A survey of SiO emission towards interstellar masers

Abstract

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 (, ) and (,) of SiO at 3 and 2 mm were measured simultaneously. With Onsala only the (,) line was accessible. Altogether 369 objects were observed and SiO was detected towards 137 of them. The detection rate is highest towards the most intense H2O masers, which probably require powerful shocks to be excited. The SiO detection rate correlates also with the integrated far-infrared flux density and the FIR luminosity of the associated IRAS point source, indicating that the occurrence of shocks is related to the amount of radiation from the 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 conditions for the production of gaseous SiO. The full widths above of the SiO() lines, which are likely to be related to the associated shock velocities, range from 2 to 60 kms-1 except for the line in Ori KL which has a full width of about 100 kms-1. The median of our sample is 19 kms-1. The SiO lines are single-peaked and the peak velocities are always close to the ambient cloud velocity as determined 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 the emission arises from turbulent wakes behind bow-shocks. However, the number of symmetric, relatively narrow profiles indicates that at least 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).] These reactions may be initiated and sustained by ionizing radiation from shocks, in the same way as the enhancement of HCO+ near Herbig-Haro objects has been explained in the model of [Wolfire & Königl (1993).] The excitation temperatures of SiO() and () transitions were determined towards three strong sources using measurements in isotopically substituted SiO. In all three sources the transitions are clearly subthermally excited, implying moderate densities ( cm-3) in the SiO emission regions.