The detection of a supernova with an unusual chemical signature may hold the key to solving the longstanding mystery that is the source of these violent explosions. Observations taken by the Magellan telescopes at Carnegie’s Las Campanas Observatory in Chile led to the detection of hydrogen emissions that makes this supernova — ASASSN-18tb — so unique.

The finding was made by a team of astronomers led by Carnegie’s Juna Kollmeier — and including Carnegie’s Nidia Morrell, Anthony Piro, Mark Phillips, and Josh Simon — their work is published in Monthly Notices of the Royal Astronomical Society.

Type Ia supernovae — a very helpful explosion

Type Ia supernovae play a crucial role in helping astronomers measure distances across the Universe — their brilliance enabling astronomers to spot from great distances and use them as cosmic mile-markers.

In addition to this, these violent explosions synthesize many of the elements and eject them into the galaxy to generate future stars and stellar systems — not to mention much of the everyday matter around us.

Despite this, the universe’s most abundant element — hydrogen — is almost never seen in Type Ia supernova explosions. This 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.

Thus the discovery of hydrogen emissions from this supernova was extremely surprising.

This cartoon courtesy of Anthony Piro illustrates three possibilities for the origin of the mysterious hydrogen emissions from the Type IA supernova called ASASSN-18tb that were observed by the Carnegie astronomers. Starting from the top and going clockwise: The collision of the explosion with a hydrogen-rich companion star, the explosion triggered by two colliding white dwarf stars subsequently colliding with a third hydrogen-rich star, or the explosion interacting with circumstellar hydrogen material ( Courtesy of Anthony Piro, Carnegie Institution for Science)

Type Ia supernovae originate from the thermonuclear explosion of a white dwarf that is part of a binary system — what exactly triggers the explosion of the white dwarf — the dead core left after a Sun-like star exhausts its nuclear fuel — is a great puzzle. A prominent theory with a great deal of support in the scientific community suggests that the white dwarf gains matter from its companion star via accretion or from stellar winds. This process could eventually trigger a massive explosion causing the white dwarf’s outer shell to be blown away. Whether or not this is the correct theory explaining these supernovae has been hotly debated for decades.

This led the research team behind this paper to begin a major survey of Type Ia supernovae — called 100IAS — spark by a discussion of the origin of these supernovae between Juna Kollmeier and study co-authors Subo Dong of Peking University and Doron Kushnir of the Weizmann Institute of Science. Along with Weizmann colleague Boaz Katz, the team put forward a new theory for Type Ia explosions that involve the violent collisions between two white dwarfs.

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, astronomers have discovered a small number of rare Type Ia supernovae that are cloaked in large clouds of hydrogen — maybe even possessing the mass of our Sun. In several respects, ASASSN-18tb is different from these previous events.

Kollmeier says: “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.”

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. ASASSN-18tb, however, occurred in a galaxy consisting of old stars.

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

Anthony Piro says: “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.”

Co-author Josh Simon adds: “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.”

Could this really be hydrogen?

Nidia Morrell — observing that night — immediately reduced the data coming off the telescope, circulating it to the team including PhD student Ping Chen, who works on 100IAS for his thesis and Jose Luis Prieto of Universidad Diego Portales, a veteran SNe observer.

Chen was the first to notice that this was not a typical spectrum, she and the rest of the team were astounded. Morrell recalls: “I was shocked, and I thought to myself ‘could this really be hydrogen?’”

Morrell met with team member Mark Phillips — a pioneer in establishing the relationship — informally named after him — that allows Type Ia supernovae to be used as standard rulers to discuss the observation. Phillips was convinced: “It is hydrogen you’ve found; no other possible explanation.”

Kollmeier adds: “This is an unconventional supernova program, but I am an unconventional observer — a theorist, in fact.

“It’s an extremely painful project for our team to carry out. Observing these things is like catching a knife, because by definition they get fainter and fainter with time! It’s only possible at a place like Carnegie where access to the Magellan telescopes allow us to do time-intensive and sometimes arduous, but extremely important cosmic experiments.

“No pain, no gain,” Kollmeier concludes.

Original research: https://academic.oup.com/mnras/article-abstract/486/3/3041/5426830?redirectedFrom=fulltext