A PRECESSING JET IN THE L-1157 MOLECULAR OUTFLOW

The blue-shifted lobe of the L1157 molecular outflow has been imaged i n the CO J=1-->0 line, and in the lambda 2.6 mm continuum, with angula r resolution of 3.5 '' x 3 '' (similar to 7 10(-3) pc at the distance of L 1157, 440 pc) at the IRAM interferometer. The data consist of a m osaic of 10 partially overlapping fields, complemented with short spac ing information from the IRAM 30-m telescope. The images reveal the pr esence of at least two prominent limb-brightened cavities which have b een likely created by the propagation of large bow-shocks due to episo dic events in a precessing, highly-collimated jet. A simple spatio-kin ematic model involving two different ejection events provides an accur ate description of the observations. We find that the outflow is incli ned by similar to 80 degrees to the line of sight, and that the axis o f the underlying jet precesses on a cone of 6 degrees opening angle, w ith a period of similar to 4000 yr. We discuss the constraints derived from the present observations on several recent models for jet-driven molecular outflows. We conclude that, similarly to what happens in ot her outflows (e.g. L 1448), the large opening angle of the L 1157 CO o utflow is originated by the large size of the propagating bow-shocks, since the precession in L 1157 happens in a narrow cone. However, the observed shape of the cavities evacuated by the bow-shock propagation are not well accounted for by current models of gas entrainment by bow -shocks.