The Absolute Magnitudes of 1991T-like Supernovae

TL;DR

This paper identifies 1991T-like supernovae using spectral and light curve features and confirms they are over-luminous compared to normal Type Ia supernovae across optical and near-infrared wavelengths.

Contribution

It introduces a method combining spectral and light curve data to classify 1991T-like supernovae and quantifies their over-luminosity relative to normal Type Ia supernovae.

Findings

1991T-like supernovae are over-luminous by 0.1-0.5 mag.

Spectral and light curve features effectively identify 1991T-like supernovae.

Over-luminosity persists across optical and near-infrared wavelengths.

Abstract

1991T-like supernovae are the luminous, slow-declining extreme of the Branch shallow-silicon (SS) subclass of Type Ia supernovae. They are distinguished by extremely weak Ca II H & K and Si II $\lambda6355$ and strong Fe III absorption features in their optical spectra at pre-maximum phases, and have long been suspected to be over-luminous compared to normal Type Ia supernovae. In this paper, the pseudo equivalent width of the Si II $\lambda$6355 absorption obtained at light curve phases from $\leq+10$ days is combined with the morphology of the $i$-band light curve to identify a sample of 1991T-like supernovae in the Carnegie Supernova Project-II. Hubble diagram residuals show that, at optical as well as near-infrared wavelengths, these events are over-luminous by $\sim$0.1-0.5 mag with respect to the less extreme Branch SS (1999aa-like) and Branch core-normal supernovae with similar $B$-band light curve decline rates.