Shells, jets, and internal working surfaces in the molecular outflow from IRAS 04166+2706

Context. IRAS 04166+2706 in Taurus is one of the closest young stellar objects whose molecular outflow contains a highly collimated, rapidly moving component. Aims. The high symmetry and pristine appearance of this outflow suggests that its study could offer unique clues about the nature of its still mysterious underlying driving wind. Methods. We observed the IRAS 04166+2706 outflow with the IRAM Plateau de Bure interferometer in CO(J = 2-1) and SiO(J = 2-1), achieving angular resolutions of between 2 '' and 4 ''. To improve the quality of the CO(2-1) images, we added single-dish data to the interferometer visibilities. Results. The outflow consists of two distinct components. At velocities of <10 km s(-1), the gas forms two opposed, approximately conical shells that have the YSO at their vertex. These shells coincide with the walls of evacuated cavities and appear to result from the acceleration of the ambient gas by a wide-angle wind. At velocities of > 30 km s(-1), the gas forms two opposed jets that travel along the center of the cavities and whose emission is dominated by a symmetric collection of at least 7 pairs of peaks. The velocity field of this component presents a sawtooth pattern with the gas in the tail of each peak moving more rapidly than the gas at its head. This pattern and a systematic widening of the peaks with distance from the central source are consistent with the emission arising from internal working surfaces traveling along the jet due to variations in the velocity field of ejection. We interpret this component to be the true protostellar wind, and find its composition to be consistent with a chemical model of this wind. Conclusions. Our results favor outflow wind models that have simultaneously wide-angle and narrow components, and suggest that the EHV peaks observed in a number of outflows consist of internally-shocked wind material.