Quasars are among the most energetic and luminous objects
known in the universe. These objects are powered by supermassive black holes
(SMBHs) accreting matter at prodigious rates in the centres of massive
galaxies. SMBHs have masses ranging from ~100 million to billions of times the
Sun’s mass. A quasar consists of a SMBH surrounded by a massive accretion disk.
The accretion disk feeds matter to the black hole, thereby powering the quasar.
A number of studies have proposed that exotic supermassive stars may form in
such an accretion disk, somewhat like how planets form in protoplanetary disks
around young stars.
.jpg)
An artist’s impression of ULAS J1120+0641, a distant quasar
powered by a black hole with 2 billion times the Sun’s mass. Credit: ESO/M.
Kornmesser.
Beyond a few hundred to a few thousand Schwarzschild radii
from the SMBH, a quasar’s accretion disk starts to become self-gravitating. The
term “Schwarzschild radius”, is a unit of measurement, where a value of one
Schwarzschild radius is the distance from a black hole where the escape speed would
equal the speed of light. For a SMBH with 100 million times the Sun’s mass, its
Schwarzschild radius would be 300 million km.
When a quasar’s accretion disk becomes self-gravitating, it
can fragment into gravitationally bound clumps and form very massive stars with
masses easily exceeding a few hundred times the Sun’s mass. The term
“supermassive” is truly justified here since such stars can attain masses of up
to perhaps ~10,000 times the Sun’s mass, making them far more massive than the
most massive stars currently known.
Once a supermassive star forms, it can clear a gap in the
quasar’s accretion disk and start migrating towards the central SMBH. The
migration timescale to the SMBH is comparable to the lifespan of the
supermassive star. During this period, the supermassive star can interrupt the
flow of gas to the SMBH and temporary dim the quasar. The final merger of the
supermassive star, or what is left of it, with the SMBH could be a strong
source of low frequency gravitational waves.
Detecting the presence of a supermassive star can be challenging.
Although supermassive stars are very luminous, they would still be overwhelmed
by the glare of the nearby quasar. Nevertheless, the presence of a supermassive
star around the SMBH of a quasar can show up as a periodic milli-magnitude
amplitude modulation of the quasar’s brightness, with a period ranging from a
few days to a few years. The physical conditions found in a quasar’s accretion
disk are much more extreme than in a conventional star-forming region. This can
favour the rapid formation of truly supermassive stars.
References:
- Jeremy Goodman and Jonathan C. Tan (2004), “Supermassive
Stars in Quasar Disks”, arXiv:astro-ph/0307361
- Yanfei Jiang and Jeremy Goodman (2010), “Star Formation in
Quasar Disk”, arXiv:1011.3541 [astro-ph.HE]