Using 1991T/1999aa-like Type Ia Supernovae as Standardizable Candles

TL;DR

This study analyzes 91T/99aa-like Type Ia Supernovae, demonstrating their potential as standardizable candles with small residuals, and explores the relationship between Si II line strength and luminosity residuals.

Contribution

It provides the first comprehensive analysis of 91T/99aa-like SNe Ia's standardizability and introduces a correlation between Si II line strength and Hubble residuals.

Findings

91T/99aa-like SNe Ia are 0.2 mag brighter than normal SNe Ia after corrections.

The Hubble residuals for 91T/99aa-like SNe Ia are around 0.25 mag, indicating good relative distance measurement.

A broken linear correlation exists between Si II pEW and Hubble residuals, affecting distance estimates.

Abstract

We present the photometry of 16 91T/99aa-like Type Ia Supernovae (SNe Ia) observed by the Las Cumbres Observatory. We also use an additional set of 21 91T/99aa-like SNe Ia and 87 normal SNe Ia from the literature for an analysis of the standardizability of the luminosity of 91T/99aa-like SNe. We find that 91T/99aa-like SNe are 0.2 mag brighter than normal SNe Ia, even when fully corrected by the light curve shapes and colors. The weighted root-mean-square of 91T/99aa-like SNe (with $z_{CMB}>0.01$) Hubble residuals is $0.25\pm0.03$ mag, suggesting that 91T/99aa-like SNe are also excellent relative distance indicators to $\pm$12%. We compare the Hubble residuals with the pseudo-equivalent width (pEW) of Si II $\lambda\lambda$6355 around the date of maximum brightness. We find that there is a broken linear correlation in between those two measurements for our sample including both 91T/99aa-like and normal SNe Ia. As the $pEW_{max}$(Si II $\lambda\lambda$6355) increasing, the Hubble residual increases when $pEW_{max}$(Si II $\lambda\lambda$6355)$<55.6$ \r{A}. However, the Hubble residual stays constant beyond this. Given that 91T/99aa-like SNe possess shallower Si II lines than normal SNe Ia, the linear correlation at $pEW_{max}$(Si II $\lambda\lambda$6355)$<55.6$ \r{A} can account for the overall discrepancy of Hubble residuals derived from the two subgroups. Such a systematic effect needs to be taken into account when using SNe Ia to measure luminosity distances.