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

We present SiO $J = 8 \hbox{--} 7$ (347.3 GHz) observations towards HH 212 (Herbig-Haro object 212) using the ASTE (Atacama Submillimeter Telescope Experiment) telescope. Our observations with a $22^{\prime\prime}$-diameter beam show that the SiO emission is highly concentrated within $1'$ of the driving source. We carefully compare the SiO observations with archival $\mathrm{H}_2$ 1–0 S(1) images and published $\mathrm{H}_2$ echelle spectra. We find that, although the SiO velocities closely match the radial velocities seen in $\mathrm{H}_2$, the distributions of $\mathrm{H}_2$ and SiO emission differ markedly. We attribute the latter to the different excitation conditions required for $\mathrm{H}_2$ and SiO emission, particularly the higher critical density ($n_{\mathrm{H}_2} \sim 10^8 \,\mathrm{cm}^{-3}$) of the SiO $J = 8 \hbox{--} 7$ emission. The kinematic similarities imply that the $\mathrm{H}_2$ and SiO are associated with the same internal working surfaces. We conclude that the SiO $J = 8 \hbox{--} 7$ emission has a potential for probing the jet/wind launching region through interferometric observations in the future, particularly for the youngest, most deeply embedded protostars where IR observations are not possible.