Can Helium-detonation Model Explain the Observed Diversity of Type Ia Supernovae?

TL;DR

This study investigates whether the helium-detonation model can explain the observed diversity in Type Ia supernovae by analyzing early spectral features, velocities, and host galaxy properties.

Contribution

The paper provides observational evidence supporting the helium-detonation model as a potential explanation for certain features of Type Ia supernovae, highlighting similarities in progenitor environments.

Findings

HV and NV supernovae show distinct early spectral features.

HV objects have higher velocities of high-velocity features than NV objects.

Some observed features deviate from helium-detonation model predictions.

Abstract

We study a sample of 16 Type Ia supernovae (SNe Ia) having both spectroscopic and photometric observations within 2 $-$ 3 days after the first light. The early $B-V$ colors of such a sample tends to show a continuous distribution. For objects with normal ejecta velocity (NV), the C~II $\lambda$6580 feature is always visible in the early spectra while it is absent or very weak in the high-velocity (HV) counterpart. Moreover, the velocities of the detached high-velocity features (HVFs) of Ca~II NIR triplet (CaIR3) above the photosphere are found to be much higher in HV objects than in NV objects, with typical values exceeding 30,000 km~s$^{-1}$ at 2 $-$ 3 days. We further analyze the relation between %velocities of Si~II~$\lambda$6355 at maximum, $v_{\rm Si,max}$, the velocity shift of late-time [Fe~II] lines ($v_{\rm [Fe~II]}$) and host galaxy mass. We find that all HV objects have redshifted $v_{\rm [Fe~II]}$ while NV objects have both blue- and redshifted $v_{\rm [Fe~II]}$. It is interesting to point out that the objects with redshifted $v_{\rm [Fe~II]}$ are all located in massive galaxies, implying that HV and a portion of NV objects may have similar progenitor metallicities and explosion mechanisms. We propose that, with a geometric/projected effect, the He-detonation model may account for the similarity in birthplace environment and the differences seen in some SNe Ia, including $B-V$ colors, C~II feature, CaIR3 HVFs at early time and $v_{\rm [Fe~II]}$ in the nebular phase. Nevertheless, some features predicted by He-detonation simulation, such as the rapidly decreasing light curve, deviate from the observations, and some NV objects with blueshifted nebular $v_{\rm [Fe~II]}$ may involve other explosion mechanisms.