Observations of high-JSiO emission along the HH211 outflow

Abstract

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 15″ 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]). 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 $n_{\rm H2} \sim2{-}5\times10^{6}$ 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 $v_{\rm s} -1 , where all the silicon released from the grains by sputtering and/or grain-grain collisions is converted into gas-phase SiO.