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1. Introduction

SiO, tex2html_wrap_inline1844, and OH masers have been found to be associated with stars on the Asymptotic Giant Branch (AGB), as well as with post-AGB stars. The masers are located in the circumstellar envelopes, with the SiO masers closest to the star (and possibly even inside the expanding envelope) and the OH masers furthest away. All three types of masers are usually found around stars presently losing mass, e.g. Mira variables and OH/IR stars. However, when the mass loss stops the tex2html_wrap_inline1844 and SiO masers disappear, whereas the OH masers remain active also during the proto-planetary (PPN) and early planetary nebula (PN) stages (Lewis 1989).

OH and tex2html_wrap_inline1844 masers have been studied in several surveys towards late-AGB and post-AGB objects (e.g., Likkel 1989; Hu et al. 1994), but relatively little work has been done on the late evolution of SiO masers. Gomez et al. (1990) observed SiO (v=1, J=1-0) and tex2html_wrap_inline1844 masers in OH/IR stars and a few PPNe and found that the detection rate of both masers drops with increasing IRAS [25-12] color, which is believed to be an evolutionary stage indicator. Observations of SiO masers in OH/IR stars were also done by Jewell et al. (1991), and Nyman et al. (1986; 1993a). SiO masers have only been found towards four objects classified as PPNe, and they are discussed in Sect. 5.

To investigate more thoroughly the evolution of SiO masers we have searched for SiO emission towards a large sample of OH/IR stars with cool envelopes, oxygen-rich PPNe, and young PNe. The search was done in three different transitions, SiO (v=1 and 2, J=1-0) at 43 GHz, and SiO (v=1, J=2-1) at 86 GHz, because, although all transitions produce bright masers in Mira variables, there are indications that the SiO (v=1, J=1-0 and J=2-1) masers are underluminous in OH/IR stars with cool envelopes (Nyman et al. 1993a).


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