Temperamental supernova appeared strangely cool before exploding

Supernova SN 2019yvr, a rare type Ib supernova, was detected by the Young Supernova Experiment. Its progenitor star in the spiral galaxy NGC 4666 was identified in earlier images from the Hubble Space Telescope. The top left inset shows a Gemini Telescope image of the supernova 67 days after it was discovered, and the lower insets show the progenitor star in the Hubble images. (Kilpatrick et al., MNRAS 2021)

A curiously yellow star has caused astrophysicists to reevaluate the possible pathways that can lead to the explosion of a massive star as a supernova.

An international team including UC Santa Cruz astrophysicists used observations from NASA’s Hubble Space Telescope taken two-and-a-half years before the star exploded to identify and study the massive star that became supernova SN2019yvr. The team presented their findings in a paper published May 5 in the Monthly Notices of the Royal Astronomical Society.

The supernova was notable for the absence of a hydrogen signal in its spectrum, making it a rare type Ib supernova, explained coauthor Ryan Foley, assistant professor of astronomy and astrophysics at UCSC.

“This class has always been intriguing, because the progenitor star has to somehow get rid of its hydrogen,” Foley said. “This particular star, however, is a little weird for the type of supernova it produced.”

Located 35 million light years from Earth in the Virgo galaxy cluster, the star appeared cool and yellow, with a much larger radius than expected. The typical progenitor for a type Ib supernova would be a hot, blue, compact stellar core stripped of its hydrogen envelope.

“We haven’t seen this scenario before,” said first author Charles Kilpatrick, who led the study as a UCSC postdoctoral researcher and is now at Northwestern University. “If a star explodes without hydrogen, it should be extremely blue—really, really hot. It’s almost impossible for a star to be this cool without having hydrogen in its outer layer. We looked at every single stellar model that could explain a star like this, and every single model requires that the star had hydrogen, which, from its supernova, we know it did not. It stretches what’s physically possible.”

In the years preceding its death, the star might have shed its hydrogen layer or lost it to a nearby companion star, Foley said.

“We have different theoretical pathways that can lead to this type of supernova, but in this case nature seems to have been more inventive than we are,” he said. “We’re still trying to put together a detailed model that can explain all of the observations.”

SN2019yvr was detected by the Young Supernova Experiment, which uses the Pan-STARSS telescope at Haleakalā, Hawaii, to catch supernovae right after they explode. Foley, who helps lead the survey, said this is one of its first science results. The team was able to precisely determine the position of the supernova in the relatively nearby spiral galaxy NGC 4666, then used the earlier Hubble images to identify which star had exploded.

“When it exploded, it seemed like a very normal hydrogen-free supernova,” Kilpatrick said. “There was nothing outstanding about this. But the progenitor star didn’t match what we know about this type of supernova.”

Several months after the explosion, however, the researchers discovered a clue. As ejecta from the star’s final explosion traveled through its environment, it collided with a large mass of hydrogen. This led the team to hypothesize that the progenitor star might have expelled the hydrogen within a few years before its death.

“Astronomers have suspected that stars undergo violent eruptions or death throes in the years before we see supernovae,” Kilpatrick said. “This star’s discovery provides some of the most direct evidence ever found that stars experience catastrophic eruptions, which cause them to lose mass before an explosion. If the star was having these eruptions, then it likely expelled its hydrogen several decades before it exploded.”

Another possibility is that a less massive companion star might have stripped away hydrogen from the supernova’s progenitor star. The team, however, will not be able to search for a possible companion star until after the supernova’s brightness fades, which could take up to 10 years.

“Unlike it’s normal behavior right after it exploded, the hydrogen interaction revealed it’s kind of this oddball supernova,” Kilpatrick said. “But it’s exceptional that we were able to find its progenitor star in Hubble data. In four or five years, I think we will be able to learn more about what happened.”

The researchers obtained follow-up observations of the supernova using the Swope Telescope at Las Campanas Observatory, the Spitzer Space Telescope, and instruments at the Gemini Observatory, Keck Observatory, and Las Cumbres Observatory.

In addition to Kilpatrick and Foley, the coauthors of the paper include Katie Auchettl, Georgios Dimitriadis, David Jones, Enrico Ramirez-Ruiz, and César Rojas-Bravo at UC Santa Cruz, as well as researchers at the University of Toronto, University of Melbourne, University of Copenhagen, University of Illinois, and Johns Hopkins University. This study was supported by NASA, the National Science Foundation, the Canadian Institute for Advanced Research, the VILLUM Foundation, and the Australian Research Council Centre of Excellence.