Helium in Double-Detonation Models of Type Ia Supernovae

TL;DR

This paper models helium line signatures in double-detonation Type Ia supernovae, predicting observable features that could confirm the explosion mechanism through spectral analysis.

Contribution

It introduces simple approximations for helium line optical depths and predicts spectral signatures in double-detonation supernova models, aiding observational identification.

Findings

He I 10830 Å line may be modestly visible near maximum light.

He I 2 μm line is expected to be very weak.

Lower luminosity models show stronger helium spectral features.

Abstract

The double-detonation explosion model has been considered a candidate for explaining astrophysical transients with a wide range of luminosities. In this model, a carbon-oxygen white dwarf star explodes following detonation of a surface layer of helium. One potential signature of this explosion mechanism is the presence of unburned helium in the outer ejecta, left over from the surface helium layer. In this paper we present simple approximations to estimate the optical depths of important He I lines in the ejecta of double-detonation models. We use these approximations to compute synthetic spectra, including the He I lines, for double-detonation models obtained from hydrodynamical explosion simulations. Specifically, we focus on photospheric-phase predictions for the near-infrared 10830 \AA~and 2 $\mu$m lines of He I. We first consider a double detonation model with a luminosity corresponding roughly to normal SNe Ia. This model has a post-explosion unburned He mass of 0.03 $M_{\odot}$ and our calculations suggest that the 2 $\mu$m feature is expected to be very weak but that the 10830 \AA~feature may have modest opacity in the outer ejecta. Consequently, we suggest that a moderate-to-weak He I 10830 \AA~feature may be expected to form in double-detonation explosions at epochs around maximum light. However, the high velocities of unburned helium predicted by the model ($\sim 19,000$~km~s$^{-1}$) mean that the He I 10830 \AA~feature may be confused or blended with the C I 10690~\AA~line forming at lower velocities. We also present calculations for the He I 10830 \AA~and 2 $\mu$m lines for a lower mass (low luminosity) double detonation model, which has a post-explosion He mass of 0.077 $M_{\odot}$. In this case, both the He I features we consider are strong and can provide a clear observational signature of the double-detonation mechanism.