OBSERVATION AND MODELING OF MAIN-SEQUENCE STAR CHROMOSPHERES .3. DIFFERENTIAL ANALYSIS OF HYDROGEN LINES VERSUS ACTIVITY LEVEL IN M DWARFS

In two previous papers we investigated in detail the formation of hydr ogen lines in high pressure dMe stellar chromospheres, constraining th e possible structures over a two orders of magnitude pressure range. I n another study, we modeled the low activity M dwarf spectra. Here, we use these constraints at the extremes in magnetic activity level to s imulate the effect of varying magnetic non-thermal heating in dM and d Me chromospheres by varying the transition region pressure and tempera ture minimum. We built four grids of model atmospheres with temperatur e minimum either at 2,660 K or 3,000 K, and a range of transition regi on pressure. We found that when decreasing the transition region press ure (i.e. the chromospheric temperature gradient), the Balmer lines ch ange rapidly from emission to strong absorption, then the profiles wea ken and become narrower until they disappear totally (zero H(alpha) st ars). The Paschen and Brackett series exhibit a qualitatively similar behaviour, but the ''emission domain'' is at a higher column mass: log (M)> -4.4 g cm-2, log(M)> -4.2 g cm-2 and log(M)> -3.8 g cm-2 respecti vely for the Balmer, Paschen and Brackett series. The Brackett lines n ever really develop a strong absorption. In opposition with other seri es, the Lyman lines show a monotonous decrease and even change to abso rption for very low density models. These differences are useful spect ral diagnostics for M dwarf atmospheres. All hydrogen series, except t he Lyman series for intermediate and high pressures, are sensitive to the temperature minimum when large changes are considered. We also inv estigated the effect of the temperature break zone and found it is imp ortant only for high pressure atmospheres. We show that the continuum emission shortward of 4000angstrom is highly dependent on the atmosphe ric pressure and rises sharply in the Balmer emission domain. Our grid s of models successfully reproduce all types of observed H(alpha) prof iles: (i) high activity with strong emission and weak self-reversal, ( ii) filled in intermediate activity with inner wings in emission and t he core in absorption, (iii) intermediate activity with strong and bro ad absorption, (iv) low activity with weak and narrow absorption, (v) ''zero activity'' with an undetectable profile. We discuss the line ch aracteristics over this wide range of physical conditions. We analyse the ionisation fraction and electron density for our series of chromos pheres. Changes in the ionisation fraction are important throughout th e pressure range. Heavy elements are the main electron donor in the ph otosphere for all models, and in the chromosphere for low pressure mod els. Back-ionisation by the chromospheric continuum emission and direc t ionisation by the photospheric radiation field are taking place at a bout the temperature minimum. Finally, we compare the chromospheric de nsity regimes for main sequence stars (M, K and G dwarfs) for the quie scent and flare states.