No plateau observed in late-time near-infrared observations of the underluminous Type Ia supernova 2021qvv

TL;DR

This study finds no late-time near-infrared plateau in the underluminous SN 2021qvv, challenging previous observations of normal Type Ia supernovae and suggesting differences in ejecta ionization or temperature.

Contribution

It provides the first late-time NIR observations of a 1991bg-like SN, showing the absence of a plateau and proposing explanations related to ejecta properties.

Findings

No NIR plateau observed in SN 2021qvv out to 250 days.

Absence of plateau may be due to higher ionization or lower temperature of ejecta.

Supports the idea of a continuum between 1991bg-like and normal SNe Ia.

Abstract

Near-infrared (NIR) observations of normal Type Ia supernovae (SNe Ia) obtained between 150 to 500 d past maximum light reveal the existence of an extended plateau. Here, we present observations of the underluminous, 1991bg-like SN 2021qvv. Early, ground-based optical and NIR observations show that SN 2021qvv is similar to SN 2006mr, making it one of the dimmest, fastest-evolving 1991bg-like SNe to date. Late-time (170-250 d) Hubble Space Telescope observations of SN 2021qvv reveal no sign of a plateau. An extrapolation of these observations backwards to earlier-phase NIR observations of SN 2006mr suggests the complete absence of a NIR plateau, at least out to 250 d. This absence may be due to a higher ionization state of the ejecta, as predicted by certain sub-Chandrasekhar-mass detonation models, or to the lower temperatures of the ejecta of 1991bg-like SNe, relative to normal SNe Ia, which might preclude their becoming fluorescent and shifting ultraviolet light into the NIR. This suggestion can be tested by acquiring NIR imaging of a sample of 1991bg-like SNe that covers the entire range from slowly-evolving to fast-evolving events ($0.2 \lesssim s_\mathrm{BV} \lesssim 0.6$). A detection of the NIR plateau in slower-evolving, hotter 1991bg-like SNe would provide further evidence that these SNe exist along a continuum with normal SNe Ia. Theoretical progenitor and explosion scenarios would then have to match the observed properties of both SN Ia subtypes.