2017A&A...600A..13A

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the CaII 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 CaII 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 CaII 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 CaII H and K lines to a close spectral continuum) into the R'_HK. We monitored magnetic activity through the CaII 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 CaII activity, as expected. Under that period, the R'_HK index saturates to approximately 10^–4. Stellar mass modulates the CaII activity, with R'_HK showing a constant basal activity above 0.6M_☉ and then decreasing with mass between 0.6M_☉ and the fully-convective limit of 0.35M_☉. 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.