OBSERVATION AND MODELING OF MAIN-SEQUENCE STAR CHROMOSPHERES .1. MODELING OF THE HYDROGEN SPECTRUM IN DME STARS

In this new series we present observations and modelling of spectral l ines formed in main sequence star chromospheres (from G0 to M8). In th is paper, we discuss the Hydrogen line formations in very active dMe s tellar chromospheres. We present the conclusions drawn from our modell ing approach to the AU Mic (dM2e) spectrum. Our final model atmosphere is described in detail in the next paper in this series. Here, we inv estigate the effects of some stellar (T(eff, upsilon sin i) and atmosp heric (turbulence, temperature structure) parameters on the line forma tion. We found that observations are best reproduced by a chromospheri c structure including a constant temperature gradient (in a log(M) sca le) in the chromosphere and transition region. We show that a very thi n transition region is required to account for the observed Ly(alpha) to H(alpha) surface flux ratio. Then, to drive the Balmer lines into e mission and reproduce the H(alpha) and H(beta) self-reversal and FWHM, the chromospheric gradient and transition region pressure must be qui te high for the most active stars. We put forward a complete set of im portant constraints on the possible structures of such active region a tmospheres from 5,000 K to 50,000 K. Notably we show that: (i) the chr omosphere/transition region temperature break zone must be located at about 8,200 +/- approximately 200 K, implying that there is rather lit tle plasma above this temperature, (ii) the temperature break zone is central to the formation of Lyman and Balmer lines, it should be smoot h and contained in a small column mass domain, (iii) the transition re gion must be very thin and at a high column mass, log(M) approximately -3, (iv) the temperature break demarcates the temperature domains of formation of Lyman and other Hydrogen series, (v) backwarming due to t he Lyman and Balmer radiation fields occurs at the top of the chromosp here, (vi) turbulence and rotational broadening have little effect on the line profiles and fluxes, (vii) rotational broadening should not h owever be neglected because H(alpha) self-reversal is a major constrai nt to the modelling, (viii) the profiles are weakly dependant on the s tellar effective temperature whose uncertainties can be neglected in f irst approximation. Finally, we further compare our results to observa tions and propose some general properties that could be tested with fu ture observations.