Chinese astronomers discover a high-velocity star ejected from the globular star cluster Messier 15

Chinese astronomers report the discovery of a new high-velocity star about 4,200 light-years away. The newly discovered star, named J0731+3717, appeared to have been ejected from the globular star cluster Messier 15 by an intermediate-mass black hole. The finding was presented in a paper published on the pre-print server on June 3 arXiv.

Globular clusters (GCs) are collections of tightly bound stars orbiting galaxies. Astronomers regard them as natural laboratories that enable research into the evolution of stars and galaxies. In particular, globular clusters could help researchers better understand the formation history and evolution of early-type galaxies, as the origin of GCs appears to be closely linked to periods of intense star formation.

Messier 15 (also known as NGC 7078 or M15), located about 35,700 light-years from Earth, is a core-collapsed GC with a radius of about 88 light-years and an estimated mass of 560,000 solar masses. It is one of the oldest (about 12.5 billion years old) and most metal-poor galactic GCs (with a metallicity of about −2.25), and one of the most densely packed GCs in our Galaxy.

The cluster is known to harbor an intermediate-mass black hole (IMBH) with an estimated mass of 1,700 to 3,200 solar masses.

The team of astronomers led by Yang Huang of the University of Chinese Academy of Sciences in Beijing recently conducted a search for high-velocity stars that may be ejected from a globular cluster. As a result, using data mainly from the Sloan Digital Sky Survey (SDSS) and from ESA’s Gaia satellite, they found a promising candidate in Messier 15.

“We report the discovery of a high-velocity star J0731+3717, whose retrograde orbit intersected that of the globular cluster M15 within the tidal radius of the cluster about 21 million years ago,” the researchers explained.

J0731+3717 has a total speed of 418.71 km/s, a mass of about 0.69 solar masses and an effective temperature of 6,062 K. The star, estimated to be 13 billion years old, is located about 4,200 light-years away from Earth and has a metallicity at a level of -2.23.

The study found that although J0731+3717 is currently 37,500 light-years away from Messier 15, their backward orbits crossed 21 million years ago with a relative velocity of 548 km/s and a shortest distance of about 189 light-years. This distance is smaller than the tidal radius of Messier 15, which is estimated to be 430 light years.

Furthermore, J0731+3717 was found to exhibit rare chemical fingerprints consistent with those of Messier 15, and to have a similar metallicity and age to that of the cluster. Therefore, the astronomers concluded that this star was originally associated with Messier 15.

The authors of the paper assume that J0731+3717 was most likely ejected tidally from as far away as 1 AU to the center of Messier 15. They added that an ejection at such a high speed would require a black hole with a mass of at least 100 solar masses. masses, meaning the cluster’s IMBH is most likely responsible for this.