Magnetic activity in the HARPS M dwarf sample

Abstract

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period ( P rot ) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the R' HK -index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs – the most numerous stars of the Galaxy – were left out of these analyses and no calibration of their Ca ii H and K emission to an R' HK exists to date. Aims. We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the R' HK -index that extends to the realm of M dwarfs, and by evaluating the relationship between R' HK and the rotation period. Methods. We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S -index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the R' HK . We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results. The R' HK index, like the fractional X-ray luminosity L X / L bol , shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R' HK index saturates to approximately 10 -4 . Stellar mass modulates the Ca ii activity, with R' HK showing a constant basal activity above 0.6 M ⊙ and then decreasing with mass between 0.6 M ⊙ and the fully-convective limit of 0.35 M ⊙ . Short-term variability of the activity correlates with its mean level and stars with higher R' HK indexes show larger R' HK variability, as previously observed for earlier spectral types.