Sunday, May 5, 2013

Hypervelocity Stellar Collisions

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.

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.

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.

Shmuel Balberg et al. (2013), “A new rare type of supernovae: hypervelocity stellar collisions at galactic centers”, arXiv:1304.7969 [astro-ph.SR]