Astronomy and Astrophysics, volume 600A, 13-13 (2017/4-1)
Magnetic activity in the HARPS M dwarf sample. The rotation-activity relationship for very low-mass stars through R'HK.
ASTUDILLO-DEFRU N., DELFOSSE X., BONFILS X., FORVEILLE T., LOVIS C. and RAMEAU J.
Abstract (from CDS):
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 (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 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 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 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.
© ESO, 2017
stars: activity - stars: late-type - stars: rotation - planetary systems - techniques: spectroscopic - techniques: spectroscopic
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<Available at CDS (J/A+A/600/A13): tablea3.dat>
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