SOLIS XVI. Mass ejection and time variability in protostellar outflows: Cep E

Context. Protostellar jets are an important agent of star formation feedback, tightly connected with the mass-accretion process. The history of jet formation and mass ejection provides constraints on the mass accretion history and on the nature of the driving source. Aims. We characterize the time-variability of the mass-ejection phenomena at work in the class 0 protostellar phase in order to better understand the dynamics of the outflowing gas and bring more constraints on the origin of the jet chemical composition and the mass-accretion history. Methods. Using the NOrthern Extended Millimeter Array (NOEMA) interferometer, we have observed the emission of the CO 2-1 and SO N-J = 5(4)-4(3) rotational transitions at an angular resolution of 1.0 '' (820 au) and 0.4 '' (330 au), respectively, toward the intermediate-mass class 0 protostellar system Cep E. Results. The CO high-velocity jet emission reveals a central component of <= 400 au diameter associated with high-velocity molecular knots that is also detected in SO, surrounded by a collimated layer of entrained gas. The gas layer appears to be accelerated along the main axis over a length scale delta(0) similar to 700 au, while its diameter gradually increases up to several 1000 au at 2000 au from the protostar. The jet is fragmented into 18 knots of mass similar to 10(-3) M-circle dot, unevenly distributed between the northern and southern lobes, with velocity variations up to 15 km s(-1) close to the protostar. This is well below the jet terminal velocities in the northern (+65 km s(-1)) and southern (-125 km s(-1)) lobes. The knot interval distribution is approximately bimodal on a timescale of similar to 50-80 yr, which is close to the jet-driving protostar Cep E-A and similar to 150-20 yr at larger distances >12 ''. The mass-loss rates derived from knot masses are steady overall, with values of 2.7 x 10(-5) M-circle dot yr(-1) and 8.9 x 10(-6) M-circle dot yr(-1) in the northern and southern lobe, respectively. Conclusions. The interaction of the ambient protostellar material with high-velocity knots drives the formation of a molecular layer around the jet. This accounts for the higher mass-loss rate in the northern lobe. The jet dynamics are well accounted for by a simple precession model with a period of 2000 yr and a mass-ejection period of 55 yr.