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Across the Universe

August 25, 2017 - 12:25am

White dwarfs are the very dense leftover cores of stars like our sun. Think of an object with the mass of the sun, but in a volume the size of the earth. Because of gravity, white dwarfs have a very specific limit on their size. If they become larger than 1.44 times the mass of the sun, then they explode in what astronomers call a Type Ia supernova. Because white dwarfs no longer create their own energy, Type Ia supernovas only occur when a white dwarf orbits close enough to a companion star for the white dwarf’s gravity to “steal” gas from the companion.

Because of the size limit on white dwarfs, astronomers can estimate the true total brightness or 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. It is thought that nothing survives this kind of explosion.

However, a new discovery by 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 speed of the Milky Way. This high speed 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, spectrograph that combines the large size of the Gemini North telescope with Espadons, the high resolution spectropolarimeter at Canada-France-Hawaii Telescope (CFHT), via a fiber optic link between the two.

All astronomers ever witness from Type Ia supernova is the aftermath of the explosion, a bright flash in the sky observable with a telescope.

“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.” said Vennes.

The team studied the white dwarf star LP40-365 for two years with telescopes around the world. The new star was first identified with the National Science Foundation’s (NSF) Mayall four-meter telescope at Kitt Peak National Observatory in Arizona.

The final data 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’s 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” Nadine Manset said, the GRACES instrument scientist at CFHT.

After collecting the data, the team analyzed the data with a state of the art computer code. The analysis proved the compact nature of the star and its exotic chemical composition. The analysis also revealed an extraordinary Galactic trajectory, an extremely high speed on a path out of our Milky Way galaxy.

Supernova models and simulations did suggest the possibility of observing surviving stellar remnants in the aftermath of a supernova explosion but LP40-365 is the first observational evidence for surviving remnants of Type Ia supernovae, making it an invaluable object to improve our understanding.

The team believes that many more of these objects are lurking in the Milky Way and awaiting discovery.

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