A multinational team of astronomers led by Dr Joel Kastner of Rochester Institute of Technology has captured stunning views of four planetary nebulae: NGC 6543, also known as the Cat’s Eye, NGC 7662 (Snowball Nebula), NGC 7009 (Saturn Nebula) and NGC 6826.
This image shows four planetary nebulae from the first systematic survey of such objects in the solar neighborhood made with NASA’s Chandra X-ray Observatory. The nebulae are NGC 6543, top left, NGC 7662, top right, NGC 7009, bottom left, and NGC 6826. X-ray emission from Chandra is colored purple, and optical emission from Hubble is colored red, green and blue (X-ray: NASA / CXC / RIT / J. Kastner et al; optical: NASA / STScI)
The team has combined observations made with NASA’s Chandra X-ray Observatory and the Hubble Space Telescope to shed light on the violent ‘end game’ of a Sun-like star’s life.
The astronomers won seven days of observing time with Chandra in 2011–12 to survey and image nearly two dozen relatively nearby planetary nebulae, resulting in the most comprehensive X-ray survey to date for such objects. They collected data for 35 planetary nebulae – 21 previously unobserved and 14 pulled from Chandra archival data – all within roughly 5,000 light years of the Sun. The same team recently won an eight-day time award with Chandra to continue its observing program, and will begin collecting new X-ray data later this year. In the new study, they will examine a total of 59 objects from among the roughly 120 planetary nebulae identified within this distance. Both the previous and upcoming series of observations are part of the Chandra X-ray Survey of Planetary Nebulae.
“Planetary nebulae have provided astrophysicists with dying star ‘laboratories’ for more than a century,” Dr Kastner said. “They provide test beds for theories of stellar evolution and give us insight into the origin of heavy elements in the universe and on Earth. Yet we still don’t fully understand why they take on such a dazzling variety of shapes.”
“An X-ray survey of this kind is completely uncharted territory in the planetary nebula world,” Dr Kastner explained. “Astronomers working in this area agreed that we need large quantities of time to look at as many planetary nebulae as possible, specifically with Chandra.”
The team is using X-ray imaging to look “under the hood” of planetary nebulae. X-rays cut through the illuminated gas and dust, allowing astronomers to investigate the last tens of thousands of years of history of the dying star that threw off its outer sheaths.
“With Chandra’s exceptional ‘X-ray vision,’ we can detect the million-degree plasma inside the discarded shells and probe the energies of the stellar winds that shape them,” Dr Kastner said. “Because they all just happen to lie relatively nearby, we think this group of objects is fairly representative of planetary nebulae in general.”
The new data reveal that the optical images of most planetary nebulae with diffuse X-ray emission display compact shells with sharp rims, surrounded by fainter halos. All of these compact shells have observed ages that are less than about 5000 years, which therefore likely represents the timescale for the strong shock waves to occur. These results were published in the Astronomical Journal (arXiv.org version).
About half of the planetary nebulae in the study show X-ray point sources in the center, and all but one of these point sources show high energy X-rays that may be caused by a companion star, suggesting that a high frequency of central stars responsible for ejecting planetary nebulae have companions.
Future studies should help clarify the role of double stars in determining the structure and evolution of planetary nebulae.
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Bibliographic information: J. H. Kastner et al. 2012. The Chandra X-Ray Survey of Planetary Nebulae (CHANPLANS): Probing Binarity, Magnetic Fields, and Wind Collisions. The Astronomical Journal 144, 58; doi: 10.1088/0004-6256/144/2/58
