We present a multiline mm-wave survey of SiO emission towards a sample of s
tar-forming regions associated with molecular and Kerbig-Haro outflows. The
sample includes sources in the northern and southern hemispheres. We exten
sively mapped some particularly interesting objects (IRAS 00338+6312, HH7-1
1 and CepA). The high detection rate in the sample (52%) confirms that the
SiO emission is closely associated with outflows. There exists a trend so t
hat the more intense SiO sources are associated with higher luminosities, w
ith an average L-SiO/L-IR ratio of 1.8 10(-10)
The SiO lines exhibit a variety of profiles, ranging from narrow lines (1-3
km s(-1) width) at ambient velocities to broad profiles (10-20 km s(-1)),
with complex profiles consisting of a blend of low and high velocity compon
ents as intermediate stages. In the regions where SiO was mapped, the low v
elocity SiO emission comes from regions definitely offset from the position
where the high velocity emission is present, indicating that the low and h
igh velocity SiO emissions trace two distinct regimes. The SiO abundances a
re different in those two regimes: we estimate that typical SiO abundances
are similar or equal to 10(-9)-10(-8) in the high velocity components, but
they decrease by two orders of magnitude (10(-11)-10(-10)) when SiO is dete
cted at low velocities.
The hydrogen volume densities estimated from the multiline SiO observations
are in the range 10(5) to fews 10(6) cm(-3), in both the low and the high
velocity regimes, indicating that all the SiO emission arises in shock-comp
ressed regions. We argue that the different observed SiO profiles could be
caused by an evolutionary effect: the SiO molecules produced at high veloci
ties could be slowed down because of their interaction with the surrounding
gas before they stick onto the dust grains. However, the possibility that
the low velocity SiO emission is due to slow shocks cannot be ruled out, bu
t this would require the presence of a small amount of silicon compounds on
the dust grain mantles.