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In 2004, astronomers using NASA’s Galaxy Evolution Explorer discovered an unusual ring-shaped “blue” nebula and the star at its center, TYC 2597-735-1. New observations of the object suggest that TYC 2597-735-1 merged with its lower-mass stellar companion less than 5,000 years ago and created a bipolar outflow of material.
The Blue Ring Nebula is located 6,197 light years away in the constellation of Hercules. The nebula is 13 light years wide and is composed of hydrogen gas (blue) that expands from a central star, which is the residual core of a stellar fusion; the red filaments are shock wave filaments from the fusion event. Image credit: NASA / JPL-Caltech / M. Seibert, Carnegie Institution for Science / K. Hoadley, Caltech / GALEX Team.
“We were observing one night, with a new spectrograph we had recently built, when we received a message from our colleagues about a particular object composed of a hazy gas that rapidly expands away from a central star,” said the co-author. Dr. Guðmundur Stefánsson, postdoctoral researcher at Princeton University’s Department of Astrophysical Sciences.
“How was it formed? What are the properties of the central star? We were immediately thrilled to help solve the mystery! “
Most of the stars in the Milky Way are found in binary star systems: pairs of stars orbiting one another. If they are close enough together, such systems can meet their end in a stellar merger event.
To test this hypothesis, Dr Stefansson and colleagues observed the Blue Ring Nebula with two different spectrographs on large ground-based telescopes: the HIRES optical spectrograph on the 10-meter Keck telescope and the Habitable-zone planet finder in the near-infrared on -m Hobby-Eberly telescope at McDonald’s observatory.
“The spectroscopic observations were instrumental in allowing us to further understand the object, from which we see that the central star is inflated, and we see signs of accretion likely from a surrounding debris disk,” said Dr. Stefánsson.
“Indeed, the spectroscopic data coupled with theoretical modeling show that the Blue Ring Nebula is consistent with the image of a merging binary star system, suggesting that the inward spiraling companion was likely a low-mass star.” said lead author Dr., a postdoctoral researcher at Caltech’s Cahill Center for Astrophysics.
Although the relics of some of these binary fusion events have been observed previously, all of these objects have been enveloped in opaque dust and clouds, obstructing the view of the properties of the central stellar remnant.
The Blue Ring Nebula is the only object that allows an unobstructed view of the central stellar remnant, offering a clear window into its properties and providing clues to the merging process.
“The Blue Ring Nebula is rare. As such, it is really exciting that we were able to find it, and we are excited about the possibility of finding more similar objects in the future, ”said Dr Hoadley.
“If so, this would allow us to gain more insight into the remnants of stellar mergers and the processes that govern them.”
“For quite a long time we thought that maybe there was a planet several times the mass of Jupiter that was being torn apart by the star, and that was throwing all that gas out of the system,” said co-author Dr. Mark Seibert, an astrophysicist at the Carnegie Institution for Science and a member of the GALEX team at Caltech.
“We see a lot of two-star systems that might merge one day and we think we have identified stars that merged maybe millions of years ago,” said co-author, Dr. Brian Metzger, a researcher in the Columbia University Physics Department. and the Center for Computational Astrophysics at the Flatiron Institute.
“But we have almost no data on what happens in between.”
“We think there are likely many young remnants of stellar mergers in our Galaxy, and the Blue Ring Nebula could show us what they are like so we can identify more of them.”
“The merger of two old stars explains the seemingly conflicting observations of the central star’s youthful luminosity and aging chemical composition,” said co-author Dr Andrew McWilliam, an astrophysicist at the Carnegie Institution for Science.
The team’s work was published in the November 19, 2020 edition of the magazine Nature.
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K. Hoadley et al. 2020. A blue ring nebula from a stellar merger several thousand years ago. Nature 587, 387-391; doi: 10.1038 / s41586-020-2893-5
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