Tuesday, April 8, 2014

Supermassive Stars around Supermassive Black Holes

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.

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.

- 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]