Planetary Nebula Survey By Chandra
October 11, 2012

Astronomers Using Chandra To Survey Nearby Planetary Nebulae

April Flowers for - Your Universe Online

An international team of astronomers using NASA's Chandra X-ray Observatory has created a sweeping new survey of dying stars — or planetary nebula — in the neighborhood of the Sun. The resulting X-ray images shed light on the violent "end game" of a Sun-like star's life.

Led by Joel Kastner from the Rochester Institute of Technology (RIT), the team won seven days of observation time with Chandra in 2011-2012 to survey and image nearly two dozen nearby planetary nebulae. This resulted in the most comprehensive X-ray survey to date for such objects. The team has won a further eight-day time award to use Chandra in continuing this survey, and will begin collecting more X-ray data later this year.

Both observation series are part of Chandra's X-ray Survey of Planetary Nebulae (ChanPlaNS). Planetary nebulae astronomers from seven countries teamed up to win the large Chandra observing time awards.

A dying red giant star that has cast off its outer layers is considered a planetary nebula. The newly exposed, hot core of the star illuminates these ejected layers. At the same time, the core's fast winds sculpt the material into a variety of shapes which are favorite targets of optical and near-infrared telescopes with names like Cat's Eye, Lemon Slice, and Blue Snowball. The core of this dying star will eventually become a white dwarf.

The image above shows four of these fantastically shaped planetary nebulae. These four are NGC 6543, known as the Cat's Eye, and NGC 7662, NGC 7009 and NGC 6826. Chandra's X-ray data is shown in purple with optical emissions observed from the Hubble Space Telescope in red, green and blue.

“Planetary nebulae have provided astrophysicists with dying star ℠laboratories´ for more than a century,” Kastner says. “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.”

This uncertainty has led to debate amongst astrophysicists and resulted in Kastner and his team making the request for a large allocation of X-ray satellite time to investigate the processes of stellar death and wind collisions that lead to these shaping processes.

“An X-ray survey of this kind is completely uncharted territory in the planetary nebula world,” Kastner adds. “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 X-ray satellite is providing an "under the hood" look at planetary nebulae. X-rays cut through the illuminated gas and dusts, thus allowing astronomers to investigate the last tens of thousands of years of the dying star's history.

“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,” Kastner says.

At the beginning stages of the project, which has been reported in The Astronomical Journal, the research team collected data for 35 planetary nebulae all within roughly 5,000 light years of the Sun. Of these 35, 21 were previously unobserved and 14 were pulled from Chandra's archival data. The new time award will bump the number of nebulae in the study up to 59 out of the roughly 120 identified planetary nebulae within this distance.

“Because they all just happen to lie relatively nearby, we think this group of objects is fairly representative of planetary nebulae in general,” Kastner says.

Within these findings, theorists will find material to refine current models describing the mechanisms that shape planetary nebulae, especially the potential influence of a stellar or planetary companion to the dying star.

About one-half of the planetary nebulae observed show X-ray point sources at their center. All but one of these point sources show high energy X-rays that may be caused by a companion star. This suggests that a high frequency of central stars responsible for ejecting planetary nebulae have companions. Future studies will be needed to determine the structure and evolution of the nebulae.

“The ChanPlaNS study provides fresh new insights into the last, dying gasps of stars like the Sun,” Kastner says. “We expect it will clarify what planetary nebulae can tell us about binary star astrophysics and stellar wind interactions.”