Rare Binary White Dwarf Systems Brought Into Focus
John P. Millis, PhD for redOrbit.com – Your Universe Online
Nearly a half-century ago, astronomers discovered a strange star in the Canes Venatici constellation. It was later determined that the unusual properties of the object arose because it was not an isolated star at all, but rather a very tight binary system.
With an orbital period of 18 minutes – for comparison the orbit of Mercury around the Sun takes about 88 Earth days – the individual members of the system are incredibly close, so much so that orbital energy of the system is dissipated in the form of gravitational waves ejected into outer space.
An interesting question arose after this discovery: what causes these types of systems to form? Such tight orbits from binary white dwarf systems are rare – especially if the companion object is also a degenerate remnant such as a white dwarf or neutron star. Other examples discovered in the intervening years have even faster orbits – some a short as five minutes.
But now, data from the x-ray satellite observatories Chandra and XMM-Newton, coupled with follow-up observations by ground based optical observatories such as the McDonald Observatory in Texas and the Mt. John Observatory in New Zealand, may have found two systems that may ultimately evolve into this rare Am CVn binary system.
In both cases these systems – known as J0751 and J1741 – are white dwarf binaries that, while not as tightly bound as Am CVn systems, may be on their way. Researchers suggest that as these systems loose energy to gravitational waves, their orbits will continue to decay. Eventually, they will orbit closely enough that the smaller white dwarf will begin accreting matter from its larger companion.
Should the mass of the smaller companion exceed the Chandrasakar limit, the remnant could ignite in a Type Ia supernova. While these important events – they allow astronomers to measure the distances to far away galaxies – have been observed throughout the Universe, this is the first time that a system has been observed prior to the outburst.
Continued study of these two systems will allow astronomers to better model how these rare, exotic systems form and potentially provide a laboratory for studying gravitational waves once new technology comes online.