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Abstract:

We present optical and near-infrared (NIR) 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 Å near 1.037 μm 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 the He i 1.083 μm line with a blueshifted velocity of 1.3 × 104 km s−1 and a FWHM of 2.1 × 103 km s−1, supported by the detection of multiple optical He i transitions at earlier epochs with a higher velocity of around 1.5 × 104 km s−1. The high velocity of the helium cannot be explained by helium external to the progenitor at the time of explosion, such as stripped surface helium from a companion star. The properties of the helium absorption in the spectra of SN 2022an instead point to unburned 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 in sub-Chandrasekhar-mass double-detonation explosions involving a surface helium shell. No theoretical calculations of modern helium-shell double detonations have been performed at epochs similar to those observed for SN 2022an to study the effects of helium on their spectra, revealing a gap between observations and theoretical calculations in our understanding of how helium manifests 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.