A Second Case of Variable Na I D Lines in a Highly-Reddened Type Ia Supernova (with Erratum)

TL;DR

This study investigates the occurrence of variable Na I D absorption lines in Type Ia supernovae, finding that such variability is rare and may be linked to dusty environments, with only SN 2006X confirmed to show significant variability after correction.

Contribution

The paper provides a large-scale analysis of Na I D line variability in SNe Ia, correcting previous claims and establishing that such variability is uncommon, with implications for understanding supernova environments.

Findings

SN 2006X shows significant Na I D variability.

SN 1999cl's variability was due to measurement error.

Variable Na I D lines are present in about 3% of SNe Ia.

Abstract

(Original) Recent high-resolution spectra of the Type Ia SN 2006X have revealed the presence of time-variable and blueshifted Na I D features, interpreted by Patat et al. as originating in circumstellar material within the progenitor system. The variation seen in SN 2006X induces relatively large changes in the total Na I D equivalent width ($\Delta\rm{EW}\approx 0.5 \unicode{x212B}$ in just over two weeks), that would be detectable at lower resolutions. We have used a large data set comprising 2400 low-resolution spectra of 450 Type Ia supernovae (SNe Ia) obtained by the CfA Supernova Program to search for variable Na I D features. Out of the 31 SNe Ia (including SN 2006X) in which we could have detected similar EW variations, only one other (SN 1999cl) shows variable Na I D features, with an even larger change over a similar ~10-day timescale ($\Delta\rm{EW} = 1.66 \pm 0.21 \unicode{x212B}$). Interestingly, both SN 1999cl and SN 2006X are the two most highly-reddened objects in our sample, raising the possibility that the variability is connected to dusty environments. (Erratum) The large variation in the Na I D equivalent width observed in SN 1999cl results in fact from a measurement error. Our new measurements show that the EW variation is significantly lower, at $0.43 \pm 0.14 \unicode{x212B}$. While the EW variation remains statistically significant (3.1$\sigma$ different from zero), it is now below the detection threshold of 0.5 $\unicode{x212B}$ derived from the Monte Carlo simulations published in the original paper. As a result, SN 1999cl should no longer be considered as an object displaying variable Na I D lines in our study. The fraction of SNe Ia in our sample displaying Na I D lines thus goes from $\sim$6% (2/31) in the original study to $\sim$3% (1/31) in the revised analysis, SN 2006X being the only SN Ia in our sample with variable Na I D lines.