Magnetic activity in the HARPS M dwarf sample⋆
The rotation-activity relationship for very low-mass stars through R'HK
1 Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
2 CNRS, IPAG, 38000 Grenoble, France
3 Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland
4 Département de Physique, Université de Montréal, CP 6128 Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada
Received: 29 July 2015
Accepted: 17 November 2016
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 (Prot) 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 LX/Lbol, 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.
Key words: stars: activity / stars: late-type / stars: rotation / planetary systems / techniques: spectroscopic
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