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.
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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.
Reference:
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]