Detailed Spectral Analysis of the Type Ib Supernova 1999dn. Paper I: Hydrogen-free Models

We present spectral fits to five epochs of the typical Type Ib supernova 1999dn using the generalized, non-LTE, stellar atmospheres code PHOENIX. Our goal is threefold: to determine basic physical properties of the supernova ejecta, such as velocity, temperature, and density gradients; to reproduce He I absorption lines by invoking non-thermal excitation; and, to investigate possible spectral signatures of hydrogen, especially a feature around 6200 Angstrom, which has been attributed to high velocity $H_\alpha$. Our models assume an atmosphere with uniform composition devoid of any hydrogen. Our model spectra fit the observed spectra well, successfully reproducing most of the features, including the prominent He I absorptions. The most plausible alternative to $H_\alpha$ as the source of the 6200 Angstrom feature is a blend of Fe II and Si II lines, which can be made stronger to fit the observed feature better by increasing the metallicity of the ejecta. High-metallicity models fit well at early epochs, but not as well as solar-metallicity models after maximum light. While this blend of metal lines is a reasonable explanation of the source of the 6200 Angstrom feature, it is still important to investigate hydrogen as the source; therefore, a second paper will present models that include a thin shell of hydrogen around the main composition structure.