Magnetic activity of M dwarfs

To do this, we used 2965 spectra in the optical range from different instruments installed in the southern and northern hemispheres to construct time series with extensions of up to 21 years. We analysed these time series with different time-domain techniques to detect cyclical patterns. In addition, using 2MASS-K_S_ and visible photometry we also studied the potential impact of chromospheric activity on surface brightness.

Using the colour index (V-KS), we calculated the chromospheric emission levels and found that most of the stars in the sample have low emission levels, indicating that most of them are inactive or very inactive stars. For 31 stars of 35, we constructed time series using the S indexes, and we detected 13 potential cycles of magnetic activity. These cycles have an approximate duration of between three and 19 years, with false alarm probabilities (FAPs) less than 0.1%.

For stars that do not show cyclic behaviour, we found that the mean value of the S index varies between 0.350 and 1.765, and its mean variability and chromospheric emission level are around 12% and -5.110dex, respectively. We do not find any impact of chromospheric activity on the surface brightness in the domain of -5.6<logR'HK<-4.5.