Modeling Type Ic Supernovae with tardis: Hidden Helium in SN 1994I?

Abstract

Abstract Supernovae (SNe) with photospheric spectra devoid of hydrogen and helium features are generally classified as SNe Ic. However, there is ongoing debate as to whether helium can be hidden in the ejecta of SNe Ic (that is, helium is present in the ejecta, but produces no obvious features in the spectra). We present the first application of the fast, 1D radiative transfer code tardis to an SN Ic, and we investigate the question of how much helium can be hidden in the outer layers of the SN Ic ejecta. We generate tardis models for the nearby, well-observed, and extensively modeled SN Ic 1994I, and we perform a code comparison to a different, well-established Monte Carlo based radiation transfer code. The code comparison shows that tardis produces consistent synthetic spectra for identical ejecta models of SN 1994I. In addition, we perform a systematic experiment of adding outer He shells of varying masses to our SN 1994I models. We find that an outer He shell of only 0.05M ⊙ produces strong optical and near-infrared (NIR) He spectral features for SN 1994I which are not present in observations, thus indicating that the SN 1994I ejecta is almost fully He deficient compared to the He masses of typical He-rich SN progenitors. Finally we show that the He i λ20851 line pseudo-equivalent width of our modeled spectra for SN 1994I could be used to infer the outer He shell mass which suggests that NIR spectral follow-up of SNe Ic will be critical for addressing the hidden helium question for a statistical sample of SNe Ic.