June 23, 2014
Astronomers May Have Found Coldest, Faintest White Dwarf Ever
Brett Smith for redOrbit.com - Your Universe Online
Using data from three different telescopes, a large international team of astronomers has identified what could be the coldest, faintest while dwarf ever, according to a new report in the Astrophysical Journal.
The report said the temperature of the stellar vestige is so low, the carbon it contains has probably undergone crystallization, a process that results in the formation of a diamond here on Earth.
“It’s a really remarkable object,” said study author David Kaplan, a professor at the University of Wisconsin-Milwaukee. “These things should be out there, but because they are so dim they are very hard to find.”
The first object identified in this system was not the white dwarf, but a pulsar known as PSR J2222-0137. Pulsars are speedily spinning neutron stars, the superdense remnant of enormous stars that have gone supernova. As neutron stars spin, lighthouse-like beams of radio waves are sent out and these waves can be picked up by radio telescopes here on Earth.
Initial observations of PSR J2222-0137 showed it was spinning over 30 times each second and was gravitationally linked to a companion star, which was initially recognized as either a different neutron star or an oddly cool white dwarf. The two were determined to orbit each other once every 2.45 days.
After two years of observations, astronomers determined its place and range from the Earth – about 900 light-years away in the direction of the constellation Aquarius. This data was fundamental in improving the model used to gauge the arrival of the pulses at the Earth with the Green Bank Telescope (GBT) in West Virginia.
Through the use of Einstein's relativity theory, the scientists studied how the gravitational forces of the companion star bent space-time, causing delays in the radio signal as the pulsar moved behind it. These slowed travel periods helped the scientists ascertain the orientation of their orbit and the specific masses of the two stars: The pulsar was found to have a mass 1.2 times that of the Sun and the companion a mass 1.05 times that of the Sun.
The information strongly revealed that the pulsar companion was not another neutron star; the orbits were too tidy for a second supernova to have occurred, the study author concluded.
Figuring a white dwarf should appear at that location, the astronomers tried to observe it in optical and infrared light, but neither the Southern Astrophysical Research (SOAR) telescope in Chile nor the 10-meter Keck telescope in Hawaii was able to pick it up.
“Our final image should show us a companion 100 times fainter than any other white dwarf orbiting a neutron star and about 10 times fainter than any known white dwarf, but we don’t see a thing,” said study author Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill. “If there’s a white dwarf there, and there almost certainly is, it must be extremely cold.”
The study team determined that the white dwarf would be no more than a comparatively cool 3,000 degrees Kelvin—about 5,000 times cooler than the center of our own Sun. At that temperature, the white dwarf would be mostly crystallized carbon, not unlike a diamond.