Observation and modelling of main sequence star chromospheres – XIII. The Na i D1 and D2, and He i D3 lines in dM1 stars

We investigate spectral lines of interest in dM1 stars, namely the Na  i D1 and D2 and He  i 5876 lines. We study in detail the line shapes of the Na  i 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 Ca  ii resonance line–mean equivalent width (EW) and v  sin  i as well as between the Na  i mean line core relative flux and v  sin  i . These correlations include low activity dM1 stars and show that the Na  i mean line core flux is a good chromospheric diagnostic. We find a good correlation between the Na  i D1 line core flux and the Na  i 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 Ca  ii lines. The Na  i D1 and D2 mean line core flux also correlates well with the Ca  ii mean EW and with the H α EW. We also compare these correlations to the available model computations. We investigate in detail the shapes of the Na  i 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 He  i 5876 line is detected in only one dM1 star in our sample. We obtain activity correlations between the He  i 5876 line EW and the Ca  ii mean EW, and the H α EW.