FORMATION OF HYDROGEN LINES IN THE ATMOSPHERES OF TYPE-II SUPERNOVAE

A systematic study of NLTE hydrogen line formation for type II superno vae (SNe II) during the photospheric phase is presented. We show that the solution of the line radiative transfer using the Sobolev approxim ation with the correct continuum incident radiation is of the same acc uracy as the results obtained with the 'comoving frame method' and thu s can be used for quantitative spectroscopy. The net emission and the blue shifted emission peak observed in the H-alpha-profiles of SNe II are explained by NLTE effects and the significant extension of the lin e forming region. For an analysis of the hydrogen lines alone, we sugg est that the velocity measured in the absorption minimum of H-delta ma y 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 ph otospheric radius on the resulting line profiles. In addition, the inf luence of a statistical velocity field of variable strength is investi gated. Statistical velocity fields with less than 10 % of the photosph eric expansion velocity have little influence on the line shapes and t he 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.