Thermal SiO emission in the ground vibrational state has been found to be associated with high-velocity flows and H2O masers in massive star forming regions (Downes et al. 1982). In low-mass star-forming regions SiO emission comes exclusively from regions shocked by energetic outflows from young stellar objects and has very little or no contribution from the surrounding quiescent dense cores (e.g. Martín-Pintado et al. 1992; Bachiller 1996). The reason for this behaviour is that in cool dense gas SiO molecules are frozen onto dust grains. Si is however released in shocks due to evaporation of the grain mantles or destruction of their silicate cores, whereafter it quickly reacts with O2 or OH to form SiO (for recent chemical studies see McKay 1995, 1996 and Schilke et al. 1997).
Molecular masers originate from shock-energized dense gas in the neighbourhood of newly born massive stars. H2O masers and Class I methanol (CH3OH) masers are often associated with powerful molecular outflows (e.g. Felli et al. 1992; Menten 1991; Xiang & Turner 1995). In the model of Elitzur et al. (1989) H2O masers form behind shocks produced when highly supersonic mass outflows from young stellar objects collide with clumps or inhomogeneities of the surrounding cloud. It seems, however, that also radiative pumping of H2O masers is possible. Evidence for this in the case of a circumstellar accretion disk in S255 has been provided by Cesaroni (1990).
OH masers at 18-cm and Class II methanol masers are frequently seen in the direction or at the projected edges of ultra compact (UC) HII regions surrounding young massive stars (e.g. Gaume & Mutel 1987; Churchwell 1991; Menten et al. 1992). The required density enhancement for maser excitation in these regions may be caused by a bow-shock around the moving ionized shell (Van Buren et al. 1990). Recent interferometric observations have shown spatial-velocity structures indicative of rotation, thereby suggesting that also OH and CH3OH masers can be associated with protostellar disks (Caswell 1997).
Although the various maser types probably occur in different physical regions, they can be spatially very close to each other (Forster & Caswell 1989; Forster et al. 1990). Therefore, besides theoretical investigation of the excitation requirements, observational studies of the maser kinematics and thermal emission from the associated shocks are important for the understanding of these phenomena.
It seems possible that the emission from rotational transitions of SiO near massive young stars comes from the regions where H2O, OH and CH3OH masers are excited. As a tracer of shocked gas, SiO is superior to the molecules often used in outflow studies, e.g. CO, CS, NH3 and SO, since the emission is not confused by the ambient cloud component. With respect to maser emission, the advantage is the thermal (or quasi-thermal) nature, which does not exhibit rapid variations.
The purpose of the present survey is to find an extensive sample of strong SiO sources which could form a basis for more detailed studies of the structure, kinematics and thermal properties of the shock fronts associated with interstellar masers. In this paper we concentrate on the SiO detections and their line profiles, whereas in a subsequent paper we shall correlate the survey results with maser characteristics and with information inferred from the associated UC HII regions.
Previous observations suggest that SiO emission can arise from the cooling regions behind bow shocks (e.g. Dutrey et al. 1997). We therefore study the validity of the bow-shock hypothesis by comparing the SiO profiles with the predictions of kinematical models. An alternative related to the chemistry model of McKay (1995, 1996) is that SiO emission could come from dense clumps irradiated by powerful shocks (Wolfire & Königl 1993). In this latter case the lines are expected to be narrow.
In Sect. 2 of this paper, we give details of the target list selection criteria, and in Sect. 3 we describe the observations and the data analysis. The results from the observations are presented in Sect. 4. In Sect. 5 we discuss three bow-shock models and compare the predicted line profiles with the observations. Further discussion of the survey results is presented in Sect. 6. We conclude with a summary in Sect. 7.
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