Kinematics of SiOJ = 8–7 Emission towards the HH 212 Jet

We present SiO J=8-7 (347.3 GHz) observations towards HH 212 using the ASTEtelescope. Our observations with a 22''-diameter beam show that the SiOemission is highly concentrated within 1' of the driving source. We carefullycompare the SiO observations with archival H_2 1-0 S(1) images and publishedH_2 echelle spectra. We find that, although the SiO velocities closely matchthe radial velocities seen in H_2, the distribution of H_2 and SiO emissiondiffer markedly. We attribute the latter to the different excitation conditionsrequired for H_2 and SiO emission, particularly the higher critical density(n_H2 ~10^8 cm^-3) of the SiO J=8-7 emission. The kinematic similarities implythat the H_2 and SiO are associated with the same internal working surfaces. Weconclude that the SiO J=8-7 emission has a potential to probe the jet/windlaunching region through interferometric observations in the future,particularly for the youngest, most deeply embedded protostars where IRobservations are not possible.Comment: 6 pages, 4 figures, accepted to PAS