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The chameleon supernova ANGLO-AUSTRALIAN OBSERVATORY NEWS RELEASE Posted: May 6, 2006 Using the Gemini South telescope in Chile, Australian astronomers have found a predicted "companion" star left behind when its partner exploded as a very unusual supernova. The presence of the companion explains why the supernova, which started off looking like one kind of exploding star, seemed to change its identity after a few weeks.
Renowned Australian supernova hunter Bob Evans first spotted supernova 2001ig in December 2001. It lies in the outskirts of a spiral galaxy, NGC 7424, which is about 37 million light-years away in the southern constellation of Grus (the Crane). The supernova was monitored over the next month by optical telescopes in Chile. Supernovae are classified according to the features in their optical spectra. SN2001ig initially showed the telltale signs of hydrogen, which had it tagged as a Type II supernova, but the hydrogen later disappeared, which put it into the Type I category. But how could a supernova change its type? Only a handful of such supernovae, classified as "Type IIb" to indicate their curious change of identity, have ever been seen. Only one (called SN 1993J) was closer than SN 2001ig. Astronomers studying SN1993J had suggested an explanation: the supernova's progenitor had a companion star that stripped material off the star before it exploded. This would leave only a little hydrogen on the progenitor - so little that it could disappear from the supernova spectrum within a few weeks. A decade after SN1993J appeared, observations with the orbiting Hubble Space Telescope and one of the Keck telescopes in Hawai'i confirmed that it did indeed have a companion. Ryder and colleagues wondered if SN2001ig might have had a companion as well. Radio light curve shows lumps & bumps Radio observations also hinted at a companion. Soon after SN2001ig was discovered, Ryder and his colleagues began monitoring it with a radio telescope, the CSIRO (Commonwealth Scientific and Industrial Research Organisation) Australia Telescope Compact Array in eastern Australia. The radio emission did not fall off smoothly over time but instead showed regular bumps and dips. This suggested that the material in space around the star that exploded - which must have been shed late in its life - was unusually lumpy. Although the lumps might have represented matter periodically shed from the convulsing star, their spacing was such that another explanation seemed more likely: that they were generated by a companion in an eccentric orbit. As it orbited, the companion would have swept material shed by the progenitor into a spiral (pinwheel) pattern, with denser lumps at the point in the orbit - periastron- where the two stars approached most closely. Such spirals have been imaged around hot, massive stars called Wolf-Rayet stars by Dr. Peter Tuthill of the University of Sydney, using the Keck telescopes. The bumps in the radio light-curve of SN2001ig were spaced in a way consistent with the curvature of one of the spirals Tuthill has imaged. "Stellar evolution theory suggests that a Wolf-Rayet star with a massive companion could produce this unusual kind of supernova," said Ryder. If the supernova progenitor had a companion, it might be visible when the supernova debris had cleared. So the astronomers put in a request to observe with the GMOS (Gemini Multi-Object Spectrograph) camera on the 8-meter Gemini South telescope. When the time came to observe, the "seeing conditions" (stability of the atmosphere) were excellent. Just an hour and a half was needed to image the supernova field - and reveal a yellow-green point-like object at the location of the supernova explosion. "We believe this is the companion," said Ryder. "It's too red to be a patch of ionized hydrogen, and too blue to be part of the supernova remnant itself." The companion has a mass of between 10 and 18 times that of the Sun. The astronomers hope to use GMOS again in coming months to get a spectrum of the companion, to refine this estimate. Binary companions could explain much of the diversity seen in supernovae, Ryder suggests. "We've been able to show the chameleon-like behavior of SN2001ig has a surprisingly simple explanation," he said. This is only the second time a companion star to a Type IIb supernova has been imaged, and the first time the imaging has been done from the ground. A paper on the observations, "A post-mortem investigation of the Type IIb supernova 2001ig", co-authored by Ryder, University of Tasmania graduate student Clair Murrowood and former AAO astronomer Dr Raylee Stathakis, was published online in Monthly Notices of the Royal Astronomical Society on May 2. |
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The flight of Spacelab 3 aboard Challenger in April/May 1985 was a week-long scientific research mission using a laboratory tucked in the shuttle's payload bay. Experiments focused on material and fluid behaviors in weightlessness, plus observations of monkeys in the lab. The crew also watched amazing auroral displays over Earth. This post-flight crew film shows the highlights of STS-51B and includes remarkable views out the shuttle cockpit window during launch showing the Chesapeake Bay, New York City and Cape Cod as Challenger soared up the eastern seaboard.
Discovery launched April 12, 1985 on the STS-51D mission. A U.S. military communications satellite, known as Leasat 3, failed to activate after its deployment from the payload bay. That set the stage for a spacewalk -- the shuttle program's first unplanned EVA -- to attach handcrafted "Flyswatter" objects on the shuttle robotic arm to hit a timing switch on the satellite. The rescue attempt did not succeed. Upon landing at Kennedy Space Center, Discovery blew a tire. The crew, including Senator Jake Garn of Utah, narrate this post-flight film of highlights from the week-long mission.
NASA managers hold this news conference April 28 to give an update on plans for the next space shuttle mission, the ongoing external fuel tank testing and debates over further modifications.
The Boeing Delta 2 rocket carrying the CALIPSO and CloudSat atmospheric research spacecraft lifts off at 3:02 a.m. local time April 28 from Vandenberg Air Force Base, California.
Inside the Vehicle Assembly Building, the external fuel tank for the STS-121 space shuttle mission is hoisted into position for attachment with the twin solid rocket boosters atop a mobile launch platform. The tank, ET-119, will carry the liquid oxygen and liquid hydrogen to feed Discovery's three main engines during launch.
In preparation for space shuttle Discovery's departure from its Orbiter Processing Facility hangar for rollover to the Vehicle Assembly Building and mating with the tank and boosters, the ship's 60-foot long payload bay doors are swung shut.
Take a virtual ride aboard the Russian Progress 21P cargo freighter as it docks with the International Space Station. This movie captures the final approach and successful linkup from a camera on the Progress craft's nose.