Distribution of Si II $λ$6355 Velocities of Type Ia Supernovae and Implications for Asymmetric Explosions

Abstract

The ejecta velocity is a very important parameter in studying the structure and properties of Type Ia supernovae (SNe Ia). It is also a candidate key parameter in improving the utility of SNe Ia for cosmological distance determinations. Here we study the velocity distribution of a sample of 311 SNe Ia from the kaepora database. The velocities are derived from the Si II 𝜆6355 absorption line in optical spectra measured at (or extrapolated to) the time of peak brightness. We statistically show that the observed velocity has a bimodal Gaussian distribution consisting of two groups of SNe Ia: Group I with a lower but narrower scatter (𝜇 1 = 11,000 km s −1 , 𝜎 1 = 700 km s −1 ), and Group II with a higher but broader scatter (𝜇 2 = 12,300 km s −1 , 𝜎 2 = 1800 km s −1 ). The population ratio of Group I to Group II is 201:110 (65%:35%). There is substantial degeneracy between the two groups, but for SNe Ia with velocity 𝑣 > 12,000 km s −1 , the distribution is dominated by Group II. The true origin of the two components is unknown, though there could be that naturally there exist two intrinsic velocity distributions as observed. However, we try to use asymmetric geometric models through statistical simulations to reproduce the observed distribution assuming all SNe Ia share the same intrinsic distribution. In the two cases we consider, 35% of SNe Ia are considered to be asymmetric in Case 1, and all SNe Ia are asymmetric in Case 2. Simulations for both cases can reproduce the observed velocity distribution but require a significantly large portion (> 35%) of SNe Ia to be asymmetric. In addition, the Case 1 result is consistent with recent polarization observations that SNe Ia with higher Si II 𝜆6355 velocity tend to be more polarized.

Multi-Dimensional Parameter Study of Double Detonation Type Ia Supernovae Originating from Thin-Helium-Shell White Dwarfs

“…The asymmetric ejecta in this work closely resembles that of the single run in Townsley et al (2019). Thus, we expect the Si ii line velocities from most of our models to resemble the line of sight-dependent model used by Zhang et al (2020) to represent those results. When comparing the Si ii λ 6355 velocities from Townsley et al (2019) to the inner Si peak ejection velocities, they are consistent within a couple thousands of km s −1 across viewing angles.…”

“…If SNe Ia in nature produce asymmetric ejecta as in our models, line of sight to an observed event would almost certainly have an effect on the observables as seen in the singular case of Townsley et al (2019). Zhang et al (2020) examined Si ii absorption velocities among observed SNe Ia and showed that the distribution is bimodal. This may be due to separate populations (symmetric and asymmetric) in SNe Ia and/or the nature of asymmetric ejecta itself.…”

“…This may be due to separate populations (symmetric and asymmetric) in SNe Ia and/or the nature of asymmetric ejecta itself. Zhang et al (2020) conducted statistical simulations using line of sight-dependent Si ii velocities parameterized by the velocity range and critical polar angle beyond which the velocity remains relatively constant. This parametrization is able to fit the line velocity results from our previous work in Townsley et al (2019) (see Figure 4 in Zhang et al 2020).…”

“…Zhang et al (2020) conducted statistical simulations using line of sight-dependent Si ii velocities parameterized by the velocity range and critical polar angle beyond which the velocity remains relatively constant. This parametrization is able to fit the line velocity results from our previous work in Townsley et al (2019) (see Figure 4 in Zhang et al 2020). It was found that the observed distribution cannot be explained with only the distribution implied by the 1.0 M progenitor and the assumption of a random line-of-sight.…”