Supermassive black holes (SMBHs) contain hundreds of thousands to billions of times the mass of our Sun. They are known to reside in the centres of most, if not all galaxies. When a binary star comes too close to a SMBH, the intense gravitational field of the SMBH can disrupt the binary system by yanking apart the two stars. When that happens, one star tends to get captured into a tight orbit around the SMBH while the other star gets ejected away with a very high velocity. It is estimated that a typical SMBH disrupts a binary star system once every 10,000 to 100,000 years. Over a period of time, the SMBH builds up a population of stars in tightly bound orbits around it. Being so close to a SMBH, these stars can travel at velocities exceeding 10,000 kilometres per second.
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Figure 1: Artist’s conception of a binary star system.
Occasionally, two stars can cross path with each other at
sufficiently high velocities to produce a hypervelocity stellar collision. The
collision can occur as an energetic head-on collision or a less energetic
grazing collision. When two stars collide at such high speeds, a very powerful
explosion is produced. The explosion brightens over a period of several days,
reaching a peak luminosity that is comparable to a supernova explosion.
Currently, there are 3 primary mechanisms for supernovae explosions - thermonuclear
explosion of a white dwarf, core-collapse in a massive star and pair-instability
explosion of a very massive star. As such, the explosion from a hypervelocity
stellar collision can serve as yet another mechanism for a supernova explosion.
After a hypervelocity stellar collision, some stellar
material will fall towards the SMBH and form an accretion disk around it. Material
in the accretion disk is expected to be heated to very high temperatures. Accretion-induced
emission can produce a later phase of observable emission following the initial
explosion from the collision. This later phase of emission involves a rise in
X-ray, ultraviolet and optical radiation over a period of several days. Nevertheless,
the phase of accretion-induced emission is sensitive to how the initial stellar
collision occurs and changes to the initial conditions can lead to very
different outcomes.
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Figure 2: Artist’s conception of an accretion disk around a
black hole.
A supernova explosion originating from a hypervelocity stellar
collision can be distinguished from other types of supernovae. Firstly, such a supernova
will tend to occur in the centre of a galaxy where a SMBH is expected to reside.
Secondly, the creation of heavy atomic nuclei from nuclear fusion is not
expected for such a supernova and this means that the evolution of its
luminosity over time will not feature the exponential luminosity decline from
radioactive decay of heavy atomic nuclei. Lastly, the ejected material from a
supernova following a hypervelocity stellar collision is mostly hydrogen gas
travelling at a higher speed than those ejected from a typical supernova.
In the near future, searches that involve surveying wide
tracts of the sky for transient phenomena could observe supernovae from
hypervelocity stellar collisions. These surveys include the Panoramic Survey
Telescope and Rapid Response System (Pan-STARRS); the Palomar Transient Factory
(PTF); and the Large Synoptic Survey Telescope (LSST). In addition to
hypervelocity stellar collisions, other combinations of hypervelocity
collisions such as planet-and-planet collisions or planet-and-star collisions
may also generate unique observable signatures.
Reference:
Shmuel Balberg et al. (2013), “A new rare type of
supernovae: hypervelocity stellar collisions at galactic centers”, arXiv:1304.7969
[astro-ph.SR]