Posted: October 06, 2008
Spitzer scientists studying hot spots near the Cassiopeia A supernova remnant say they are light echoes that contain the memory of the blast’s very first moments.
In the October 1 edition of the journal The Astrophysical Journal, scientists Eli Dwek of NASA's Goddard Space Flight Center in Greenbelt and Richard Arendt of the University of Maryland say these light echoes are powered by radiation from the supernova shock wave that blew the star apart some 11,000 years ago. "We're seeing the supernova's first flash," says Dwek.
The Cassiopeia A supernova's first flash of radiation makes six clumps of dust (circled) unusually hot. Credit: NASA/JPL-Caltech/E. Dwek and R. Arendt.
The hot spots are made up of hot dust, and the scientists used Spitzer to study these glowing regions in more detail. Six knots of silicate dust near the remnant show temperatures between -173° and -123° Celsius, hot compared to typical interstellar dust. Dwek and Arendt claim that the only event that could make the grains this hot is the powerful and short-lived pulse of ultraviolet radiation and X-rays that heralded the death of the star.
"They've identified the precise event during the demolition of the star that produces the echo we see," says Michael Werner, the Project Scientist for Spitzer at NASA's Jet Propulsion Laboratory.
When a massive star, greater than about eight times the mass of the Sun, runs out of nuclear fuel, its core collapses into a superdense neutron star. As the neutron star forms, it stiffens and rebounds. “The bounce produces a shock wave that travels through the star,” explains Dwek. “When the shock breaks out through its surface it creates an intense flash of X-ray and ultraviolet radiation, one hundred billion times brighter than the Sun, but this flash lasts for less than a day or so.”
Light from the explosion reached Earth in the 17th century, but no one noticed, and so the Spitzer find gives astronomers a second chance to study the supernova as it unfolds. Although the initial explosion originally escaped detection, its aftermath - a hot, expanding gas cloud known as Cassiopeia A (Cas A, for short) - is one of the best-studied supernova remnants. The blast zone lies 11,000 light-years away in the constellation Cassiopeia.
“The echoes offer us a glimpse of an extremely rare astronomical event lasting for the equivalent of a blink of the eye in a human lifetime,” says Dwek. “Analysis of such events can tell us a lot about the structure of the star before it exploded, its size and density profile, and on the structure of its immediate surroundings. Understanding such stars is extremely important because these supernova explosions disperse into space the heavy elements (carbon, silicon, iron, etc) that were cooked in their interior or made in these violent explosions. These elements provide the raw material out of which planets and ultimately living beings are formed.”
Cassiopeia A is among the best-studied supernova remnants. This image blends data from NASA's Spitzer (red), Hubble (yellow), and Chandra (green and blue) observatories. Credit: NASA/JPL-Caltech/STScI/CXC/SAO.
The infrared echoes from Cas A arise from dust clouds about 160 light years farther away than the remnant. Once heated, the dust's infrared energy had to make up the same distance, so the extra travel time results in a 320-year offset between the supernova's initial outward-moving flash and arrival of the dust's infrared echo at Earth.
“The flash of light propagates through a given cloud, and how long a given cloud remains visible depends on how long it takes the light pulse to travel through it, that is, the size of the cloud,” says Dwek. “Some small clouds have disappeared after a year or so, indicating that their size is less than about one light year. However, many such clouds are part of larger complexes which have therefore remained visible for over thirty years. These complexes may remain visible for decades in the future. However, the medium around the exploding star has many clouds around it, located at different distances from the supernova. So after traversing one cloud the pulse of light may encounter at some later time, which depends on the distance of the cloud from the supernova, another different cloud and light that one up. So light echoes will be appearing and disappearing at unpredictable locations in the sky, much like the twinkling lights on a christmas tree.”