Mid-Mass Black Hole Hurls Star Across The Milky Way


Maunakea, Hawaii – An international team of astronomers has pinpointed the origin of a runaway high-velocity star named PG 1610+062 and determined that it was likely ejected from its birth cluster with the help of a mid-mass black hole (MMBH).

The findings are published in the journal Astronomy & Astrophysics.

In order to put tight constraints on PG 1610+062’s projected rotational velocity, its radial velocity, as well as measure its chemical composition accurately, the team needed spectral data of the star, but its distance and position in the sky made W. M. Keck Observatory’s Echellette Spectrograph and Imager (ESI) the only tool for the job.

“In the northern hemisphere, only the combination of Keck Observatory and ESI gave us what we needed. The collecting area of Keck allowed us to gather enough photons for our object and ESI has exactly the right resolution, which is high enough to resolve all the spectral features,” says co-author Thomas Kupfer, a Kavli Institute for Theoretical Physics Postdoctoral Scholar at the University of California, Santa Barbara.

While formerly considered an old star with half a solar mass, typical for the galactic halo, the Keck Observatory data revealed that PG1610+062 is actually a surprisingly young star that’s ten times more massive, ejected from the Galactic disk almost at the escape velocity from the Milky Way.

Some even faster stars, called hyper-velocity stars (HVSs), do exist – the first three were discovered in 2005. Among them is the unique star US 708, which was found from observations using the Low Resolution Imaging Spectrometer (LRIS) on the Keck I telescope; it was going so fast it escaped the Milky Way’s gravitational pull. To achieve such velocities requires an extremely dramatic slingshot event.

Young, massive stars like PG 1610+062 in the Milky Way’s galactic halo live far from our galaxy’s star-forming regions. Astronomers are trying to understand how these ‘runaway stars’ were forced to leave their birth place. New observations of PG 1610+062 suggest that a mid-sized black hole in the Milky Way may be responsible for evicting the star from its home cluster. IMAGE CREDIT: A. IRRGANG, FAU

posted Sep 12, 2019, 6:53 AM by Shifu RC

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