Characterisation of magnetic activity of M dwarfs

Context. M dwarfs are an ideal laboratory for hunting Earth-like planets, and the study of chromospheric activity is an important part of this task. On the one hand, according to the study of short-term activity their high levels of magnetic activity can affect habitability and make it difficult to detect exoplanets orbiting around them. On the other hand, however, long-term activity studies can show whether or not these stars exhibit cyclical behaviour in their activity, facilitating the detection of planets in periods of low magnetic activity.Aims. The long-term cyclical behaviour of magnetic activity can be detected by studying several spectral lines and explained by different stellar dynamo models (such as αΩ or α^{2 dynamos). In the present work, we studied the Mount Wilson S index to search for evidence of long-term activity possibly driven by a solar-type dynamo.Methods. We studied a sample of 35 M dwarfs with different levels of chromospheric activity and spectral classes ranging from dM0 to dM6. 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.Results. 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 −5.110 dex, respectively. We do not find any impact of chromospheric activity on the surface brightness in the domain of −5.6 < log R′H K < −4.5.}}