Astronomers using the Magellan telescopes at Las Campanas Observatory, Chile, have detected the faint signal emitted by hydrogen gas in the remnant of the Type Ia supernova ASASSN-18tb. The researchers believe the detection may provide new insights into elusive progenitors of these violent explosions.

Type Ia supernovae play a crucial role in helping astronomers understand the Universe. Their brilliance allows them to be seen across great distances and to be used as cosmic mile-markers. These explosions synthesize many of the elements that make up the world around us, which are ejected into the galaxy to generate future stars and stellar systems.

Although hydrogen is the most-abundant element in the Universe, it is almost never seen in Type Ia supernova explosions. In fact, the lack of hydrogen is one of the defining features of this category of supernovae and is thought to be a key clue to understanding what came before their explosions.

Type Ia supernovae originate from the thermonuclear explosion of a white dwarf that is part of a binary system. But what exactly triggers the explosion of the white dwarf is a great puzzle.

A prevailing idea is that, the white dwarf gains matter from its companion star, a process that may eventually trigger the explosion, but whether this is the correct theory has been hotly debated for decades.

Astronomers eagerly study the chemical signatures of the material ejected during these explosions in order to understand the mechanism and players involved in creating Type Ia supernovae.

In recent years, they have discovered a small number of rare Type Ia supernovae that are cloaked in large amount of hydrogen — maybe as much as the mass of our Sun.

But in several respects, ASASSN-18tb — also known as SN 2018fhw — is different from these previous events.

“It’s possible that the hydrogen we see when studying ASASSN-18tb is like these previous supernovae, but there are some striking differences that aren’t so easy to explain,” said Dr. Juna Kollmeier, an astronomer with the Observatories of the Carnegie Institution for Science.

First, in all previous cases these hydrogen-cloaked Type Ia supernovae were found in young, star-forming galaxies where plenty of hydrogen-rich gas may be present. But ASASSN-18tb occurred in an early-type galaxy dominated by old stellar populations.

Second, the amount of hydrogen ejected by ASASSN-18tb is significantly less than that seen surrounding those other Type Ia supernovae. It probably amounts to about one-hundredth the mass of our Sun.

“One exciting possibility is that we are seeing material being stripped from the exploding white dwarf’s companion star as the supernova collides with it. If this is the case, it would be the first-ever observation of such an occurrence,” said Dr. Anthony Piro, from the Observatories of the Carnegie Institution for Science.

“I have been looking for this signature for a decade! We finally found it, but it’s so rare, which is an important piece of the puzzle for solving the mystery of how Type Ia supernovae originate,” said Dr. Josh Simon, also from the Observatories of the Carnegie Institution for Science.

The team’s work was published in the Monthly Notices of the Royal Astronomical Society.

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Juna A. Kollmeier et al. 2019. H α emission in the nebular spectrum of the Type Ia supernova ASASSN-18tb. MNRAS 486 (3): 3041-3046; doi: 10.1093/mnras/stz953