Astronomers Solve the Case of the Unknown Star Explosion, Discover Rare Coupling of Elements
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 9, 1999
Greenbelt, Md. -- Astronomers have pieced together the scene of a crime that no one saw: a 700-year-old star explosion nearly as bright as a full moon, undocumented by early stargazers and unknown to modern-day astronomers until only very recently. Clues came in the form of two radioactive elements never before seen together in such explosions, a rare event that offers a new avenue to test star explosion theories.
Drs. Wan Chen and Neil Gehrels at NASA's Goddard Space Flight Center, Greenbelt, Md., calculated the explosion source to most likely be a star 15-times more massive than the Sun and a mere 500 light-years away. The star, the astronomers said, must have depleted its nuclear fuel, experienced a core collapse, and exploded.
While the fact that no one documented this explosion until last year is interesting in itself, Drs. Chen and Gehrels were intrigued to find that radioactive titanium and aluminum detected in the region came from the same source. Previously, astronomers assumed they could only see one element or the other in an explosion remnant.
"Different models of supernova explosions offer different yields of titanium, aluminum and other elements," said Dr. Chen. "When we have one source producing both titanium and aluminum emission, it provides tighter calculation constraints, and we can pretty much figure out what kind of star exploded."
The astronomers' calculations were based on X-ray light from hot gas in the remnant and gamma rays produced by decaying titanium-44, detected in 1998. Gamma ray astronomers had known about decaying aluminum-26 in the region as early as 1992 and assumed the emission came from the Vela supernova, an older and more powerful, more distant explosion. Drs. Chen and Gehrels' interpretation of the older data showed that the aluminum-26 most likely originated in the more recent explosion, and this supported their initial calculations.
Dr. Chen said no one ever observed radioactive decay from two elements together in the same star (supernova) explosion because the elements' half-lives (the time required for half the amount of an element to undergo radioactive decay to a new form) are so different. The half-life of titanium, for example, is 60 years; for aluminum, it is 700,000 years. That means that after an explosion, decaying titanium is plentiful, but decaying aluminum is not. After about a thousand years, most of the titanium is gone. So to see both elements at the same time, the explosion must be close enough to Earth to detect the small traces of decaying aluminum and recent enough to detect decaying titanium before it vanishes.
"The rate for supernovae in the entire Galaxy is about one in every 50 to 100 years," said Dr. Chen. "If 'close' means within 1,500 light-years from Earth, then given the size of the Milky Way, we'll have to wait about 20,000 years for the next close event."
The newly discovered explosion is now known as supernova remnant GRO/RX J0852. The source has shown astronomers that rare events do exist, and that other supernova remnants could harbor the remains of other combinations of radioactive elements, which could further bolster supernova modeling.
Dr. Anatoli Lyudin of the Max Planck Institute of Extraterrestrial Physics led an international team in first observing titanium emission from GRO/RX J0852 region in 1998 with the COMPTEL gamma-ray telescope on NASA's Compton Gamma Ray Observatory. Dr. Bernd Aschenbach, also at Max Planck, first observed the remnant in X-rays with instruments aboard the German-operated ROSAT satellite (a German/US/UK joint venture). Drs. Chen and Gehrels based their calculations on these data.
Astronomers had never noticed the elusive remnant because a massive fiery gas cloud behind it from yet another supernova explosion, the 11,000-year-old Vela remnant, overshadowed it. Observing the region in a higher X-ray energy band brought out the features of the younger explosion. Likewise, observing the region in gamma rays revealed only the radioactive titanium from GRO/RX J0852, for Vela had long since lost its titanium.
The supernova explosion was close enough within the Milky Way to be seen by medieval astronomers near or below the equator for months, even during daylight hours. Yet there is no written record of the event. Dr. Chen is collecting theories from other astronomers on why this happened. These theories include, among many, cultural upheaval in Central and South America, social unrest and war in China, or simply records lost over time.
"It's a real mystery," said Dr. Chen. "This supernova was very bright. People had to have seen it, but we haven't found any written records yet."
The results of Drs. Chen and Gehrels' work appear in the April 1 issue of the Astronomical Journal and are presented at the April 15 meeting of the High Energy Astrophysics Division of the American Astronomical Society.
-30-