Low and high velocity SiO emission around young stellar objects

We present a multiline mm-wave survey of SiO emission towards a sample of star-forming regions associated with molecular and Herbig-Haro outflows. The sample includes sources in the northern and southern hemispheres. We extensively mapped some particularly interesting objects (IRAS 00338+6312, HH7–11 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 that the more intense SiO sources are associated with higher luminosities, with an average LSiO/LIR 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 components as intermediate stages. In the regions where SiO was mapped, the low velocity SiO emission comes from regions definitely offset from the position where the high velocity emission is present, indicating that the low and high velocity SiO emissions trace two distinct regimes. The SiO abundances are different in those two regimes: we estimate that typical SiO abundances are ' 10−9-10−8 in the high velocity components, but they decrease by two orders of magnitude (10−11-10−10) when SiO is detected at low velocities. The hydrogen volume densities estimated from the multiline SiO observations are in the range 105 to fews 106 cm−3, in both the low and the high velocity regimes, indicating that all the SiO emission arises in shock-compressed regions. We argue that the different observed SiO profiles could be caused by an evolutionary effect: the SiO molecules produced at high velocities 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, but this would require the presence of a small amount of silicon compounds on the dust grain mantles.