Determining the shapes of dying stars

A University astronomer has for the first time determined the shapes of newly formed shells of dying stars. Susan Trammell, a Grainger Postdoctoral Fellow in Astronomy & Astrophysics, presented the results of her research last week at the American Astronomical Society meeting in Tucson, Ariz.

Trammell was trying to answer some long-standing questions in stellar evolution: Why do these shells -- called planetary nebulae -- come in shapes that vary from near-perfect spheres to very elongated hourglasses? When is the shape determined? And what do the different shapes tell about the life cycles of stars?

"The development of a planetary nebula is a basic part of stellar evolution that astronomers do not yet fully understand," Trammell said. "This experiment may shed some light on that process."

Trammell used a technique called optical spectropolarimetry to "see" the shapes of distant planetary nebulae far too small to be imaged clearly by any ground-based telescope. Optical spectropolarimetry had never before been used to study these dying stars.

Planetary nebulae are the end stage in the evolution of such stars as our sun. Called "planetary" nebulae because of their resemblance to planets when early astronomers viewed them through small telescopes, "planetaries" are actually shells of gas and dust expelled when a star that has run out of fuel expands and cools off.

These shells slowly expand outward billions of miles into space, leaving behind a remnant core -- called a white dwarf -- that is so dense a teaspoon of it would weigh a ton. What puzzles astronomers is that the shapes of these outer shells can vary from spherical to highly asymmetric. Clues as to why planetary nebulae adopt these different shapes may lie in the stars that created them, or in the factors that influenced their evolution.

"We don't understand when and how the asymmetry develops," Trammell said. "Theory suggests that the shapes are determined very early in the formation of a planetary nebula, but, unfortunately, there is little observational evidence to support this claim. We don't understand the early evolution of the planetaries or how the early mass loss shapes their later evolution."

Trammell and her colleagues sought to learn more about the evolutionary process by studying stars that were on the verge of becoming planetary nebulae. The stars they studied were within the Milky Way, at a distance of 3,000 to 9,000 light-years from Earth.

"These young planetaries are very small, so you can't just take a picture to see what they look like," said Trammell. "We knew these stars were in this particular stage of evolution, and the question was, what's their shape? If we saw aspherical structure in these objects, we would know it develops early in the formation of a planetary, not later on."

To determine the shapes of these young planetaries, Trammell used optical spectropolarimetry.

Trammell explained that the dust grains in a planetary nebula are like tiny mirrors that reflect light. When light is reflected by a dust grain, the light becomes polarized. "If the dust grains are distributed in a spherical pattern in these spatially unresolved objects, the polarizations from these different mirrors cancel each other out," Trammell explained. Spherical nebulae would appear unpolarized, while aspherical shells would have a net polarization.

Trammell and her colleagues used the Texas spectropolarimeter on the 107-inch telescope at the University of Texas McDonald Observatory. They used it to observe 31 spatially unresolved planetary nebulae and found that 14 of them were spherical and 17 were asymmetric. Trammell said most of these asymmetric nebulae are probably elliptical, but two had very aspherical, hourglass shapes -- specifically GL1403 in the constellation Leo Minor and IRAS 8005-2356 in the constellation Puppis.

"These findings support the idea that aspherical structure is present early on in the planetary-nebula formation process," said Trammell. "The confirmation of theory and a new knowledge of the shapes of these objects will lead to a better understanding of the early phases of the development of a planetary nebula."

Trammell would eventually like to use the Hubble Space Telescope to create images of the planetaries to confirm her polarimetry results.

She collaborated on the project with Harriet Dinerstein, who had been her thesis adviser at the University of Texas, and Robert Goodrich of the Space Telescope Science Institute. The work was funded by the National Science Foundation.

Trammell received her Ph.D. from the University of Texas in September 1994 and came to Chicago in October as the recipient of a two-year Grainger Postdoctoral Fellowship in Experimental Physics. She plans to continue studying planetary nebulae, as well as star formation, using infrared polarimetry and imaging techniques.

-- Diana Steele