R. Deacon (University of Sydney); J. Chapman (ATNF); A. Green (University of Sydney)

Planetary nebulae are the remnants of low-to-middle mass stars. When stars, like our own Sun, run out of core-burning hydrogen, they go through a sequence of structural changes. The outer layers greatly expand while the stellar cores decrease in size and become hotter. Of particular interest in the giant stages is that most stars shed a large fraction of their mass through slow, dense stellar winds. Such winds transfer more than half of a star's original mass back into the interstellar medium. After most of the outer layers of gas have been blown away, stars change from losing mass in slow dense winds to losing mass in hot, low-density, fast winds. The hot winds sweep up the remaining material surrounding the stars and the swept-up shells may then be seen as glowing ionised nebulae. Stars in this stage are known as planetary nebulae.

Planetary nebulae can be spectacularly beautiful and reveal many different forms. While some appear circular, others have the shapes of butterfly wings, or show elliptical or bipolar shapes, in some cases with complex, filamentary structures. The causes for this proliferation of geometries are not yet known, but several different theories are being debated. One possible explanation is that magnetic fields from the stars constrain the stellar winds to flow in some directions only. Alternatively, companion stars or planets may cause gravitational effects, or rotation of the central stars may be important.

We are studying a sample of stars that are the immediate precursors to planetary nebulae. These stars are known as post-asymptotic giant branch (post-AGB) stars and have evolved beyond the second giant phase known as the AGB. The different geometries seen in planetary nebulae are also seen in post-AGB stars and it seems likely that the shaping of non-spherical planetary nebulae winds begins early in the post-AGB phase of stellar evolution. By studying the post-AGB stars we may be able to determine what causes the butterfly-wing geometries and other complicated structures seen in some planetary nebulae.

To investigate this question, we have selected a well-defined sample of 85 post-AGB stars from a previous study of OH maser sources in the Galactic plane (Sevenster et al.). For this sample, we are observing radio emission from hydroxyl, water and silicon monoxide molecules that are located in the outflowing stellar winds. Each molecule requires different physical conditions to exist and provides information on the physical gas conditions in a layer around the central star. The radiation from these molecules is produced by a maser effect - the microwave equivalent of lasers. The maser spectra also provide accurate information on the wind speeds, and on whether the winds are likely to be spherically symmetrical or distorted.

Figure 1 shows examples of OH maser spectra, for sources in our sample, at a frequency of 1612 MHz. We have classified the spectra using six different categories, depending on the shape of the spectral profiles. Of the 85 sources, 57 exhibit a classic double-peaked spectrum ("D-type") at 1612 MHz, with steep outer edges and sloping inner edges between the two peaks. This spectral profile is characteristic of an expanding spherical shell, with the two peaks being emitted from small caps on the front and rear of the shell. In our sample we see several variations on the classic double-peaked profiles: The "De" spectra have one emission peak which is very much stronger than the other this shows that the maser emission is much stronger on one side of the star than on the other. The "Dw" stars have OH 1612-MHz spectra with sloping outer edges as well as sloping inner edges. These are expected to be stars with bipolar shells. More unusual is the "DD" source with four emission peaks. Only one other source with this characteristic is known.

In our sample we also find five "S-type" sources. These show only a single peak of maser emission, but otherwise have characteristics in common with AGB and post-AGB stars. Finally the "I" or irregular spectra have multiple emission peaks and a larger than usual velocity range. Post-AGB stars with such irregular spectra are usually associated with exotic envelope geometries.

From our maser data, together with still-to-come measurements of the stellar magnetic fields and detailed radio images of the outflowing winds, we hope to clarify why some stars evolve to have asymmetric winds and others do not.

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