January 6, 2010
Astronomers announced Monday that they have uncovered evidence that a massive, explosive white dwarf star in a binary star system with a Sun-like star in our Milky Way Galaxy is growing in mass and is much closer to our solar system than previously thought. The report was presented by professors Edward M. Sion and Patrick Godon as well as student Timothy McClain of Villanova University at the 215th meeting of the American Astronomical Society in Washington, D.C. This result is of special interest because it may shed light on the still unidentified type of stellar objects that explode as type Ia supernovae, the kind of supernova that has been used to demonstrate the accelerated expansion of the universe.
The close binary system T Pyxidis, located in the Southern Hemisphere constellation Pyxis the Compass, is known as a recurrent nova because its massive white dwarf star has suffered thermonuclear (nova) explosions approximately every 20 years. Its previous recorded nova explosions occurred in 1890, 1902, 1920, 1944, and 1967, making it 44 years overdue for its next thermonuclear explosion. Nobody understands why the system is has stopped its thermonuclear explosions.
Hydrogen-rich gas transferred to the white dwarf star by the very close Sun-like star triggers the thermonuclear explosions. An important unanswered question about such close binary stars is whether the mass-receiving white dwarf continually grows in mass despite the nova explosions or decreases in mass because the nova explosions eject more mass from the white dwarf than it accumulates from the Sun-like star. If the mass of the white dwarf in such a binary star system increases with time, it will eventually reach the so-called Chandrasekhar limit and undergo instantaneous gravitational collapse, resulting in a powerful thermonuclear detonation that would completely destroy the white dwarf and leave no stellar remnant such as a pulsar or black hole. This catastrophic event, known as a type Ia supernova (or "white dwarf supernova"), releases 10 million times more energy than a nova explosion, or is equivalent to 20 billion, billion, billion megatons of TNT.
The Villanova team analyzed far ultraviolet spectra of T Pyxidis obtained with the International Ultraviolet Explorer spacecraft and modeled the spectra for the first time with state-of-the-art theoretical models of accretion disks and white dwarf atmospheres. They found that the radiation emitted by a luminous accretion disk enshrouding the white dwarf dominates the light emitted by the system. But the system is at a distance within only 1,000 parsecs (3,260 light-years), far closer to our solar system than anyone previously thought.
The theoretical model that best matches the observed spectra corresponds to a white dwarf very close to the Chandrasekhar limit, an orbital tilt to our line of sight of 18° and a rate of mass accretion by the white dwarf of 2 × 1017 grams per second (3 × 10-9 solar masses per year). But the distance of the system must be less than 1,000 parsecs. The closer distance makes the disk less luminous, the accretion rate lower, and the white dwarf mass even closer to the Chandrasekhar limit.
The closer distance means the ejected nova shells imaged by Hubble have smaller masses than previously thought, which makes them consistent with the small amount of accreted mass needed to trigger a thermonuclear explosion on a massive white dwarf. This is important because it would mean the white dwarf mass is increasing with time, not decreasing. If the mass of the ejected shells was greater than the mass accumulated by the white dwarf, then the white dwarf would decrease its mass with time and not become a supernova by reaching the Chandrasekhar limit.
If a type Ia supernova explosion occurs within 1,000 parsecs (3,260 light-years) of Earth, then the gamma radiation emitted by the supernova would fry Earth, dumping as much gamma radiation (about 100,000 ergs per square centimeter) into our planet, which is equivalent to the gamma-ray input of 1,000 solar flares simultaneously. The production of nitrous oxides in Earth's atmosphere by the supernova's gamma rays would completely destroy the ozone layer if the supernova went off within 1,000 parsecs.