Citizen scientists and professional astronomers have teamed up to detect a rare hypervelocity star racing through our Milky Way. At its current speed and trajectory, it may one day be able to escape the galaxy forever.
The discovery was only possible thanks to the legions of volunteers who lend their time to the Backyard Worlds: Planet 9 project. These volunteers help astronomers by analyzing more than 14 years of data from NASA's Wide-field Infrared Survey Explorer (WISE) mission, looking for objects that move between images. Astronomers can then follow up on interesting findings to learn more.
Burgasser et al. / AAS 244
At the recent 244th national meeting of the American Astronomical Society in Madison, Wisconsin, Adam Burgasser (University of California, San Diego) announced the discovery of a fast-moving object known as CWISE J124909+362116.0 (J1249+36 for short), approx. 400 light years from Earth. He estimates that it travels through the Milky Way at about 1 million miles per hour (450 km/s).
Burgasser used the WM Keck Observatory on Maunakea, Hawai'i, to obtain the infrared spectrum of J1249+36. The spectrum matched atmospheric models created by Roman Gerasimov (also of the University of California, San Diego) of a class of low-mass stars known as L subdwarfs. These cool stars are some of the rarest and oldest in the universe.
Burgasser then combined data collected from the spectra with images from ground-based telescopes to measure the star's position and velocity. “This is where the source became very interesting, as its speed and trajectory showed that it was moving fast enough to potentially escape the Milky Way,” says Burgasser.
We may know where he's headed, but where does this high-speed runner come from?
One option is that it once orbited a white dwarf star, which subsequently exploded as a Type Ia supernova. “In this type of supernova, the white dwarf is completely destroyed, so its companion breaks free and flies off at whatever orbital speed it was originally moving at, plus a little kick from the supernova explosion,” says Burgasser. . If this happened, it happened so long ago that there are no supernova remnants left to hunt.
The second possibility is that the star began deep in a dense group of stars known as a globular cluster. He then found a pair of black holes located deep in the cluster. “When a star encounters a binary black hole, the complex dynamics of this three-body interaction can eject that star out of the globular cluster,” says team member Kyle Kremer (also from UC San Diego).
A. Burgasser et al. / AAS 244
There is a way astronomers could decide between these two options. “Basically, we are looking for a chemical signature that can identify which system this star comes from,” says Gerasimov. But that would require a more detailed spectrum of J1249+36. Such a spectrum could show that the subdwarf was contaminated with elements ejected by the supernova. On the other hand, it could show a close correspondence with the chemistry of stars in globular clusters; Because globulars are so old, their stars contain very few elements beyond hydrogen and helium.
“This type of object is certainly rare and has not been found before,” says Sergey Koposov (University of Edinburgh, UK), who was not involved in the research. “But we have many objects of much larger masses that are moving at similar speeds.” Koposov also doesn't think the list of the star's possible backstories is exhaustive, meaning his exact story remains a major puzzle to solve.
However, one thing is clear, says Koposov: “The discovery would not have been possible without citizen scientists.”