A molecular survey of outflow gas: velocity-dependent shock chemistry
  and the peculiar composition of the EHV gas
  (Abridged) We present a molecular survey of the outflows powered by L1448-mm
and IRAS 04166+2706, two sources with prominent wing and extremely high
velocity (EHV) components in their CO spectra. The molecular composition of the
two outflows presents systematic changes with velocity that we analyze by
dividing the outflow in three chemical regimes, two of them associated with the
wing component and the other the EHV gas. The analysis of the two wing regimes
shows that species like H2CO and CH3OH favor the low-velocity gas, while SiO
and HCN are more abundant in the fastest gas. We also find that the EHV regime
is relatively rich in O-bearing species, as is not only detected in CO and SiO
(already reported elsewhere), but also in SO, CH3OH, and H2CO (newly reported
here), with a tentative detection in HCO+. At the same time, the EHV regime is
relatively poor in C-bearing molecules like CS and HCN. We suggest that this
difference in composition arises from a lower C/O ratio in the EHV gas. The
different chemical compositions of the wing and EHV regimes suggest that these
two outflow components have different physical origins. The wing component is
better explained by shocked ambient gas, although none of the existing shock
models explains all observed features. The peculiar composition of the EHV gas
may reflect its origin as a dense wind from the protostar or its surrounding
disk.
