Exploring the spectral diversity of low-redshift Type Ia supernovae using the Palomar Transient Factory

TL;DR

This study analyzes optical spectra of 264 low-redshift Type Ia supernovae to understand spectral diversity, focusing on velocity and equivalent width measurements, revealing the prevalence of high-velocity components and unburnt material in early spectra.

Contribution

It provides a comprehensive analysis of spectral features in a large sample of SNe Ia, highlighting the role of high-velocity components and early unburnt material detection, which were less characterized before.

Findings

High-velocity Ca II NIR triplet component present in ~95% before -5 days

Broader light curves correlate with stronger high-velocity components

>40% of early spectra show signs of unburnt carbon in SNe Ia

Abstract

We present an investigation of the optical spectra of 264 low-redshift (z < 0.2) Type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory, an untargeted transient survey. We focus on velocity and pseudo-equivalent width measurements of the Si II 4130, 5972, and 6355 A lines, as well those of the Ca II near-infrared (NIR) triplet, up to +5 days relative to the SN B-band maximum light. We find that a high-velocity component of the Ca II NIR triplet is needed to explain the spectrum in ~95 per cent of SNe Ia observed before -5 days, decreasing to ~80 per cent at maximum. The average velocity of the Ca II high-velocity component is ~8500 km/s higher than the photospheric component. We confirm previous results that SNe Ia around maximum light with a larger contribution from the high-velocity component relative to the photospheric component in their Ca II NIR feature have, on average, broader light curves and lower Ca II NIR photospheric velocities. We find that these relations are driven by both a stronger high-velocity component and a weaker contribution from the photospheric Ca II NIR component in broader light curve SNe Ia. We identify the presence of C II in very-early-time SN Ia spectra (before -10 days), finding that >40 per cent of SNe Ia observed at these phases show signs of unburnt material in their spectra, and that C II features are more likely to be found in SNe Ia having narrower light curves.