When stars collide: Solving the 16-year mystery of the Blue Ring Nebula
November 18, 2020
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Beautiful stellar object.
Enlarge / The Blue Ring Nebula was discovered by the Galaxy Evolution Explorer, or GALEX, in 2004, but it took 16 years of observations with other telescopes, both on the ground and in space, to finally pin down its cause

It took 16 long years, but astronomers have finally solved the puzzle of the mysterious Blue Ring Nebula, according to a new paper published in Nature. First spotted in 2004, the star with an unusual ultraviolet ring appears to be the result of two stars merging, spewing out debris in opposite directions and forming two glowing cones. It appears to us as a blue ring because one of those cones points directly at Earth. That makes this the first observation of a rare stage of stellar evolution, just a few thousand years into the process, akin to capturing a baby’s first steps.

The story begins with the so-called GALEX (Galaxy Evolution Explorer) mission, an all-sky survey in the ultraviolet band of the electromagnetic spectrum that was in operation from 2003 to 2013. Caltech physicist Chris Martin was the PI for GALEX when his team spotted an unusual object: a large, faint blob of glowing gas with a star at its center. GALEX makes measurements in both the far UV and near UV bands, but while most objects GALEX observed showed up in both bands, the stunning blue ring around the star dubbed TYC 2597-735-1 only showed up in the far UV.

Intrigued, Martin decided to investigate further, confident that he and his team could come up with a viable explanation by the end of the year. He thought the Blue Ring Nebula was most likely a supernova remnant or perhaps a planetary nebula formed from the remains of a star roughly the size of our Sun, even though these typically emit light in multiple wavelengths outside the UV range. But it turned out to be a far knottier problem.

Martin and his team spent the next few years studying the nebula using as many different space and terrestrial telescopes as they could. They realized there were actually two rings, offset from the central part of the surrounding nebula—also consistent with it being a shock nebula. A perusal of archival data on the central star showed that there was excess emission in the infrared, suggesting the presence of a disk of dust absorbing the star’s light and re-radiating it in the infrared.

“This was extremely surprising because this nebula looks like something that is created after a star has aged and stopped burning hydrogen and is maybe turning into a white dwarf, like a planetary nebula,” said Martin during a virtual press conference. “But the circumstellar disk looked like something we would see in a young star.”

The next step was to use the high-resolution spectrograph at the W.M. Keck Observatory in Hawaii to search for evidence of a companion around the star. The team didn’t find anything as massive as a star but caught hints of what might be a “hot Jupiter” planet near the central star. Although the evidence was ambiguous, “Our working hypothesis became that a hot Jupiter had spiraled into the star and created some sort of conflagration, which caused what we thought was a bipolar outflow,” said Martin. “But it became impossible to come up with a scenario to explain all these [conflicting] observations, so after a few years, we went on to do other things, and the project lay dormant.”

Back on the case

Fast forward to 2017, when Keri Hoadley joined Martin’s team as a postdoctoral fellow. Hoadley took on the task of putting all the pieces of the puzzle together to explain this paradoxical object. She soon determined that the glow from the nebula resulted from hydrogen atoms becoming excited as the shock front formed, causing them to glow with visible light. Meanwhile, a reverse shockwave moved inward, causing hydrogen molecules (rather than atoms) to also become excited and glow in the ultraviolet regime. “We’ve seen other objects in the universe that emit these same molecules in the same way, and it wasn’t our initial guess of what was exciting the molecules in this case,” Hoadley said during the press conference.

As for the hot Jupiter working hypothesis, collaborative work with colleagues at the Habitable Zone Planet Finder on the Hobby-Eberly Telescope in Texas confirmed there was no compact object orbiting the star. “We didn’t think we were seeing a planet there after all,” Hoadley said, which pushed them to consider other alternatives.

Animation of the geometry of the Blue Ring Nebula. It is thought to be the product of two stars merging into one. The collision of the bodies ejected a cloud of hot debris into space. A disk of gas orbiting the larger star cut the cloud in half, creating two cones that are moving away from the star in opposite directions.

To help tie everything together, Hoadley et al. turned to theoretical physicist Brian Metzger of Columbia University, who agreed that the evidence didn’t favor a star-planet collision since the nebula was moving too fast and had too much mass. He thought a stellar collision was the most likely explanation for the Blue Ring Nebula and that the unusual properties astronomers had observed were because they had caught the merger at precisely the right stage. Stellar evolution models confirmed that hypothesis.

So what really happened? Astronomers now believe that a few thousand years ago, a star about the size of our Sun had a smaller star orbiting around it. As it aged, the larger sun puffed up, bringing its outer layers ever-closer to its companion star. That smaller star siphoned debris off its bigger partner, forming a disk, but it was eventually consumed by the larger star. This merger then launched a cloud of debris into space.

That lovely blue ring is the outflow of all that debris, forming two cones that fan outward in opposite directions. From Earth’s perspective, we see one cone head on, with the second rear-facing cone directly behind it. (See animation above.) According to Metzger, a likely mechanism for this biconical geometry is that the initial disk was created around the smaller star as it began spiraling into the bigger star.

But at the final plunge, so to speak, more material was ejected in all directions. That new material hit the initial disk, which essentially sliced it in two, redirecting all that ejected matter in a bipolar outflow. As the millennia passed, the expanding cloud of debris gradually cooled and formed molecules and dust, including hydrogen molecules that collided with the interstellar medium. This caused the hydrogen molecules to emit far-UV light, which eventually became bright enough for GALEX to spot it.

A “Rosetta Stone” for stellar evolution?

Hoadley admits that some team members were disappointed with the conclusions, since they had their heart set on a “planetary destruction scenario.” A two-star collision seemed less interesting.

“But that’s not true at all for this case,” said Hoadley. “We’re catching the Blue Ring Nebula and its central stellar merger remnant at a time where we have never seen an example of this before. Stellar mergers peak in brightness and then quickly fade because when a star merges with another star, it ejects a lot of mass, and this mass cools and condenses into dust and other molecules. That quickly blocks any view we have of what’s happening at the core of where the merger happened.”

With the Blue Ring Nebula, the merger hasn’t just happened, but it remains “in a state where things are highly unstable and the whole system is still reeling from this event,” she continued. “So it’s this prime, beautiful system we can study to test our theories about the evolution of these systems, to explain the different stellar populations we see out there.” And she and her colleagues have plenty of time in which to continue studying it. The Blue Ring Nebula is expected to last anywhere between 1,000 to tens of thousands of years.

That’s why Metzger described the Blue Ring Nebula as a “Rosetta Stone” type of object, given that astronomers believe stellar mergers are actually quite common. “We sort of have an idea about how a single star evolves throughout its life, and mergers can have a dramatic impact on how stars evolve,” he said during the press conference. Astronomers occasionally spot unusual types of stars and speculate that they could be the result of stellar mergers. “But we have no way to confirm that because by the time we see these stars, the equivalent Blue Ring Nebula of those mergers has long ago dissolved.”

The next step is to hopefully find more such midpoint events, preferably involving stars with different solar masses. “By those different combinations of mergers, we might be able to further test our ideas about how diverse these outcomes on mergers are,” said Metzger.

DOI: Nature, 2020. 10.1038/s41586-020-2893-5 (About DOIs).

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