1995A&A...294..773H

In two previous papers we investigated in detail the formation of hydrogen lines in high pressure dMe stellar chromospheres, constraining the possible structures over a two orders of magnitude pressure range. In another study, we modeled the low activity M dwarf spectra. Here, we use these constraints at the extremes in magnetic activity level to simulate the effect of varying magnetic non-thermal heating in dM and dMe chromospheres by varying the transition region pressure and temperature minimum. We built four grids of model atmospheres with temperature minimum either at 2,660K or 3,000K, and a range of transition region pressure. We found that when decreasing the transition region pressure (i.e. the chromospheric temperature gradient), the Balmer lines change rapidly from emission to strong absorption, then the profiles weaken and become narrower until they disappear totally (zero H_α stars). The Paschen and Brackett series exhibit a qualitatively similar behaviour, but the "emission domain" is at a higher column mass: log(M)>-4.4gcm^–2, log(M)>-4.2gcm^–2 and log(M)>-3.8gcm^–2 respectively for the Balmer, Paschen and Brackett series. The Brackett lines never really develop a strong absorption. In opposition with other series, the Lyman lines show a monotonous decrease and even change to absorption for very low density models. These differences are useful spectral diagnostics for M dwarf atmospheres. All hydrogen series, except the Lyman series for intermediate and high pressures, are sensitive to the temperature minimum when large changes are considered. We also investigated the effect of the temperature break zone and found it is important only for high pressure atmospheres. We show that the continuum emission shortward of 4000A is highly dependent on the atmospheric pressure and rises sharply in the Balmer emission domain. Our grids of models successfully reproduce all types of observed H_α profiles: (i) high activity with strong emission and weak self-reversal, (ii) filled in intermediate activity with inner wings in emission and the core in absorption, (iii) intermediate activity with strong and broad absorption, (iv) low activity with weak and narrow absorption, (v) "zero activity" with an undetectable profile. We discuss the line characteristics over this wide range of physical conditions. We analyse the ionisation fraction and electron density for our series of chromospheres. Changes in the ionisation fraction are important throughout the pressure range. Heavy elements are the main electron donor in the photosphere for all models, and in the chromosphere for low pressure models. Back-ionisation by the chromospheric continuum emission and direct ionisation by the photospheric radiation field are taking place at about the temperature minimum. Finally, we compare the chromospheric density regimes for main sequence stars (M, K and G dwarfs) for the quiescent and flare states.