Unlike OH and masers, SiO masers are believed to be located not in the
expanding circumstellar envelope, but in a region called the extended
atmosphere, i.e., between the photosphere of the star and the dust
formation point. This is a complex region with both outflowing and infalling gas
as seen in infrared lines of OH and
(Hinkle et al. 1982),
and there is also evidence
of periodic shocks, driven by stellar pulsations, passing through
(Willson 1982). There are several arguments supporting the view that
SiO masers are located
here:
Observational evidence that SiO masers appear when the star first starts to pulsate is given by Haikala et al. (1994). In their Fig. 6a the number of all detected SiO masers as function of IRAS LRS class is shown. A large number of SiO masers are found in classes 14-17 with a peak at class 15, but no masers were found in classes 18 and 19. Van der Veen & Habing (1988) showed that objects in LRS classes 17-19 are normally non-variable stars with photospheric temperatures larger than 2000 K, whereas classes 14-16 consist of variable stars with a small amount of circumstellar material. This indicates that there is a close correlation between variability and the probability of finding SiO masers.
During the subsequent evolution along the AGB, SiO masers have a very high detection rate, and they are found in Mira variables and OH/IR stars with the characteristics given above. Hall et al. (1990) found a strong correlation between the presence of SiO masers and the optical variability of the central star. In their sample, stars with a visual amplitude larger than 2.5 mag were almost invariably detected in SiO.
In this work we show that also at the end of the AGB there is a close correlation between variability and the presence of SiO masers. SiO masers were found only towards variable objects, including the detected PPNe, but with the possible exception of OH 15.7 (tentatively detected, Sect. 5) which however is an object that only very recently may have evolved into a PPNe and shows irregular mass loss.
It is not evident how variability and SiO masers are connected. Most likely it is the variability that produces the extended atmosphere through mass motions. Here the conditions are favorable for the formation of SiO masers. When the variability decreases, or even stops, the conditions in the masing region change, prohibiting strong SiO masers. Since the latter most probably is a very sensitive process, it is not clear how dramatic the changes in the physical conditions are required to be to give this effect. It may be, once our understanding of the SiO maser mechanism has improved, that the SiO masers can be used as sensitive probes of the changing atmospheric conditions during the very last phase of AGB evolution. Apart from the disappearance of mass motions, also the increase in effective temperature of the central star during the post-AGB evolution will eventually increase the photodissociation rate of the SiO molecules, making the existence of masers impossible.
Acknowledgements
This research has made use of the Simbad database, operated at CDS, Strasbourg, France. We thank the referee, J. Alcolea, for helpful comments that improved the paper. The Swedish-ESO Submillimetre Telescope, SEST, is operated jointly by ESO and the Swedish National Facility for Radioastronomy, Onsala Space Observatory at Chalmers University of Technology. Parkes Observatory is part of the Australia Telescope which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.