Thursday, October 30, 2014

A Black Hole Lurks in the Trapezium

The Orion Nebula Cluster (ONC) is a young star cluster whose age is estimated to be less than ~3 million years. Due to its proximity, the ONC is one of the best studied star clusters. At the heart of the ONC is the Trapezium, a tight cluster of several massive OB stars. A study by Subr et al. (2012) suggests that a massive black hole with more than 100 times the mass of the Sun might be lurking in the Trapezium. This is due to the large velocity dispersion observed for the 4 brightest Trapezium stars - Θ1A, Θ1B, Θ1C and Θ1D.

Considering the small number of stars in the sample, such a velocity dispersion measurement is not particularly robust. Nevertheless, the large velocity dispersion indicates there is more mass holding the cluster together than can be accounted for by just the stars. As a result, the presence of a black hole with more than 100 times the Sun’s mass is hypothesized. A black hole of this mass would represent the collapsed remnant of what was once a massive runaway-mass star. At 1300 light years away, this black hole would be the closest known to Earth.

This image shows the heart of the ONC. Clearly visible are the 4 Trapezium stars. These 4 hot and massive stars dominate the core of the ONC. Image credit: Hubble Legacy Archive, Robert Gendler

The ONC was once more compact that it currently is. For instance, the ONC does not contain wide stellar binaries with separations larger than ~1000 AU which would have been disrupted when the cluster was more compact. A consequence of being more compact is that gravitational interactions among massive OB stars would have been frequent enough to cause a fraction of them to be ejected from the cluster and a fraction of them to undergo ‘runaway’ stellar collisions to form a massive runaway-mass star. Subsequently, the massive runaway-mass star collapses directly to form a black hole, which can still continue to experience runaway growth by accreting more mass.

Interestingly, the observed number of massive OB stars in the ONC appears to be fewer than predicted. This is consistant with the loss of massive OB stars through ejection from the cluster and ‘runaway’ stellar collisions, leading to a deficit in massive OB stars. If a massive black hole is indeed lurking in the Trapezium, it probably isn’t growing much at present. The intense radiation from the hottest and most massive stars in the ONC would have driven much of the star-forming gas out of the cluster, causing the cluster to expand in size as there is less mass keeping the cluster together. As the cluster swells, stars collide less frequently and the runaway growth of the black hole slows to a halt.

Subr et al. (2012), “Catch me if you can: is there a runaway-mass black hole in the Orion Nebula Cluster?”, arXiv:1209.2114 [astro-ph.GA]