The state of the lone star: Tracking a low-mass star speeding through the Milky Way

While the sun may appear to stand 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 half a million miles per hour. That may seem quick, but when a faint red star was spotted speeding across the sky at an astonishing speed, scientists took notice. Thanks to the efforts of a citizen science project called “Backyard Worlds: Planet 9” and a team of astronomers from across the country, a rare L-class superfast subdwarf star has been discovered hurtling through the Milky Way. What’s even more remarkable is that the star may be on an orbit that will take it away from the Milky Way entirely. The study, led by Adam Burgasser, professor of astronomy and astrophysics at the University of California, San Diego, was announced today at a press conference during the 244th National Meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.

The star, charmingly named CWISE J124909+362116.0 (“J1249+36”), was first discovered by some of the more than 80,000 volunteer citizen scientists participating in the Backyard Worlds: Planet 9 project, sifting through reams of data collected over the past 14 years by NASA’s Wide-field Infrared Survey Explorer (WISE) mission. The project leverages the unique human ability to look for patterns and detect anomalies in ways that computer technology can’t match. Volunteers mark moving objects in data files, and when enough volunteers mark the same object, astronomers study it. J1249+36 was quickly noticed because of its speed across the sky, initially estimated to be about 600 kilometers per second (1.3 million miles per hour). At this speed, the star would be fast enough to escape the gravity of the Milky Way galaxy, potentially making it a “hypervelocity star.” Dream. The data showed that the object is a rare L subdwarf star – a class of stars with very low mass and temperature. Subdwarf stars are the oldest stars in the Milky Way galaxy. Insights into the composition of J1249+36 were made possible by a set of new atmospheric models created by graduate student Roman Gerasimov of the University of California, San Diego. He worked with UC LEADS Fellow Efrain Alvarado III to create a model specifically for the study of L subdwarf stars. “We were excited to see that our model was able to accurately reproduce the observed spectrum,” said Alvarado, who is presenting his modeling results at the AAS meeting. Spectral data and images from several ground-based telescopes allowed the team to precisely measure J1249+36’s position and speed in space, which allowed them to predict its trajectory through the Milky Way. “Now this source became really interesting, because its speed and trajectory indicated that it was moving fast enough that it could potentially escape the Milky Way,” Burgasser explained.

What struck 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 a very close orbit with a white dwarf, the white dwarf can transfer mass, triggering periodic explosions called novas. If the white dwarf accumulates too much mass, it can collapse and explode as a supernova. “In this type of supernova, the white dwarf is completely destroyed, so the companion is freed and flies away at its original orbital velocity, plus a small acceleration due to the supernova explosion,” Burgasser said. “Our calculations show that this scenario works. However, since the white dwarf is no longer there and the remnants of the explosion that probably occurred millions of years ago have already disappeared, we do not have conclusive evidence that this is its origin.” In the second scenario, J1249+36 was originally a member of a globular cluster, a collection of closely connected stars immediately recognizable by their distinct spherical shape. The centers of these clusters are expected to contain black holes with a wide range of masses. These black holes can also form binaries, and such systems have proven to be excellent catapults for stars that get too close to the black holes. “When a star encounters a black hole binary, the complex dynamics of this three-body interaction can knock the star out of the globular cluster,” explained Kyle Kremer, a future assistant professor in the Department of Astronomy and Astrophysics at the University of California, San Diego. . Kraemer ran a series of simulations and found that this type of interaction could occasionally eject a low-mass subdwarf star from a globular cluster, putting it into an orbit similar to J1249+36. “This is a proof of concept. But we don’t actually know which globular cluster this star came from. If we trace J1249+36 back, it might be in a very crowded part of the sky,” Kraemer said. It contains star clusters that are yet to be discovered. To determine whether one of these scenarios, or another mechanism, can explain J1249+36’s orbit, Burgasser said the research team would like to study its elemental composition in more detail. For example, a white dwarf’s explosion would produce heavy elements that may have “polluted” J1249+36’s atmosphere as it escaped. Stars in globular clusters and satellite galaxies in the Milky Way also show different abundance patterns, which could shed light on the origin of J1249+36.

Source:University of California – San Diego