Successive ejection events in the L1551 molecular outflow

We report CO J = 2 to 1 strip maps along the axis and across the lobes of the L1551 molecular outflow. The overall structure of the outflow is more complicated than previously believed. In particular, we find at least four successive high-velocity features along the axis of the blueshifted lobe which are very suggestive of different ejection episodes. The well-known 'shell structure' is restricted to the feature closest to the exciting source, indicating that a simple hollow shell model is inappropriate to explain the overall outflow structure. In contrast, our data are more consistent with recent jet-driven models for bipolar outflows. The presence of different high-velocity features suggest that the entraining jet could be of intermittent nature. We have also carried out high-sensitivity observations of (13)CO toward selected positions in the outflow, and we infer from them that self-absorption effects in the CO wing profiles are unimportant.