‘Fireworks’ sparked by survivor of stellar collision

When two white dwarfs in a binary star system eventually spiral in toward each other and collide, the result is usually mutually assured destruction: a thermonuclear explosion that consumes both stars and scatters their remains into the cosmos.

But astronomers have found one case where such a collision resulted in fireworks of a different kind.

New observations of the faint nebula Pa 30 have revealed that it is surrounded by filaments of glowing sulfur gas, appearing like the trails of sparks blown outward by an exploding fireworks shell. Astronomers think this scene was caused when two white dwarfs collided — and managed to not destroy each other. Instead, they apparently merged and formed a magnetic monster of a star that blows its own material into space, whisking debris from the merger outward to form the sulfuric, streaming contrails.

Researchers say the nebula and its central star comprise a unique object with scarcely any observational precedent. “I’ve worked on supernova remnants for 30 years and I’ve never seen anything like this,” said Robert Fesen, of Dartmouth College in Hanover, New Hampshire. Fesen was speaking Jan. 12 in Seattle at the winter meeting of the American Astronomical Society (AAS), where he presented his team’s results at a press conference. “There’s nothing like this in our galaxy.” A draft of their report is available on the arXiv preprint server and has been accepted for publication by The Astrophysical Journal Letters.

It is “a really interesting” object, said Benson Guest, an X-ray astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who wasn’t involved with the study. “These things are very hard to detect because they’re not very bright compared to a normal supernova, so you’re looking for a very faint transient [object].”

The new imagery bolsters the case that Pa 30 is what astronomers call a Type Iax supernova — a type of “failed” supernova that results in a relatively tepid burst of light and leaves behind a surviving star. These have been observed in distant galaxies, but “this would be the first one we’ve ever found” in the Milky Way that we can easily study, said Guest. “Any time you can say that in astronomy, that’s something that’s really cool.”

What’s more, the new observations also pins down the object’s age — and give it a strong case for being the solution to a 900-year-old astronomical mystery.

Overlooked gem

Pa 30 lies just 7,500 light-years away in Cassiopeia and spans roughly 3′ (or about one-tenth the width of the Full Moon). It was discovered by amateur astronomer Dana Patchik in 2013 as he was searching archival data from NASA’s Wide Infrared Survey Satellite (WISE). In that data, the object had a rather conventional circular, doughnut-like appearance, resembling a planetary nebula — an object formed when an aging star sheds its outer layers of gas into space and then irradiates that gas, exciting it and causing it to glow.

Over the next few years, multiple professional observatories conducted follow-up observations, including the 10-meter Gran Telescopio Canarias (GTC) on La Palma. But they barely detected any emission from hydrogen, oxygen, or nitrogen gas. With apparently nothing to study, the researchers never fully scrutinized the data.

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