A SURVEY OF SIO EMISSION TOWARDS INTERSTELLAR MASERS - I - SIO LINE CHARACTERISTICS

We have conducted a survey of SiO emission towards galactic H2O and OH masers and ultracompact HII regions using the 15-m SEST and the 20-m Onsala telescope. With the SEST the transitions (v = 0; J = 2 - 1) and (v = 0, J = 3-2) of SiO at 3 and 2 mm were measured simultaneously. W ith Onsala only the (v = 0; J = 2 - 1) line was accessible. Altogether 369 objects were observed and SiO was detected towards 137 of them. T he detection rate is highest towards the most intense H2O masers, whic h probably require powerful shocks to be excited. The SiO detection ra te correlates also with the integrated far-infrared flux density and t he FIR luminosity of the associated IRAS point source, indicating that the occurrence of shocks is related to the amount of radiation from t he central stellar source(s), For flux and luminosity limited samples the SiO detection rate is higher in the inner 7 kpc from the galactic centre than elsewhere. This suggests that dense cores belonging to the so called ''molecular ring'' provide particularly favourable conditio ns for the production of gaseous SiO. The full widths above 2 sigma of the SiO(J = 2 - 1) lines, which are likely to be related to the assoc iated shock velocities, range from 2 to 60 km s(-1) except for the lin e in Ori KL which has a full width of about 100 km s(-1). The median o f our sample is 19 km s(-1). The SiO lines are single- peaked and the peak velocities are always close to the ambient cloud velocity as dete rmined from published CS observations. These line characteristics are compared with the predictions of kinematical bow-shock models. The SiO line shapes correspond with the model of Raga & Cabrit (1993) where t he emission arises from turbulent wakes behind bow-shocks. However, th e number of symmetric, relatively narrow profiles indicates that at le ast in some of the observed sources SiO emission arises also from the quiescent gas component. We suggest that this is due to evaporation of silicon compounds from grain mantles and their reprocessing to SiO in dense quiescent gas according to the model of McKay (1995, 1996). The se reactions may be initiated and sustained by ionizing radiation from shocks, in the same way as the enhancement of HCO+ near Herbig-Haro o bjects has been explained in the model of Wolfire & Konigl (1993). The excitation temperatures of SIO(J = 2 - 1) and (J = 3 - 2) transitions were determined towards three strong sources using measurements in is otopically substituted SiO. In all three sources the transitions are c learly subthermally excited, implying moderate densities (< 3 10(6) cm (-3)) in the SiO emission regions.