Observations of High-J sio Emission along the HH211 Outflow

Spectra of the pure rotational SiO J=11-10 and J=8-7 lines, at 477.5 GHz and 347.3 GHz respectively, have been obtained along the HH211 protostellar jet. Bright emission has been observed localized inside about 15arcsec of projected distance from the central source, where a compact and collimated SiO jet was previously discovered by means of SiO J=1-0 interferometric observations (Chandler & Richer \cite{Chandler2001}). The detection of the high-J lines testifies for the extreme conditions of density and temperature of the SiO emission. Values of T>250 K and nH2 ~ 2-5*E6 cm-3 are inferred from the observed line ratios, while a SiO abundance in the range ~ 10-7{-}10-6 has been estimated through a comparison with the CO rotational lines at J>14 observed by the ISO Long Wavelength Spectrometer. Both the estimated physical conditions and abundance are in agreement with a picture in which the observed SiO emission directly arises at the front of a C-type shock with vs < 35 km s-1, where all the silicon released from the grains by sputtering and/or grain-grain collisions is converted into gas-phase SiO.