Model chromospheres of flare stars. I - Balmer-line profiles

In preparation for using the Balmer lines in flare stars as chromospheric diagnostics, we have computed a grid of model chromospheres by superposing prescribed temperature rises on published models for M dwarf photospheres. The chromospheric equation of state is dominated by the non-LTE ionization of hydrogen, which is treated by using a model hydrogen atom with five bound levels. The radiative transfer equation is solved explicitly for the Lyman continuum and the Balmer lines Ha, Hfl, and H)'. In the absence of a chromosphere, Ha, Hfl, and H)' appear as weak absorption lines. As the amount of chromospheric material (between temperatures Te = 5500 K and 50,000 K) increases, these absorption lines first become deeper, then develop emission peaks on the outer edges of their wings, and finally, when the chromosphere is sufficiently massive, the Balmer lines become strong emission lines. The results obtained here will be used to interpret the profiles and decrements of the Balmer lines in flare stars, in both quiescent and flaring states. Subject headings: line profiles - stars: atmospheres - stars: chromospheres - stars: flare - stars: late-type