While it may seem like the Sun is standing still while the planets move in their orbits, it actually revolves around the Milky Way at an astonishing speed of about 220 kilometers per second, or about 500,000 miles per hour. It may seem fast, but when a faint red star was discovered moving even faster across the sky at about 1.3 million miles per hour (600 kilometers per second), scientists took notice.
This rare star is the first ultra-low-mass “hypervelocity” star discovered thanks to the efforts of a team of citizen scientists and astronomers from across the United States using multiple telescopes, including two in Hawaii: the M. Keck Observatory on Mauna Kea, Hawaii, and the University of Hawaii Institute of Astronomy Pan-STARRS on Haleakala, Maui. Located just 400 light-years from Earth, it is the closest hypervelocity star we know of. Even more remarkable is that the star may be on an unusual orbit that could take it away from the Milky Way galaxy entirely.
The study, led by Adam Burgasser, professor of astronomy and astrophysics at the University of California (UC) San Diego, was recently accepted for publication in The Astrophysical Journal Letters and is available as a preprint on arXiv. The star, called CWISE J124909+362116.0 (or “J1249+36” for short), was first discovered by some of the more than 80,000 volunteer citizen scientists involved in the Backyard Worlds: Planet 9 project, who were combing through the vast amount of data collected over the past 14 years by NASA’s Wide-field Infrared Survey Explorer (WISE) mission. The project harnesses the unique human ability, programmed by evolution, to look for patterns and detect anomalies in ways that computer technology cannot achieve.
Volunteers mark moving objects in a data file, and once enough volunteers have marked the same object, astronomers study it. J1249+36 was immediately noticeable because it moves at about 0.1% of the speed of light. “This is where this source became very interesting, because its speed and orbit indicated that it was moving fast enough that it could potentially escape the Milky Way galaxy,” Burgasser says. To better understand the nature of this object, Burgasser utilized Keck Observatory’s Near Infrared Echelette Spectrometer (NIRES) to measure its infrared spectrum.
The data showed that the object was an L subdwarf star, a class of stars with much lower mass and lower temperature than the Sun. Subdwarf stars are the oldest stars in the Milky Way. The research team compared Keck Observatory’s findings on the composition of J1249+36 with a set of new atmospheric models created by Roman Gerasimov, a graduate student at the University of California, San Diego. He worked with UC LEADS Fellow Efrain Alvarado III to develop models specifically for the study of L subdwarf stars. “We were excited to see that our models accurately reproduced the spectra obtained with Kecks NIRES,” Alvarado says.
The spectral data, along with imaging data from Pan-STARRS and several other ground-based telescopes, allowed the team to precisely measure J1249+36’s position and speed in space, thereby predicting its orbit through the Milky Way. What shocked this star? The researchers focused on two possible scenarios to explain J1249+36’s unusual orbit. In the first scenario, J1249+36 was originally a low-mass companion to a white dwarf. A white dwarf is the remaining core of a star that has run out of nuclear fuel and disappeared. If a stellar companion is in very close orbit around a white dwarf, it can transfer mass, triggering periodic explosions called novas. If the white dwarf accumulates too much mass, it can collapse and explode as a supernova. “When a star encounters a binary black hole system, the complex dynamics of this three-body interaction can cause the star to be ejected from the globular cluster,” says Kyle Kremer, an incoming assistant professor in the Department of Astronomy and Astrophysics at the University of California, San Diego.
Kremer ran a series of simulations and found that this type of interaction could, in rare cases, eject a low-mass subdwarf star from a globular cluster and send it into an orbit similar to J1249+36. “This is a proof of concept,” Kremer says. “But we don’t actually know which globular cluster this star came from. If we trace J1249+36 back, it’s in a very crowded part of the sky.” There may be star clusters hiding within it that we haven’t yet discovered. To determine whether one of these scenarios, or another mechanism, can explain J1249+36’s orbit, Burgasser says the team wants to study its elemental composition in more detail. For example, the explosion of a white dwarf star would produce heavy elements that may have “polluted” J1249+36’s atmosphere as they escaped. Stars in the Milky Way’s globular clusters and satellite galaxies also show distinct elemental abundance patterns that could shed light on J1249+36’s origins.
“We are essentially looking for a chemical fingerprint that will determine exactly which star system this star came from,” Gerasimov said. Gerasimov’s modeling work allowed them to measure elemental abundances in cool stars in several globular clusters, he said. Whether J1249+36’s rapid migration was due to a supernova, a chance encounter with a binary black hole system, or some other scenario, its discovery offers astronomers a new opportunity to learn more about the history and dynamics of the Milky Way.
source:https://dx.doi.org/10.48550/arxiv.2407.08578