SiO J = 5-4 in the HH 211 protostellar jet imaged with the submillimeter array

We have mapped the SiO J = 5-4 line at 217 GHz from the HH 211 molecular outflow with the Submillimeter Array (SMA). The high-resolution map (1″.6 × 0″.9) shows that the SiO J = 5-4 emission comes from the central narrow jet along the outflow axis with a width of ∼0″.8 (∼250 AU) FWHM. The SiO jet consists of a chain of knots separated by 3″-4″ (∼1000 AU), and most of the SiO knots have counterparts in shocked H ₂ emission seen in a new, deep VLT near-infrared image of the outflow. A new, innermost pair of knots have been discovered at just ±2″ from the central star. The line ratio between the SiO J = 5-4 data and the upper limits from the SiO 7=1-0 data of Chandler & Richer suggests that these knots have a temperature in excess of 300-500 K and a density of (0.5-1) × 107 cm^-3. The radial velocity measured for these knots is ∼30 km s^-1, comparable to the maximum velocity seen in the entire jet. The high temperature, high density, and velocity structure observed in this pair of SiO knots suggest that they are closely related to the primary jet launched close to the protostar.