1996A&A...307..891G

The blue-shifted lobe of the L1157 molecular outflow has been imaged in the CO J=1-0 line, and in the λ 2.6 mm continuum, with angular resolution of 3.5"x3"(∼7x10^–3pc at the distance of L1157, 440pc) at the IRAM interferometer. The data consist of a mosaic of 10 partially overlapping fields, complemented with short spacing information from the IRAM 30-m telescope. The images reveal the presence of at least two prominent limb-brightened cavities which have been likely created by the propagation of large bow-shocks due to episodic events in a precessing, highly-collimated jet. A simple spatio-kinematic model involving two different ejection events provides an accurate description of the observations. We find that the outflow is inclined by ∼80deg to the line of sight, and that the axis of the underlying jet precesses on a cone of 6deg opening angle, with a period of ∼4000yr. 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 other outflows (e.g. L1448), the large opening angle of the L1157 CO outflow is originated by the large size of the propagating bow-shocks, since the precession in L1157 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.