Activity indicators in M dwarfs

To do so, we employed 1796 spectra from different instruments with a median time span of observations of 21 years. The spectra have a wide spectral range that allowed us to compute the chrosmospheric activity indicators based on CaII and H{alpha}. In addition, we complemented our data with photometric observations from the TESS space mission for better stellar characterisation and short-term analysis.

We obtained a good and significant correlation ({rho}=0.91) between the indexes defined from the two lines for the whole set of stars in the sample. However, we found that there is a deviation for faster rotators (with Prot<4-days) and higher flare activity (at least one flare per day). For the individual analysis, we found that the indexes computed individually for each star correlate independently of the level of chromospheric emission and the rotation period.

There is an overall positive correlation between CaII and H{alpha} emission in dM stars, except during flare events. In particular, we found that low-energy high-frequency flares could be responsible for the deviation in the linear trend in fast-rotator M dwarfs. This implies that the rotation period could be a fundamental parameter to study the stellar activity and that the rotation could drive the magnetic dynamo in low-mass active stars.