Submillimeter CO emission from shock-heated gas in the L1157 outflow

We present the CO J = 6-5, 4-3, and 3-2 spectra from the blueshifted gas of the outflow driven by the low-mass class 0 protostar in the L1157 dark cloud. Strong submillimeter CO emission lines with T-mb > 30 K have been detected at 63 (similar to0.13 pc) south from the protostar. It is remarkable that the blue wings in the submillimeter lines are stronger by a factor of 3-4 than that of the CO J = 1-0 emission line. The CO line ratios suggest that the blueshifted lobe of this outflow consists of moderately dense gas of n(H-2) = (1-3) x 10(4) cm(-3) heated to T-kin = 50-170 K. It is also suggested that the kinetic temperature of the outflowing gas increases from similar to 80 K near the protostar to similar to 170 K at the shocked region in the lobe center, toward which the largest velocity dispersion of the CO emission is observed. A remarkable correlation between the kinetic temperature and velocity dispersion of the CO emission along the lobe provides us with direct evidence that the molecular gas at the head of the jet-driven bow shock is indeed heated kinematically. The lower temperature of similar to 80 K measured at the other shocked region near the end of the lobe is explained if this shock is in a later evolutionary stage, in which the gas has been cooled mainly through radiation of the CO rotational lines.