2009MNRAS.400..238H

We investigate spectral lines of interest in dM1 stars, namely the Nai D1 and D2 and Hei 5876 lines. We study in detail the line shapes of the Nai D1 and D2 lines. We find that these lines are strong and broad in normal dM1 stars and become weaker and narrower when metallicity is low, although our sample is insufficient in order to find out an empirical correlation between these parameters.

We find correlations between the Caii resonance line-mean equivalent width (EW) and vsini as well as between the Nai mean line core relative flux and vsini. These correlations include low activity dM1 stars and show that the Nai mean line core flux is a good chromospheric diagnostic.

We find a good correlation between the Nai D1 line core flux and the Nai D2 line core flux. This correlation shows that the line core optical depths decrease with an increasing activity level, that is the opposite of what was found for the Caii lines. The Nai D1 and D2 mean line core flux also correlates well with the Caii mean EW and with the Hα EW. We also compare these correlations to the available model computations. We investigate in detail the shapes of the Nai D1 and D2 lines through the full line widths at 85 per cent, 62 per cent and 35 per cent of the continuum. The significant differences from one star to another cannot be explained at this stage. Detailed modelling of the stellar photospheres will be necessary to interpret the observed differences.

The Hei 5876 line is detected in only one dM1 star in our sample. We obtain activity correlations between the Hei 5876 line EW and the Caii mean EW, and the Hα EW.