Legendary exploding star helps reveal secret of supernova origins

Astronomers might now know the cause of an historic supernova explosion – the supernova known as Tycho’s Star – whose appearance in Earth’s skies over 18 months in 1572 was tracked by the legendary Danish astronomer Tycho Brahe. Knowing the cause of the supernova might also help astronomers investigate dark energy in the universe. NASA’s Chandra X-ray Observatory enabled astronomers to conduct this study, which provides strong evidence that a star can survive the explosive impact generated when its companion star goes supernova. Results of the study will appear in the May 1, 2011 issue of The Astrophysical Journal.

The new study examined the remnant of a supernova – which has been dubbed simply Tycho, for short. This cloud in space – which has been expanding for centuries and which is 13,000 light-years away – was created by what today’s astronomer’s classify as a Type 1a supernova. These sorts of objects are useful in measuring astronomical distances because of their reliable brightnesses. Type 1a supernovae have been used to determine that the universe is expanding at an accelerating rate, an effect attributed to dark energy – an invisible, repulsive force throughout space.

Tycho supernova remnant. The small blue arc, lower left, represents material blown from companion star. Credit: NASA/CXC/Chinese Academy of Sciences/F. Lu et al

A team of researchers analyzed a deep Chandra observation of the Tycho supernova remnant and found an arc of X-ray emission. Evidence supports the conclusion that a shock wave created the arc when a white dwarf star – the star that existed before the supernova – exploded and blew material off the surface of a nearby companion star.

Fangjun Lu of the Institute of High Energy Physics, Chinese Academy of Sciences in Beijing, said:

There has been a long-standing question about what causes Type 1a supernovas. Because they are used as steady beacons of light across vast distances, it is critical to understand what triggers them.

Click on image to view larger. Image Credit: NASA/CXC/M.Weiss

One popular scenario for Type 1a supernovae involves the merger of two white dwarfs. In this case, no companion star or evidence for material blasted off a companion should exist.

In the other main competing theory, a white dwarf pulls material from a sun-like companion star until a thermonuclear explosion occurs. Both scenarios may actually occur under different conditions, but the latest Chandra result from the 1572 supernova remnant supports the latter idea.

In addition, the recent study of the Tycho supernova remnant seems to show the remarkable resiliency of stars. That is, the supernova explosion of 1572 appears to have blasted very little material off the companion star. How do we know which star among billions in space was the companion to the white dwarf that created the 1572 supernova? Earlier studies with optical telescopes revealed a star within the supernova remnant that is moving much more quickly than its neighbors, hinting that it could be the missing companion.

Q. Daniel Wang of the University of Massachusetts in Amherst, explained:

It looks like this companion star was right next to an extremely powerful explosion, and it survived relatively unscathed. Presumably, it was also given a kick when the explosion occurred. Together with the orbital velocity, this kick makes the companion now travel rapidly across space.

Low x-ray image of Tycho. Image Credit: NASA/CXC/Chinese Academy of Sciences/F. Lu et al

Using the properties of the X-ray arc and the candidate stellar companion, the team determined the orbital period and separation between the two stars in the binary system before the explosion. The period was estimated to be about 5 days, and the separation was only about a millionth of a light year, or less than a tenth the distance between our sun and Earth. In comparison, the supernova remnant itself is currently about 20 light years across.

Other details of the arc support the idea that it was blasted away from the companion star. For example, the X-ray emission of the remnant shows an apparent “shadow” next to the arc, consistent with the blocking of debris from the explosion by the expanding cone of material stripped from the companion.

Lu said:

This stripped stellar material was the missing piece of the puzzle for arguing that Tycho’s supernova was triggered in a binary with a normal stellar companion. We now seem to have found this piece.

The shape of the arc is different from any other feature seen in the remnant. Other features in the interior of the remnant include recently announced stripes, which have a different shape and are thought to be features in the outer blast wave caused by cosmic ray acceleration.

Summary: A team of scientists has studied the remnant of Tycho’s Star, the famous supernova of 1572. The new study used data from NASA’s Chandra X-ray Observatory to conclude that this expanding remnant – from Type 1a supernova – supports the theory that supernovae explode after a white dwarf pulls material from sun-like companion. These astronomers will publish their findings in the May 1, 2011 issue of The Astrophysical Journal.

Via Chandra

Chandra images and multimedia

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April 27, 2011

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