A SEARCH FOR SIO, OH, CO AND HCN RADIO-EMISSION FROM SILICATE-CARBON STARS

We report upper limits for radio emission of SiO at 86 and 43 GHz, of OH at 1612 and 1665/1667 MHz, of CO at 115 GHz and HCN at 88.6 GHz in the silicate-carbon stars. These upper limits of SiO imply that oxygen -rich material has not been detected within 2R(star) of a central star even though the detected emission from silicate dust glains, H2O and OH maser establishes the presence of oxygen-rich material from about t ens to thousands of AU of a central star. The upper limit of the SiO a bundance is consistent with that found in oxygen-rich envelopes. Upper limits of the mass loss rate (based on the CO data) are estimated to be between 10(-6) to 10(-7) M(.) yr(-1) assuming a distance of 1.5 kpc for these stars. The absence of HCN microwave emission implies that n o carbon-rich material can be detected at large distances (thousands o f AU) from a central star. The lack of detections of SiO, CO and HCN e mission is most likely due to the large distances of these stars. A nu mber of C stars were detected in CO and HCN, but only the M supergiant VX Sgr was detected in CO.