According to a new study, a rare glimpse of a star just before it exploded in a fiery supernova appears to be quite different from what astronomers expected. Hubble Space Telescope images reveal that a relatively cool, puffy star died in a hydrogen-depleted supernova. It was believed that supernovas without hydrogen could only come from extremely hot, compact stars.

Theorists have some ideas about how massive stars behave just before they explode, but such massive stars are few and far between in the local universe. 

It was retroactively identifying the star that caused a supernova to test how stars evolve just before exploding.

However, Charlie Kilpatrick, an astronomer at Northwestern University in Evanston, Illinois, explains that locating those stars is difficult. In the years preceding the supernova, a telescope must have focused on that specific region of the sky. And the explosion must have occurred close enough to the source star for light from it to reach a telescope.

Although both requirements are challenging to fulfil, Kilpatrick is undeterred by the hunt. Following the discovery of a supernova in December 2019 in the galaxy NGC 4666, which is about 46 million light-years away, he and colleagues rushed to verify previous Hubble observations from the same region of the sky. They sought to identify the star responsible for the explosion, dubbed SN 2019yvr.

After poring over images and comparing their findings to those from ground-based telescopes, the team discovered their quarry: a star in the exact location as the supernova observed approximately 2.6 years before the explosion. It appeared to be a yellow star with an extreme temperature and a diameter much bigger than the sun.

“I was kind of puzzled by all that,” Kilpatrick admits. Because the supernova SN 2019yvr lacked hydrogen, its progenitor was expected to be deficient in hydrogen as well. However, “if a star lacks a hydrogen envelope,” Kilpatrick explains, “then you expect to be seeing the deeper inside of the star to the hotter layers.” That is, the star should have appeared extremely hot, blue, and compact — perhaps 10,000 to 50,000 degrees Celsius and no more than 50 times the diameter of the sun. On the other hand, the cool, enormous, yellow progenitor of SN 2019yvr appeared to be well-padded with hydrogen. The researchers published their findings in the Monthly Notices of the Royal Astronomical Society on May 5.

To produce a supernova like SN 2019yvr, this type of star must have ejected a large amount of hydrogen prior to exploding, Kilpatrick says. However, how?

He and his colleagues have proposed several scenarios. The star may have ejected a large amount of hydrogen into space via violent eruptions, possibly caused by internal instability or interference from another nearby star. Alternatively, the hydrogen in the star could have been stripped away by another star in orbit around it.

Journal Reference:

Charles D Kilpatrick, Maria R Drout, Katie Auchettl, Georgios Dimitriadis, Ryan J Foley, David O Jones, Lindsay DeMarchi, K Decker French, Christa Gall, Jens Hjorth, Wynn V Jacobson-Galán, Raffaella Margutti, Anthony L Piro, Enrico Ramirez-Ruiz, Armin Rest, César Rojas-Bravo, A cool and inflated progenitor candidate for the Type Ib supernova 2019yvr at 2.6 years before explosionMonthly Notices of the Royal Astronomical Society. Published online May 5, 20201. doi: 10.1093/mnras/stab838.

Main Image credit: ESA/Hubble & NASA