ZTF SN Ia DR2: High-velocity components in the Si II $\lambda$6355

TL;DR

This study analyzes high-velocity features in the Si II $ extlambda$6355 line in Type Ia supernovae spectra from the ZTF DR2, revealing their prevalence, evolution with phase, and lack of correlation with host or light-curve properties.

Contribution

It introduces a robust MCMC and BIC-based method to classify and analyze high-velocity features in SN Ia spectra, providing new insights into their behavior and ubiquity.

Findings

Approximately 75% of spectra before -11 days show HVFs.

HVFs decrease in strength with phase, fading earlier at larger velocities.

No correlation between HVFs and SN Ia host or light-curve parameters.

Abstract

The ZTF SN Ia Data Release 2 provides a perfect opportunity to perform a thorough search for, and subsequent analysis of, high-velocity components in the Si II $\lambda$6355 feature in the pre-peak regime. The source of such features remains unclear, with potential origins in circumstellar material or density/abundance enhancements intrinsic to the SN ejecta. Therefore, they may provide clues to the elusive progenitor and explosion scenarios of SNe Ia. We employ a MCMC fitting method followed by BIC testing to classify single and double Si II $\lambda$6355 components in the DR2. The detection efficiency of our classification method is investigated through the fitting of simulated features, allowing us to place cuts upon spectral quality required for reliable classification. These simulations were also used to perform an analysis of the recovered parameter uncertainties and potential biases in the measurements. Within the 329 spectra sample that we investigate, we identify 85 spectra exhibiting Si II $\lambda$6355 HVFs. We find that HVFs decrease in strength with phase relative to their photospheric counterparts - however, this decrease can occur at different phases for different objects. HVFs with larger velocity separations from the photosphere are seen to fade earlier leaving only the double components with smaller separations as we move towards maximum light. Our findings suggest that around three quarters of SN Ia spectra before -11 d show high-velocity components in the Si II $\lambda$6355 with this dropping to around one third in the six days before maximum light. We observe no difference between the populations of SNe Ia that do and do not form Si II $\lambda$6355 HVFs in terms of SALT2 light-curve parameter x1, peak magnitude, decline rate, host mass, or host colour, supporting the idea that these features are ubiquitous across the SN Ia population.