Pulsating White Dwarf Discovered By UT Astronomers
June 20, 2013

Burnt-Out, Pulsating White Dwarf Discovered By Texas Astronomers

Lee Rannals for redOrbit.com - Your Universe Online

A group of astronomers have discovered pulsations from the crystallized remnant of a burnt-out star. The researchers recently reported in The Astrophysical Journal Letters that they were able to use the 2.1-meter Otto Struve Telescope at the University of Texas McDonald Observatory to take a look at the white dwarf GD 518 and discover the pulsations. This finding will allow scientists to see below the star's atmosphere and into its interior, analogous to how earthquakes help geologists study the composition of Earth below the surface.

"GD 518 is special because it is a very massive white dwarf: It has about 1.2 times the mass of the Sun, packed into a volume smaller than Earth," said lead author J. J. Hermes, a graduate student at The University of Texas at Austin. "Few white dwarfs are endowed with so much mass, and this is by far the most massive white dwarf discovered to pulsate."

The team wrote that the star most likely has an interior composed of heavier elements than those found in typical burnt-out stars. GD 518 is a white dwarf, which is a star at the end of its life cycle which essentially has a burnt-out core. The interiors of these dying stars can become crystallized similar to the slow formation of glaciers in a cooling ocean.

The star that died to become white dwarf GD 518 was at one time more than seven times the Sun's mass and it burned elements heavier than carbon and oxygen. Astronomers believe that GD 518 is now composed of predominantly oxygen and neon nuclei.

Pulsations are periodic brightness changes on the surface of a star that keep a regular tune every 400 to 600 seconds. The discovery of GD 518's pulsations will give astronomers the opportunity to see what makes up the star's interior.

"Like a child at a museum, astronomers are only allowed to look, not touch, when they perform experiments," said team member Barbara Castanheira, a postdoctoral researcher with McDonald Observatory. "This means we usually can only understand the surface of a star. Pulsations, like the sound of a bell, tell us more of the story, since they can unravel secrets about the much deeper interior of a star."

White dwarf stars do not fuse elements in their interior to generate energy, but they cool like coal embers removed from a fire. At a certain point the atomic nuclei in the star's interior gets cool enough to begin to settle into a lattice structure and crystallize. This happens sooner in the interiors of more massive white dwarfs.

Astronomers now have to match the pulsation periods observed in the star with those predicted by different models of the structure of its interior.

"We see evidence that the strength of pulsations in this star are very inconsistent; some nights the star is as still as a whisper," Hermes said. "This could be because the white dwarf is highly crystallized, and the pulsations are only allowed to propagate in a tiny bit of the outermost parts of the star. They thus have little inertia, and are more susceptible to changes than the pulsations in a typical pulsating white dwarf."