stellar activity indexes can be used to trace the stellar magnetic cycle in solar-type stars. Likewise, we find clear indications that different parameters of the HARPS cross-correlation function (BIS, FWHM, and contrast) are also sensitive to activity level variations. Finally, we show that, although in a few cases slight correlations or anti-correlations between radial-velocity and the activity level of the star exist, their origin is still not clear. We can, however, conclude that for our targets (early-K dwarfs) we do not find evidence of any radial-velocity variations induced by variations of the stellar magnetic cycle with amplitudes significantly above ~1 m/s.