Star survives companion’s explosion

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Star survives
companion’s explosion
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: 27 April 2011

Astronomers studying the famous Tycho supernova remnant have found evidence that the dying star’s companion survived the blast.

The finding was made with the Chandra X-ray Observatory, which detected an arc of X-ray emission in the supernova remnant. The observation is consistent with the idea that the shock wave from the exploding star blasted material off the surface of a nearby star.

Artist’s impression describing the supernova explosion that created Tycho’s remnant, and the X-ray arc of the companion star, which itself is racing away from the site of the supernova explosion. The arc has blocked debris from the explosion, creating a "shadow" behind the arc. Image: NASA/CXC/M.Weiss.

The supernova event is classified as a Type Ia, which can result from one of two processes: either two white dwarf stars merge together to cause a cataclysmic explosion, leaving no trace of the companion star, or, a white dwarf pulling material off a neighbouring Sun-like star star triggers the explosion. The latest Chandra observations support the latter theory.

“There has been an ongoing long-standing question about what causes Type Ia supernovas,” says Fangjun Lu of the Institute of High Energy Physics, Chinese Academy of Sciences in Beijing. “Because they are used as steady beacons of light across vast distances, it is critical to understand what triggers them.”

Type Ia supernovae are favoured for measuring astronomical distances because of their reliable brightness – they have been used to determine that the Universe is expanding at an accelerating rate, an effect attributed to the prevalence of an invisible, repulsive force throughout space called dark energy.

X-ray image of the supernova remnant showing the arc of material towards the bottom left. Image: NASA/CXC/Chinese Academy of Sciences/F. Lu et al.

The new observations suggest that relatively little material was ripped from the companion star. Combined with previous observations of a nearby star racing through the supernova remnant much faster than its neighbours, it seems that the companion star is still alive and well.

“It looks like this companion star was right next to an extremely powerful explosion and it survived relatively unscathed,” says Q. Daniel Wang of the University of Massachusetts. “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.”

By studying the properties of the X-ray arc and the candidate companion, the astronomers could work backwards to determine the orbital period and separation between the two stars in the binary system before the explosion, arriving at a period of five days, and a separation of just one-tenth the equivalent distance between the Earth and the Sun.

“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,” says Lu. “We now seem to have found this piece.”

The discovery of the X-ray arc follows the recent announcement of stripes within the supernova, thought to be features in the outer blast wave caused by cosmic ray acceleration (read more in our news story here).

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