2017A&A...599A..46B

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 HeI lines in the ejecta of double-detonation models. We use these approximations to compute synthetic spectra, including the HeI lines, for double-detonation models obtained from hydrodynamical explosion simulations. Specifically, we focus on photospheric-phase predictions for the near-infrared 10830Å and 2µm lines of HeI. We first consider a double detonation model with a luminosity corresponding roughly to normal SNeIa. This model has a post-explosion unburned He mass of 0.03M_☉ and our calculations suggest that the 2µm feature is expected to be very weak but that the 10830Å feature may have modest opacity in the outer ejecta. Consequently, we suggest that a moderate-to-weak HeI 10830Å 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 (∼19000km/s) mean that the HeI 10830Å feature may be confused or blended with the CI 10690Å line forming at lower velocities. We also present calculations for the HeI 10830Å and 2µm lines for a lower mass (low luminosity) double detonation model, which has a post-explosion He mass of 0.077M_☉.In this case, both the HeI features we consider are strong and can provide a clear observational signature of the double-detonation mechanism.