Molecular Jet of IRAS 04166+2706

The molecular outflow from IRAS 04166+2706 was mapped with the Submillimeter Array (SMA) at 350 GHz continuum and CO J = 3$-$2 at an angular resolution of ~1 arcsec. The field of view covers the central arc-minute, which contains the inner four pairs of knots of the molecular jet. On the channel map, conical structures are clearly present in the low velocity range (|V$-$V$_0$|$<$10 km s$^{-1}$), and the highly collimated knots appear in the Extremely High Velocity range (EHV, 50$>$|V$-$V$_0$|$>$30 km s$^{-1}$). The higher angular resolution of ~1 arcsec reveals the first blue-shifted knot (B1) that was missing in previous PdBI observation of Sant\'iago-Garc\'ia et al. (2009) at an offset of ~6 arcsec to the North-East of the central source. This identification completes the symmetric sequence of knots in both the blue- and red-shifted lobes of the outflow. The innermost knots R1 and B1 have the highest velocities within the sequence. Although the general features appear to be similar to previous CO J = 2$-$1 images in Sant\'iago-Garc\'ia et al. (2009), the emission in CO J = 3$-$2 almost always peaks further away from the central source than that of CO J = 2$-$1 in the red-shifted lobe of the channel maps. This gives rise to a gradient in the line-ratio map of CO J = 3$-$2/J = 2$-$1 from head to tail within a knot. A large velocity gradient (LVG) analysis suggests that the differences may reflect a higher gas kinetic temperature at the head. We also explore possible constraints imposed by the non-detection of SiO J = 8$-$7.