Using the 2.1 metre Otto Struve Telescope at McDonald Observatory, astronomers at the University of Texas have predicted and confirmed the existence of a new type of variable star: a pulsating carbon white dwarf.
SDSS J142625.71+575218.3: the first pulsating carbon white dwarf star, discovered by McDonald observatory astronomers. Image: Sloan Digital Sky Survey Collaboration.
A white dwarf star is the ultra dense left over remains of a Sun-like star that has burned all of the nuclear fuel in its core, and the majority of stars in the Universe will end their lives in this way. Until recently, two main types of white dwarf were thought to exist: those that have an outer layer of hydrogen, and the rarer forms which have an outer layer of helium, whose hydrogen shells have somehow been stripped away.
But last year, University of Arizona astronomers Patrick Dufour and James Liebert discovered a third type of white dwarf star: hot carbon white dwarfs, which have both their hydrogen and helium shells stripped off, leaving their carbon layer exposed. It’s thought that these could be among the most massive white dwarfs of all, and are the remnants of stars slightly too small to end their lives in a supernova explosion.
Following this discovery, Michael Montgomery, Kurtis Williams and Steven DeGennaro of the University of Texas began a systematic survey of carbon white dwarfs, following predictions that pulsations in these stars should be possible. Pulsating stars are of interest to astronomers because the changes in their light output can reveal what goes on in their interiors, a study known as asteroseismology, similar to the way in which geologists use seismic waves to understand what goes on in the Earth’s interior.
DeGennaro discovered a pulsating carbon white dwarf star 800 light years away in Ursa Major, which has about the same mass as our Sun, but a diameter smaller than the Earth's. Its light intensity was found to vary regularly by nearly two percent every eight minutes.
The light curve shows the changes in light output, or pulsations, of the white dwarf over time. Image: K. Williams/T. Jones/McDonald Observatory.
"The discovery that one of these stars is pulsating is remarkably important," says National Science Foundation astronomer Michael Briley. "This will allow us to probe the white dwarf's interior, which in turn should help us solve the riddle of where the carbon white dwarfs come from and what happens to their hydrogen and helium."
The astronomers speculate that the pulsations are caused by changes in the star's carbon outer envelope as the star cools down from its formation as a hot white dwarf. The ionized carbon atoms in the star's outer layers return to a neutral state, triggering the pulsations. Even if the variations are caused by other processes, however, studying this new breed of star will shed light on the currently unknown process that strips away their surface layers of hydrogen and helium to lay bare their carbon interiors.