A survey of SiO 5 → 4 emission towards outflows from massive young stellar objects

Results are presented of a survey of SiO 5 -> 4 emission observed with the James Clerk Maxwell Telescope towards a sample of outflows from massive young stellar objects. The sample is drawn from a single-distance study by Ridge & Moore and allows the reasons that govern the detectability of SiO to be explored without the bias introduced by observing sources at different distances. This is the first such unbiased survey of SiO emission from massive outflows. In a sample of 12 sources, the 5 -> 4 line was detected in five, a detection rate of 42 per cent. This detection rate is higher than that found for a sample of low-luminosity outflow sources, although for sources of comparable luminosity, it is in good agreement with the results of a previous survey of high-luminosity sources. For most of the detected sources, the 5 -> 4 emission is compact or slightly extended along the direction of the outflow. NGC 6334I shows a clear bipolar flow in the 5 -> 4 line. Additional data were obtained for W3-IRS5, AFGL 5142 and W75N for the 2 -> 1 transition of SiO using the Berkeley-Illinois-Maryland Association millimetre interferometer. There is broad agreement between the appearance of the SiO emission in both lines, though there are some minor differences. The 2 -> 1 emission in AFGL 5142 is resolved into two outflow lobes which are spatially coincident on the sky, in good agreement with previous observations. In general, the SiO emission is clearly associated with the outflow. Simple analysis and radiative transfer modelling of the detected sources yields similar SiO column densities. The abundance of SiO is similar to 0.1-7.0 x 10(-9), and the H-2 number density is within a factor of 2 of 10(5) cm(-3). However, the temperature is not constrained over the range 50-150 K. The primary indicator of SiO 5 -> 4 detectability is the outflow velocity, that is, the presence of SiO is an indicator of a high-velocity outflow. This result is consistent with the existence of a critical shock velocity required to disrupt dust grains and subsequent SiO formation in post-shock gas. There is also weak evidence that higher luminosity sources and denser outflows are more likely to be detected.