Feb. 5, 2001 Champaign, IL — Hot gas from a shocked stellar wind is responsible for the complex shape of a planetary nebula known as the Cat’s Eye, say astronomers using NASA’s Chandra X-ray Observatory. In addition to identifying the hot gas by its telltale X-rays, the scientists also found a surprising X-ray source at the central star in the planetary nebula.
“The X-ray data reveal a bright central star surrounded by a cloud of multimillion-degree gas,” said You-Hua Chu, a professor of astronomy at the University of Illinois. “Studying the X-ray emission from the Cat’s Eye may provide insights on the formation of planetary nebula and the deaths of stars.” The Cat’s Eye Nebula has one of the most intriguing and complex shapes among planetary nebulae, Chu said. “Optical images reveal an inner elliptical shell surrounded by an envelope with multiple, interlocking lobes bounded by sharp, filamentary structures. Despite the complex appearance of the Cat’s Eye, the X-ray emission shows that hot gas is driving the expansion.”
A planetary nebula consists of shells of gas thrown off by a red giant star as it nears the end of its life. A fast stellar wind emanating from the remaining hot core then slams into the ejected gas, pushing it outward and creating the graceful knots, bubbles and filaments seen in optical telescopes. “In the Cat’s Eye, the stellar wind is traveling at about 4 million miles per hour,” Chu said. “When this ‘fast wind’ encounters gas that was previously expelled, it is shock-heated to hundreds of millions of degrees, generating the X-rays we see with Chandra.”
To better understand the physical structure and formation mechanism of the Cat’s Eye, Chu and her colleagues – Martin Guerrero, Robert Gruendl and James Kaler at the UI, and Rosa Williams at the NASA Goddard Space Flight Center – compared the Chandra X-ray image with an optical image taken with the Hubble Space Telescope.
“The diffuse X-ray emission showed excellent correspondence with some of the optical features in the denser and cooler outer shell,” Chu said. “The thermal pressure of the hot gas is about twice as high as the pressure in the cool nebular shell. Clearly, the hot gas is playing an essential role in the structure and evolution of the Cat’s Eye Nebula.”
The Chandra observations also revealed a previously unknown X-ray source at the central star. “This was unexpected, because the star’s temperature is only about 50,000 degrees Celsius, which is too cool to be emitting X-rays,” Chu said. “We don’t yet understand how these X-rays are being formed.”
The Cat’s Eye Nebula, also known as NGC 6543, is about 3,000 light years from Earth. The nebula was formed about 1,000 years ago and is currently about 400 times larger than the Solar System. Chu presented her team’s findings at the national meeting of the American Astronomical Society, held Jan. 7-11 in San Diego. A paper has been submitted to the Astrophysical Journal.
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