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A SEARCH FOR SIO EMISSION FROM P.N.

A search has been conducted unsuccessfully for thermal, 86.8 GHz, J = 2 - 1 SiO emission from seven planetary nebulae. The observed sigma limit was a peak antenna temperature of less than or equal to 0.02 K, except for NGC 7027 where the peak antenna temperature was less than or equal to 0.01 K. It is argued that most planetary nebulae do not have sufficient midinfrared flux to significantly excite the SiO molecule. This is least likely for NGC 7027, where the lack of emission is probably due to low abundance of the molecule. Following the more detailed models of highly evolved stars, the SiO is most likely tied up in grains.