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Fast jets are thought to be a crucial ingredient of star formation becausethey might extract angular momentum from the disk and thus allow mass accretiononto the star. However, it is unclear whether jets are ubiquitous, andlikewise, their contribution to mass and angular momentum extraction duringprotostar formation remains an open question.  Our aim is to investigate the ejection process in the low-mass Class 0protostar L1157. This source is associated with a spectacular bipolar outflow,and the recent detection of high-velocity SiO suggests the occurrence of a jet.  Observations of CO 2-1 and SiO 5-4 at 0.8" resolution were obtained with theIRAM Plateau de Bure Interferometer as part of the CALYPSO large program. Thejet and outflow structure were fit with a precession model. We derived thecolumn density of CO and SiO, as well as the jet mass-loss rate and mechanicalluminosity.  High-velocity CO and SiO emission resolve for the first time the first 200 auof the outflow-driving molecular jet. The jet is strongly asymmetric, with theblue lobe 0.65 times slower than the red lobe. This suggests that thelarge-scale asymmetry of the outflow is directly linked to the jet velocity andthat the asymmetry in the launching mechanism has been at work for the past1800 yr. Velocity asymmetries are common in T Tauri stars, which suggests thatthe jet formation mechanism from Class 0 to Class II stages might be similar.Our model simultaneously fits the inner jet and the clumpy 0.2 pc scale outflowby assuming that the jet precesses counter-clockwise on a cone inclined by 73degree to the line of sight with an opening angle of 8 degree on a period of1640 yr. The estimated jet mass flux and mechanical luminosity are 7.7e-7Msun/yr, and 0.9 Lsun, indicating that the jet could extract at least 25% ofthe gravitational energy released by the forming star.

First image of the L1157 molecular jet by the CALYPSO IRAM-PdBI survey