Magnetic activity in the HARPS M dwarf sample - The rotation-activity relationship for very low-mass stars through R'HK

摘要

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 -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 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 -index that extends to the realm of M dwarfs, and by evaluating the relationship between 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 . We monitored magnetic activity through the Ca? ii H and K emission lines in the HARPS M dwarf sample. Results. The 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 index saturates to approximately 10~(-4) . Stellar mass modulates the Ca? ii activity, with 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 indexes show larger variability, as previously observed for earlier spectral types.