Helium in double-detonation models of type Ia supernovae

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 nearinfrared 10830 Å and 2 µm 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 and our calculations suggest that the 2 µm feature is expected to be very weak but that the 10830 Å feature may have modest opacity in the outer ejecta. Consequently, we suggest that a moderate-to-weak He i 10830 Å 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 (∼ 19, 000 km s −1 ) mean that the He i 10830 Å feature may be confused or blended with the C i 10690 Å line forming at lower velocities. We also present calculations for the He i 10830 Å and 2 µm lines for a lower mass (low luminosity) double detonation model, which has a post-explosion He mass of 0.077 M . In this case, both the He i features we consider are strong and can provide a clear observational signature of the double-detonation mechanism.

SNe Ia from double detonations: Impact of core-shell mixing on the carbon ignition mechanism

“…Models M1a and 3m by Kromer et al (2010) lead us to conclude that the mixing of carbon to the helium shell has an impact on the yields as it results in the production of heavier elements. However, our models show that it does not solve the problem of a significant amount of 4 He being unburned and the redness of the synthetic spectra (Boyle et al 2017;Botyánszki et al 2018). On the other hand the mixing is critical for the details of the detonation ignition mechanism as stated above.…”

“…Previous work by Kromer et al (2010), Boyle et al (2017), and Botyánszki et al (2018), for example, points out that the synthetic spectra of the sub-Chandrasekhar mass models are too red. Further He is not present in the observed spectra.…”

A hybrid type Ia supernova with an early flash triggered by helium-shell detonation

“…For example, in the W7 model, LTE and NLTE simulations yield indistinguishable light curves (except for the U-band) until ∼ 25 days after the explosion, corresponding to ∼ 5 days after the B-maximum 45,46 . In addition, we do not include the non-thermal excitation of He; generally He absorption lines in optical wavelength are invisible for the DDet models even with this effect 24 .…”

“…Vast numbers of absorption lines of these elements are very effective in blocking the flux in the blue part of the optical spectrum, thus leading to a relatively red B − V color evolution in general. Indeed, although a substantial amount of He is left after the detonation, the expected spectrum would not show a trace of He in the optical wavelength 24 . By assuming a progenitor star with a WD mass of 1.03 M and a He-shell mass as low as ∼ 0.054 M (as required to trigger the He detonation on the surface of a 1.03 M WD 12,23 ), the prominent early flash, peculiar early color evolution and Ti II trough feature are reproduced simultaneously (Figures 2-4).…”

Carnegie Supernova Project-II: The Near-infrared Spectroscopy Program

“…In the sub-Chandrasekhar helium detonation scenario, unburned helium left over from the surface helium layer can potentially produce strong NIR He I λ1.0830 and λ2.0581 µm lines, as we have shown for other supernova types in Figure 4. Boyle et al (2017) explored these features, and the possibility of confusion with the C I λ1.0693 µm line. This emphasizes the importance of identifying multiple lines of the same ion when searching for unburned carbon, as was done for SN 1999by (Höflich et al 2002) and iPTF13ebh (Hsiao et al 2015).…”