Detection of persistent helium absorption in the 91bg-like type Ia Supernova 2022an

Abstract

We present optical and near-infrared observations of the fast-declining Type Ia supernova (SN Ia) 2022an. The photometric and spectroscopic properties identify it as a standard 91bg-like event; however, our data reveal a relatively narrow absorption feature with a full width at half maximum (FWHM) of 75 angstroms near $1.037\,μ$m in the rest frame of the observed spectra that persists from around 30 days to nearly 90 days after maximum light. We attribute this feature to He I $1.083\,μ$m line with a blueshifted velocity of $1.3\times10^{4}$ km s$^{-1}$ and a FWHM of $2.1\times10^{3}$ km s$^{-1}$, supported by the detection of multiple optical He I transitions in earlier epochs at a higher velocity around $1.5\times10^{4}$ km s$^{-1}$. The high velocity of the helium could not be explained by helium external to the progenitor at the explosion, such as the stripped surface helium from a companion star. The properties of the helium absorption in SN 2022an spectra instead point to unburnt material in the outer ejecta, thus providing the most compelling evidence to date for helium-bearing ejecta in a 91bg-like SN Ia. Such helium has been predicted for sub-Chandrasekhar-mass double-detonation explosions involving a surface helium shell. No theoretical calculations of modern helium-shell double detonation have been performed at epochs similar to those observed for SN 2022an to study the effect of helium on their spectra, revealing a gap between observations and theoretical calculations in understanding the manifestation of helium in SNe Ia. Nevertheless, the discovery of persistent helium absorption in SN 2022an demonstrates the diagnostic power of NIR spectroscopy for understanding thermonuclear supernova explosions by probing the abundance and structure of their ejecta.