Nonthermal excitation of helium in type Ib supernovae

Abstract

The presence of strong He I absorption troughs in the optical spectra of type Ib SNs is attributed to non-thermal excitation. As for analogous in the premaximum spectra of SN 1987A, the early importance of nonthermal excitation in the Ib's is fundamentally due to a large-scale instability that mixes 56 Ni out into their envelopes shortly after the explosion. Because of this mixing, the helium-rich envelopes of these SNs are, even at maximum light, not strongly shielded form the γ-rays emitted by 56 Co nuclei. A non-LTE treatment of this non-thermal excitation of helium is given using a recent model of a type Ib SN that invokes mixing of 56 Ni in order to reproduce light curves.