Spatial structure of the dust shells around dying stars

Collaborators

The question we're trying to answer is: How do stars, which are spherically symmetric (left), evolve into planetery nebulae with such complicated geometry (right, as imaged by other astronomers on the Hubble Space Telescope)?

We believe the answer lies on the Asymptotic Giant Branch (AGB), the evolutionary stage where a star ejects most of its envelope and enshrouds itself in a thick circumstellar dust shell. When the star has blown off enough of its mass, it exposes its hot interior, which will ionize the inner parts of the envelope, producing a planetary nebula.

It is possible that the AGB sources themselves are not spherically symmetric (despite the drawing above), or, as evidence suggests, the dust shells produced by these sources are asymmetric. An asymmetric dust distribution might lead naturally to the asymmetric geometries seen in planetary nebulae.

We are tackling this question by imaging sources on the AGB and sources which are in transition to planetary nebulae at mid-infrared wavelengths, where the dust shell shines most strongly. We plan to follow the imaging up with long-slit spectroscopy These techniques will enable us to study the geometry and chemistry of the circumstellar dust distribution around these sources. Not only are we searching for deviations from spherical symmetry, but we will try to understand what the geometry actually is and how the character of the dust changes with location within it.

The key to our observations is the ability to spatially resolve the dust shell. Generally, this is easier for supergiants than giants, because they are much, much larger. Despite the differences in the ultimate fate of supergiants (they will probably go supernova instead of producing a planetery nebula), recent theoretical work predicts that their evolution follows the same stages as lower mass stars on the AGB (although much more quickly). This similarity allows us to talk about a Super-AGB populated by higher mass stars. As the series of images from the Hubble Space Telescope below illustrate, the late stages of evolution of low and high mass stars look remarkably similar.

Stage:	I.	II.		III.		IV.
	Low mass star			NGC 7027		MyCn 18 (The Hourglass)
	High mass star			eta Carina		SN 1987A

Comments on the stages

II. The HST has resolved the disks of Mira (omicron Ceti), the prototypical AGB source, and Betelgeuse (alpha Ori), the prototypical red supergiant. Neither source appears symmetrical, although with these optical images it is difficult to distinguish between actual asymmetries of the star and apparent asymmetries resulting from a non-uniform dust distribution.

The sources we wish to image are between Stages II and III.

III. A young planetary nebula compared to supergiant in its pre-supernova phase. In both, the hot central source had illuminated the double-lobed dust shell from within.

IV. An evolved planetary nebula compared to a supernova remnant. In the former case, the multi-ringed structure results when a fast ionized wind plows into the older remnant envelope. In the latter case, we see very similar structure produced by the shock waves from a superova explosion. In both cases, the winds and shocks simply illuminate the remnant circumstellar dust envelope. Whether produced on the AGB or super-AGB, the morphology of the circumstellar dust shell appears to be the same.

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Last modified 10 July, 2000. © Gregory C. Sloan.