Friday, July 15, 2011

Tight Stellar Binary

The discovery of a detached pair of white dwarfs with a 12.75 minutes orbital period has been published by Warren R. Brown et al. 2011 in a paper entitled: “A 12 minute Orbital Period Detached White Dwarf Eclipsing Binary”. This stellar system is designated SDSS J065133.33+284423.3 or just J0651, and it is the tightest white dwarf binary system yet discovered. J0651 is located at a distance of over 3000 light years from the Sun. Both white dwarfs are racing around each other at over 600 kilometers per second. The visible primary is a 0.25 solar mass tidally distorted helium white dwarf while the unseen secondary is a 0.55 solar mass carbon-oxygen white dwarf.

Credit: David A. Aguilar (CfA)

Both white dwarfs are separated by a mean distance of less than one-third the separation between our Earth and the Moon, and they are on the brink of a merger. The two white dwarfs are expected to merge in 900 thousand years from the loss of energy and angular momentum via the emission of gravitational wave radiation. This will eventually lead to a massive rapidly spinning white dwarf, the formation of a stable interacting binary, or possibly an explosion as an underluminous type Ia supernova. The orientation of the orbits of both white dwarfs in the binary system is such that eclipses of each white dwarf by the other are observable and this allows accurate measurements of the orbital parameters, masses and radii of the white dwarfs.

The eclipse of one white dwarf by the other occurs like clockwork, at a very predictable rate. Observers on a hypothetical planet which orbits around this star system will see one of their two suns disappear every 6 minutes or so. The shrinking of the orbits of both white dwarfs via the emission of gravitational wave radiation is expected to be measurable from observing changes in the eclipse timings. This provides a remarkable opportunity to test for the existence of gravitational waves that are predicted by Einstein’s general theory of relativity.