Calibration Of The H Alpha Age-Activity Relation For M Dwarfs

In this work, we calibrate the relationship between H alpha emission and M dwarf ages. We compile a sample of 892 M dwarfs with Ha equivalent width (H alpha EW) measurements from the literature that are either comoving with a white dwarf of known age (21 stars) or in a known young association (871 stars). In this sample we identify 7 M dwarfs that are new candidate members of known associations. By dividing the stars into active and inactive categories according to their H alpha EW and spectral type (SpT), we find that the fraction of active dwarfs decreases with increasing age, and the form of the decline depends on SpT. Using the compiled sample of age calibrators, we find that H alpha EW and fractional Ha luminosity (L-H alpha/L-bol) decrease with increasing age. H alpha EW for SpT <= M7 decreases gradually up until similar to 1 Gyr. For older ages, we found only two early M dwarfs that are both inactive and seem to continue the gradual decrease. We also found 14 mid-type M dwarfs, out of which 11 are inactive and present a significant decrease in H alpha EW, suggesting that the magnetic activity decreases rapidly after similar to 1 Gyr. We fit L-H alpha/L-bol versus age with a broken power law and find an index of -0.11(-0.01)(+0.02) for ages less than or similar to 776 Myr. The index becomes much steeper at older ages, but a lack of field age-calibrators (>= 1 Gyr) leaves this part of the relation far less constrained. Finally, from repeated independent measurements for the same stars, we find that 94% of them have a level of H alpha EW variability <= 5 angstrom at young ages (<1 Gyr).