A match flares from both ends. First the head ignites, and a moment later the flame engulfs the wooden stick. Something similar may happen in some Type Ia supernovae — except the match is a white dwarf (a star the size of Earth), and the flame is a thermonuclear explosion.
In 2022, astronomers observed supernova 2022an — a dim, rapidly fading flash similar to the 91bg subtype events. But in its spectrum, in the near-infrared, they spotted an unusual detail: a narrow absorption line at a wavelength of about 1.037 microns. It persisted from day 30 to day 90 after maximum brightness. This line is the "fingerprint" of helium atoms flying toward us at 13,000 kilometers per second. For comparison: at that speed, you could travel from Earth to the Moon in half a minute.
Where did this gas come from? In such systems, helium is usually "stripped" from the surface of a companion star. But then its velocity would be much lower, like a breeze, not a hurricane. Here, the speeds are enormous, and the helium line is narrow, as if the material were ejected in a single dense clump. So the helium is part of the explosion itself — the unburned remnants of the white dwarf's outer shell. This is the first direct evidence that 91bg supernovae can retain helium layers, as predicted by double detonation models: first, a thin helium shroud on the star explodes, and the shock wave ignites the carbon core.
The discovery exposes a gap in theory: current calculations of helium double detonations do not model spectra at such late times as 2022an. Observations have outpaced theory. But it is infrared spectroscopy, capable of peering through dust and gas, that has become the tool to "read" the composition of the ejected material. It seems that to understand how stars die, we need to look at them in light invisible to the eye.
as written by the authors arXiv:2605.07827
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 $$1.037\,\mu$$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 $$1.083\,\mu$$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.
The helium line astronomers saw is only 75 angstroms wide — about 100 times thinner than the diameter of a human hair, if we could stretch it across the spectrum.