Astronomers have for the first time observed a nova-producing system turn into a supernova, a finding that indicates the universe has more than one way to create a nova. A normal Type Ia supernova is a rare event, occurring perhaps once or twice every century. The type of supernova observed by a team of astronomers led by astronomer Ben Dilday of UC Santa Barbara is estimated to occur about one time in every 1,000 supernovae. The findings are important because supernovae are generally all considered to have the same intrinsic brightness, making them what astronomers call "standard candles" used for estimating distances across the cosmos. If some supernovae have different brightnesses because they have a different origin, that could lead to errors in distance measurements.
The new supernova, called PTF 11kx, was discovered initially by the Palomar Transient Factory, which uses a robotic telescope on the 48-inch Samuel Oschin Telescope at Palomar Observatory to scan the sky nightly looking for sudden increases in light that indicate the presence of a supernova or nova. Peter Nugent of the Lawrence Berkeley National Laboratory first observed PTF 11kx, which lies about 600 million light years away in the constellation Lynx, on Jan. 16, 2011. The signals were so unusual that Nugent and his Berkeley colleagues called for what is known as a "target of opportunity" observation with the Keck Telescope in Hawaii. "We basically called up a fellow UC observer and interrupted their observations in order to get time-critical spectra," Nugent said. The team has been observing the system regularly ever since. "For several months, almost every new observation showed something we had never seen before," Dilday said.
Indirect evidence has previously suggested that the majority of Type Ia supernovae arise from the collision of two white dwarf stars, producing a massive burst of light and energy. Regular novae, which produce far less energy, are produced from systems in which a white dwarf star -- essentially the corpse of a sun-like star -- orbits a red giant. Gas emitted from the red giant is captured by the white dwarf. When the gas builds up to a high enough concentration, a brilliant explosion occurs: a nova. The white dwarf then goes quiescent, accumulating more gas until another nova erupts, typically about 20 years later. Because the recurrent novae cause the white dwarf to lose more mass than is gained from the red giant, it was thought that such a dwarf could never accumulate enough to produce a supernova. But this system did, the team reported this week in the journal Science.
PTF 11kx is close enough to Earth that researchers could study it in more detail than most supernovae. The team observed that the system is surrounded by several shells of gas that were expanding outward at a rate too fast to be the result of solar wind, but too slow to have been caused by a supernova. Researchers have observed similar shells of gas around other stars, but have not been sure of their origin. The team now believes that they are the remnants of previous novae. The researchers reasoned that the expanding shells were slowing down as they encountered solar winds from the red giant star. If that were the case, the more energetic gases from the new supernova should catch up and collide with them.
When PTF 11kx went supernova, it produced a strong calcium signal, which is definitive of supernovae and partially how it was detected. As the team monitored the supernova, the calcium signal slowly subsided, as expected. Then, 58 days after the supernova was detected, the team once again received a strong calcium signal. The supernova's gases had, indeed, collided with the expanding shells, confirming the researchers' theory.
"Because we've looked at thousands of systems and PTF 11kx is the only one we've found that looks exactly like this, we think it is probably a rare phenomenon," said co-author Jeffrey Silverman of UC Berkeley. "However, these systems could be somewhat more common, and nature is just hiding their signal from us."
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