SEARCHING FOR HYDROGEN IN TYPE Ib SUPERNOVAE

We present synthetic spectral fits of the typical Type Ib SN 1999dn and the hydrogen-rich Ib SN 2000H using the generalized non-local thermodynamic equilibrium stellar atmospheres code PHOENIX . We fit model spectra to five epochs of SN 1999dn ranging from 10 days pre-maximum light to 17 days post-maximum light and to the two earliest epochs of SN 2000H available, maximum light and six days post-maximum. Our goal is to investigate the possibility of hydrogen in Type Ib supernovae (SNe Ib), specifically a feature around 6200 Å which has previously been attributed to high-velocity Hα. In earlier work on SN 1999dn we found the most plausible alternative to Hα to be a blend of Si ii and Fe ii lines which can be adjusted to fit by increasing the metallicity. Our models are simple; they assume a power-law density profile with radius, homologous expansion, and solar compositions. The helium core is produced by “burning” 4H→He in order to conserve the nucleon number. For models with hydrogen the outer skin of the model consists of a shell of solar composition. The hydrogen mass of the standard solar composition shell is M H ≲ 10 −3 M ☉ in SN 1999dn and M H ≲ 0.2 M ☉ for SN 2000H. Our models fit the observed spectra reasonably well, successfully reproducing most features including the characteristic He i absorptions. The hydrogen feature in SN 1999dn is clear, but much more pronounced in SN 2000H. We discuss a possible evolutionary scenario that accounts for the dichotomy in the hydrogen shell mass between these two SNe.