1995A&A...297..802D

A systematic study of NLTE hydrogen line formation for type II supernovae (SNe II) during the photospheric phase is presented. We show that the solution of the line radiative transfer using the Sobolev approximation with the correct continuum incident radiation is of the same accuracy as the results obtained with the `comoving frame method' and thus can be used for quantitative spectroscopy. The net emission and the blue shifted emission peak observed in the H_α-profiles of SNe II are explained by NLTE effects and the significant extension of the line forming region. For an analysis of the hydrogen lines alone, we suggest that the velocity measured in the absorption minimum of H_δ may be used for the determination of the photospheric expansion velocity. A parameter study is performed, proving the strong influence of the logarithmic density gradient and the weak effect of a change in the photospheric radius on the resulting line profiles. In addition, the influence of a statistical velocity field of variable strength is investigated. Statistical velocity fields with less than 10% of the photospheric expansion velocity have little influence on the line shapes and the total line emissivity. With increasing v_stat, the resulting line profiles can be characterized by red shifted decreasing emission peaks and an increase in absorption strength.