Underluminous 1991bg-like Type Ia supernovae are standardizable candles

TL;DR

This study demonstrates that underluminous 1991bg-like Type Ia supernovae can be standardized as candles using the colour stretch parameter $s_ ext{BV}$, enabling their use in cosmological distance measurements.

Contribution

The paper introduces the use of the $s_ ext{BV}$ parameter to standardize 1991bg-like SNe Ia, resolving previous issues with the width-luminosity relation for these supernovae.

Findings

Significant correlation between peak absolute magnitudes and $s_ ext{BV}$ in 1991bg-like SNe Ia.

Broad consistency of the relation with preliminary data from the Carnegie Supernova Project.

Potential to use these supernovae for an independent measurement of the Hubble constant.

Abstract

It is widely accepted that the width-luminosity relation used to standardize normal Type Ia supernovae (SNe Ia) breaks down in underluminous, 1991bg-like SNe Ia. This breakdown may be due to the choice of parameter used as a stand-in for the width of the SN Ia light curve. Using the colour stretch parameter $s_\mathrm{BV}$ instead of older parameters resolves this issue. Here, I assemble a sample of 14 nearby 1991bg-like SNe Ia from the literature, all of which have independent host-galaxy distance moduli and little to no reddening. I use Gaussian process regression to fit the light curves of these SNe in $U/u$, $B$, $V$, $g$, $R/r$, $I/i$, and $H$, and measure their peak absolute magnitudes. I find statistically significant ($>5\sigma$ confidence level in the optical and $>4\sigma$ in the near-infrared) correlations between the peak absolute magnitudes of the 1991bg-like SNe Ia and their $s_\mathrm{BV}$ values in the range $0.2<s_\mathrm{BV}<0.6$. These correlations are broadly consistent with fits to $s_\mathrm{BV}<0.7$ SNe Ia with preliminary $B$- and $V$-band peak absolute magnitudes from the Carnegie Supernova Project and significantly inconsistent with similar fits to normal and transitional SNe Ia (with $0.7<s_\mathrm{BV}<1.1$). The underluminous width-luminosity relation shown here needs to be properly calibrated with a homogeneous sample of 1991bg-like SNe Ia, after which it could be used as a rung on a new cosmological distance ladder. With surface-brightness fluctuations (or another non-Cepheid method) used to calibrate distances to nearby 1991bg-like SNe Ia, such a ladder could produce an independent measurement of the Hubble-Lema\^{i}tre Constant, $H_\mathrm{0}$.