  Results are presented of a survey of SiO 5-4 emission observed with the James
Clerk Maxwell Telescope (JCMT) towards a sample of outflows from massive young
stellar objects. The sample is drawn from a single-distance study by Ridge &
Moore. In a sample of 12 sources, the 5-4 line was detected in 5, 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. NGC6334I shows
a clear bipolar flow in the 5-4 line. Additional data were obtained for
W3-IRS5, AFGL5142 and W75N for the 2-1 transition of SiO using the
Berkeley-Illinois-Maryland Association (BIMA) 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 AFGL5142 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. The primary indicator of SiO 5-4
detectability is the outflow velocity, i.e. 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.
