Variable H$\hat{I}\pm$ Emission in the Nebular Spectra of the Low-Luminosity Type Ia SN2018cqj/ATLAS18qtd

Abstract

We present optical photometry and spectroscopy of the Type Ia supernova SN2018cqj/ATLAS18qtd. The supernova exploded in an isolated region at $\sim 65$~kpc from the S0 galaxy IC~550 at $z=0.0165$ ($D\approx 74$~Mpc) and has a redshift consistent with a physical association to this galaxy. Multicolor photometry show that SN2018cqj/ATLAS18qtd is a low-luminosity ($M_{B_{max}}\approx -17.9$ mag), fast-declining Type Ia with color stretch $s_{BV}\approx 0.6$ and $B$-band decline rate $\hat{I}”m_{15}(B)\approx 1.77$ mag. Two nebular-phase spectra obtained as part of the 100IAS survey at +193 and +307 days after peak show the clear detection of a narrow H$\hat{I}\pm$ line in emission that is resolved in the first spectrum with $\mathrm{FWHM}\approx 1200$ km s${}^{-1}$ and $L_{H\hat{I}\pm }\approx 3.8\times 10^{37}$ erg s${}^{-1}$. The detection of a resolved H$\hat{I}\pm$ line with a declining luminosity is broadly consistent with recent models where hydrogen is stripped from the non-degenerate companion in a single-degenerate progenitor system. However, the amount of hydrogen consistent with the luminosities of the H$\hat{I}\pm$ line would be $\sim 10^{-3}$ M${}_{\odot }$, significantly less than theoretical model predictions in the classical single-degenerate progenitor systems. SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining Type Ia SN after SN2018fhw/ASASSN-18tb that shows narrow H$\hat{I}\pm$ in emission in its nebular-phase spectra.

Accepted for publication in ApJ