The Little Star That Survived a Supernova

August 16, 2017

By combining the light collecting power of Gemini North's 8-meter mirror with a high-resolution spectrograph at the neighboring Canada-France-Hawaiʻi Telescope on Maunakea, astronomers probe a speeding bullet in our galaxy. The researchers suggest that the celestial speedster is likely a white dwarf star expelled from a supernova explosion and sent hurtling through our galactic neighborhood some 50 million years ago.

The following text is a joint press release from Gemini and CFHT, and can be found verbatim in CFHTʻs press release:

An international team of astronomers led by Stephane Vennes at the Astronomical Institute in the Czech Republic have identified a white dwarf moving faster than the escape velocity of the Milky Way. This high velocity star is thought to be shrapnel thrown away millions of years ago from the site of an ancient, peculiar Type Ia supernova explosion. The team used telescopes located in Arizona, the Canary Islands and Maunakea’s GRACES, a high resolution spectrograph that combines the large aperture of the Gemini North Telescope with Espadons, the high resolution spectropolarimeter at CFHT, via a 250m optical fiber link.

Type Ia supernovae play an important role important in our understanding of the Universe. They act as standard candles, astronomical objects for which astronomers have a decent estimate of their intrinsic brightness or luminosity. Astronomers can estimate the true total luminosity of a Type Ia supernovae and use that information to determine the distance. Despite astronomers’ understanding of the luminosity and distance relationship for Type Ia supernovae, very little is known about the explosions themselves. Astronomers build models aimed at a deeper understanding of the engine powering these explosions.

One of these models suggest that at the heart of a Type Ia supernova is a compact star known as a white dwarf. If the white dwarf has a close companion star, over time the gravity of the white dwarf may attract gas from the other star. This continuous feeding compresses the white dwarf to such a high density and temperature that the white dwarf is engulfed in a thermonuclear explosion. It is thought that nothing survives this kind of explosion. However, a new class of models called “Subluminous type Ia Supernova also known as type Iax” can leave a partially burnt remnant that is instantly ejected at high velocity.

"Such a cataclysmic binary star has never been caught feeding and getting just ready for the explosion," commented Stephane Vennes, leading author of the Science article. "All we ever witness is the aftermath of the explosion, that is the bright flash in the distant Universe that even outshines the galaxy hosting that event. But now, with the discovery of a surviving remnant of the white dwarf itself, we have direct clues to the nature of the most important actor involved in these events."

The team studied the white dwarf star LP40-365 for two-years with telescopes located in Arizona, the Canary Islands, and Hawaii. The new star was first identified with the National Science Foundation's (NSF) Mayall four-meter telescope at Kitt Peak National Observatory in Arizona. "We selected this object for observation with the spectrograph at the four-meter telescope because of its large apparent motion across the celestial sphere. Thousands of objects like this one are known, but the sky was partly cloudy on that night and we had to go for the brightest star available which turned out to be LP40-365," said team member Adela Kawka, underpinning the importance of serendipity in astronomy. "We alerted team members J.R. Thorstensen and E. Alper at Dartmouth College, and P. Nemeth at the Karl Remeis Observatory for urgent follow-up observations."

A final, crowning data set was obtained with the help of team member Viktor Khalack at the Université de Moncton using a unique instrument, GRACES on Maunakea. GRACES is a collaboration between the Canada-France-Hawaii Telescope and the NSF Gemini Observatory. When GRACES is in use, CFHT’s spectropolarimeter Espadons receives light fed by an optical fiber hooked to its neighbor on the summit, the eight-meter Gemini North telescope. “GRACES provides astronomers the best of both worlds, the light collecting power of the Gemini observatory combined with a state of the art instrument like Espadons. The combination packs a powerful punch and creates opportunities for discoveries like this one” says Nadine Manset, the GRACES instrument scientist at CFHT.

After collecting the data, the team used state of the art computer codes for analysis. The analysis proved the compact nature of the star and its exotic chemical composition. "The extreme peculiarity of the atmosphere required a lengthy and complex model atmosphere analysis which crunched several weeks of computing time. But the results proved very exciting. Such a peculiar atmosphere devoid of hydrogen and helium is rare indeed," commented team member Peter Nemeth. The analysis also revealed an extraordinary Galactic trajectory. “The extremely high velocity of this star puts it on a path out of the Milky Way with no return ever,” said team member Lilia Ferrario.

Supernova models and simulations did entertain the possibility of observing surviving stellar remnants in the aftermath of a supernova explosion. The unique object LP40-365 is the first observational evidence for surviving bound remnants of failed supernovae and therefore it is an invaluable object to improve our understanding of these cosmological standard candles.

Many more of these objects are lurking in the Milky Way and awaiting discovery. The recent ESA/Gaia mission may well help us discover many more of these objects and help us understand how a little white dwarf star can survive supernova explosions.