Berkeley Supernova Ia Program V: Late-Time Spectra of Type Ia Supernovae

TL;DR

This study analyzes late-time optical spectra of 34 low-redshift Type Ia supernovae, measuring nebular emission features to explore their velocity properties and correlations with early-time spectral characteristics.

Contribution

It provides the largest dataset of late-time SN Ia spectra, including new measurements of nebular line velocities and their correlations with early spectral features.

Findings

Nebular line velocities are mostly constant over time.

A correlation exists between nebular velocities and early-time ejecta velocity.

No correlation between nebular velocity and light-curve decline rate.

Abstract

In this work we analyse late-time (t > 100 d) optical spectra of low-redshift (z < 0.1) Type Ia supernovae (SNe Ia) which come mostly from the Berkeley Supernova Ia Program dataset. We also present spectra of SN 2011by for the first time. The BSNIP sample studied consists of 34 SNe Ia with 60 nebular spectra, to which we add nebular spectral feature measurements of 20 SNe Ia from previously published work (Maeda et al. 2011; Blondin et al. 2012), representing the largest set of late-time SN Ia spectra ever analysed. The full width at half-maximum intensity (FWHM) and velocities of the [Fe III] {\lambda}4701, [Fe II] {\lambda}7155, and [Ni II] {\lambda}7378 emission features are measured in most observations of spectroscopically normal objects where the data have signal-to-noise ratios >20 px^-1 and are older than 160 d past maximum brightness. The velocities of all three features are seen to be relatively constant with time, increasing only a few to ~20 km/s/d. The nebular velocity (v_neb, calculated by taking the average of the [Fe II] {\lambda}7155 and [Ni II] {\lambda}7378 velocities) is correlated with the near-maximum-brightness velocity gradient and early-time ejecta velocity. Nearly all high velocity gradient objects have redshifted nebular lines while most low velocity gradient objects have blueshifted nebular lines. No correlation is found between v_neb and {\Delta}m_15(B), and for a given light-curve shape there is a large range of observed nebular velocities. The data also indicate a correlation between observed (B-V)_max and v_neb.