The 1.05- m feature in the spectrum of the Type Ia supernova 1994D: He in SNe Ia?

Abstract

A P Cygni profile with absorption at 1.05 μm was observed in three pre-maximum J-band spectra of the Type Ia supernova (SN) 1994D. The feature was not present in two post-maximum spectra. The line was attributed to He 10830 ... or MgII 10926 ..., based on a local thermodynamic equilibrium (LTE) treatment. The detection of He in the ejecta of a SN Ia would be useful for determining the pre-SN evolution and the explosion mechanism of SNe Ia. In this paper, synthetic spectra are presented for both the He and Mg models. The population of the He levels has been computed in non-local thermodynamic equilibrium (NLTE), including non-thermal excitation and ionization effects resulting from the deposition of γ-rays from the decay of 56Ni and 56Co. The J-band feature in the pre-maximum spectra can be reproduced either assuming the presence of a narrow shell, between 10000 and 12500 km s−1, containing about 0.01 M⊙ of He, or increasing the abundance of Mg by about a factor of 5 with respect to the W7 value, implying a Mg mass of about 0.08 M⊙ above 10000 km s−1. Both models are in good agreement with the optical spectrum. In particular, a strong HeI 10830-... line does not imply a strong 5876-... line, because the departure coefficients of the 2p and 2s levels of HeI differ by about an order of magnitude. Unfortunately, neither model is able to reproduce the sudden disappearance of the J-band feature in the post-maximum spectra. Possible explanations are discussed.