T Pyxidis Soon To Be A Type Ia Supernova



by Staff Writers



Washington DC (SPX) Jan 05, 2010



Astronomers 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 is being presented by Drs. Edward M. Sion, Patrick Godon and student Timothy McClain of Villanova University at the 215th meeting of the American Astronomical Society in Washington, DC.

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 which has been used to demonstrate that the expansion of the universe is accelerating.

The close binary system T Pyxidis, located in the southern hemisphere constellation Pyxis ("the Compass Box"), is known as a recurrent nova because its massive white dwarf star has suffered thermonuclear (nova) explosions approximately every 20 years with its previous recorded nova explosions occurring in 1890, 1902, 1920, 1944 and 1967, making it 44 years overdue for its next thermonuclear explosion. Nobody understands why it is has stopped its thermonuclear explosions.

The thermonuclear explosions are triggered by hydrogen-rich gas transferred to the white dwarf star by the very close Sun-like star. An extremely 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, then it will eventually reach the so-called Chandrasekhar Limit and will undergo instantaneous gravitational collapse resulting in an unimaginably powerful thermonuclear detonation which completely destroys the white dwarf and leaves no stellar remnant such as a pulsar (i.e., spinning neutron star) or a black hole.

This catastrophic event, known as a Type Ia supernova (or "white dwarf supernova"), releases ten million times more energy than a nova explosion or is equivalent to twenty 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 that the system is at a distance within only 1,000 parsecs (3,260 light years) which is far closer to our solar system than anyone previously thought.

The theoretical model which 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 degrees and a rate of mass accretion by the white dwarf of 2 x 10^17 grams/second (3 x 10^-9 solar masses/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 is lower and the white dwarf mass even closer to the Chandrasekhar limit.

The closer distance means that 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 extremely 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.

An interesting, if a bit scary, speculative sidelight is that if a Type Ia supernova explosion occurs within 1,000 parsecs (1 parsec = 3.26 light-years) of Earth, then the gamma radiation emitted by the supernova would fry the Earth, dumping as much gamma radiation (~100,000 ergs/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.