OH masers, planetary nebulae and butterfly wings

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. In the asymptotic giant branch (AGB) stage most stars shed a large fraction of their mass through slow, dense stellar winds. After most of the outer layers of gas have blown away, stars change from losing mass in slow dense winds to losing mass in much hotter, faster 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 exert 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 so-called post-AGB stars are in an intermediate stage of evolution between the AGB and planetary nebulae. The different geometries seen in planetary nebulae are also evident 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 complicated structures seen in some planetary nebulae.

To investigate this question, we have selected a well-defined sample of 86 post-AGB stars from a previous study of OH 1612-MHz maser sources in the Galactic plane (Sevenster et al., 1997; 2001). For this sample, we are observing radio emission from hydroxyl (OH), water (H₂O) and silicon-monoxide (SiO) molecules that are located in the outflowing stellar winds. Each molecule requires different physical conditions to exist and the different molecules together probe a range of locations within the circumstellar winds. The molecular radiation is produced by a maser effect - the microwave equivalent of lasers. Maser spectra provide information on the wind velocities, and on whether the winds are likely to be spherically symmetrical or distorted.

So far we have observed 85 of the 86 sources in our sample in the four ground-state OH maser transitions at 1612, 1665, 1667 and 1720 MHz. The data were taken using the Parkes radio telescope in several observing sessions between September 2002 and February 2003. All observations were made with the H-OH receiver, with the multibeam correlator backend configured with 8192 channels and one beam. A linear feed with a radio frequency hybrid was used to generate two circular polarizations. A bandwidth of 4 MHz was used for all observations. After Hanning smoothing, the final velocity resolution of the spectra was 0.18 km/s per channel.

Figure 1 shows examples of OH 1612-MHz maser spectra for sources in our sample. We have classified the OH 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. We also find a small number of "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 irregular spectra have previously been associated with exotic envelope geometries.

Figure 1: Some examples of OH 1612-MHz spectra of post-AGB stars, obtained from data taken with the Parkes radio telescope in September 2002. The catalogue name of each star is given in the top right-hand corner with a category type given below the name. The 85 stars in this study have been classified into six different types depending on whether the OH maser profiles are double (D, De, Dw and DD-types), single (S-type) or irregular (I-type).

From our Parkes data we have also detected 1720-MHz maser emission towards several sources. Higher resolution follow-up observations are needed to determine whether these are associated with the stars or the interstellar medium. OH 1720-MHz maser emission has so far been confirmed for only one post-AGB star (Sevenster and Chapman 2001).

Table 1 lists the number of sources detected in the OH 1612, 1665 and 1667-MHz lines, for each category. The fraction of sources detected at the 1665 and 1667-MHz lines are around 25% and 50% respectively. (As this is an OH 1612-MHz selected sample, all sources have 1612-MHz emission). A significant result is that of the 1665-MHz detections, 64% are classified as irregular, in contrast to the OH 1612 and 1667-MHz spectra where 13% and 29% of maser spectra are found to be irregular.

Sevenster (2002a,b) has recently discussed the far-infrared properties of a group of evolved stars referred to as the `LI' sources. These are thought to be unusually massive stars and the precursors to the more extreme bipolar planetary nebulae. From our OH data we find that none of the LI objects was detected at 1665 MHz, with only a few detections at 1667 MHz.

As future work, we plan further observations to obtain spectra of the H₂O and SiO maser lines, and high-resolution OH-polarization imaging for some of the more unusual sources.

References

Sevenster, M. N., Chapman, J., Habing, H., Killeen, N., Lindquist, M., 1997, A&AS, 124, 509
Sevenster, M. N., Van Langevelde, H., Chapman, J., Habing, H., Killeen, N., 2001, A&A, 366, 481
Sevenster, M. N. and Chapman, J. M., 2001, ApJ, 546, L119.
Sevenster M. N., 2002a, AJ, 123, 2772
Sevenster M. N., 2002b, AJ, 123, 2788

Rachel Deacon, Jessica Chapman and Anne Green
(Rachel.Deacon@.csiro.au)