Star Zips Through the Milky Way at Record Speed

Los Angeles CA (SPX) Aug 16, 2024 - Though the Sun orbits the Milky Way at a staggering speed of 220 kilometers per second, a newly discovered faint red star is racing across the sky at an even more astonishing pace - approximately 1.3 million miles per hour (600 kilometers per second). This makes it the first known "hypervelocity" very low-mass star, identified by a collaboration between citizen scientists and astronomers utilizing multiple telescopes, including those at W. M. Keck Observatory on Maunakea and the University of Hawai'i Institute for Astronomy's Pan-STARRS on Haleakala, Maui. Located just 400 light-years away, it is the closest hypervelocity star ever observed.

This exceptional star may be on a path that could eventually take it out of the Milky Way galaxy. The research, led by Professor Adam Burgasser of UC San Diego, has been accepted for publication in *The Astrophysical Journal Letters* and is currently available on arXiv.org.

Named CWISE J124909+362116.0 (or J1249+36 for short), the star was first noticed by volunteers from the Backyard Worlds: Planet 9 project, which has more than 80,000 participants who sift through data from NASA's Wide-field Infrared Survey Explorer (WISE) mission. These volunteers, relying on their natural ability to spot patterns, tag moving objects in the data. Once an object is flagged by enough volunteers, astronomers step in to investigate.

J1249+36 stood out due to its remarkable speed - about 0.1 percent of the speed of light. "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," said Burgasser.

Burgasser and his team used Keck Observatory's Near-Infrared Echellette Spectrograph (NIRES) to analyze the star's infrared spectrum, revealing that it is an L subdwarf, a category of ancient stars with low mass and cooler temperatures than the Sun. These subdwarfs are some of the oldest stars in the Milky Way.

The team compared the spectral data from Keck Observatory with atmosphere models created by UC San Diego alumnus Roman Gerasimov, in collaboration with UC LEADS scholar Efrain Alvarado III, which are specifically designed to study L subdwarfs. "It was exciting to see that our models were able to accurately match the spectrum obtained with Keck's NIRES," said Alvarado.

With the help of additional imaging data from Pan-STARRS and other ground-based telescopes, the team could accurately determine J1249+36's position, velocity, and potential orbit within the Milky Way.

To explain J1249+36's unusual trajectory, researchers considered two main scenarios. In one, the star was once the low-mass companion to a white dwarf. If the white dwarf accumulated too much mass, it could have exploded as a supernova, propelling J1249+36 outward at high speed. "Our calculations show this scenario works," Burgasser explained, though he noted the lack of direct evidence due to the explosion occurring millions of years ago.

Another possibility is that J1249+36 was once part of a globular cluster, where interactions with a black hole binary could have flung it out of the cluster and onto its current high-speed course. Kyle Kremer, incoming Assistant Professor at UC San Diego, ran simulations supporting this theory but acknowledged that the star's original cluster remains unidentified. "It demonstrates a proof of concept," Kremer said.

Further research may focus on analyzing the star's elemental composition to uncover clues about its origin. "We're essentially looking for a chemical fingerprint that would pinpoint what system this star is from," said Gerasimov, who specializes in measuring the element abundances of stars in globular clusters.

Whether launched by a supernova, a black hole binary, or another mechanism, J1249+36 offers astronomers a unique opportunity to deepen their understanding of the Milky Way's history and dynamics.

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