THE OUTFLOW IN THE L1157 DARK CLOUD - EVIDENCE FOR SHOCK HEATING OF THE INTERACTING GAS

In the L1157 dark cloud, we have discovered a well-collimated bipolar CO outflow associated with a cold IRAS source 20386+6751. The gas kinetic temperature toward the blue lobe of the outflow rises to approximately 30 K from the temperature of the surrounding gas (less-than-or-equal-to 10 K); this high temperature region is very localized within the blue lobe. The HCO+, HCN, and NH3 lines show blueshifted and broad-line profiles toward the blue CO lobe. Furthermore, their distributions are similar to that of the blue lobe. Analysis of the energy of the outflow and dense gas suggests that a strong shock caused by the outflow is responsible for the temperature enhancement. Such shock heating has rarely been observed in low-mass star-forming regions. Our data show that the outflow has an influence on the thermal properties of the parent cloud at least locally through shock processes in its early evolutionary stage.