  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 the nature
of the driving source. We want to 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.
We have observed the emission of the CO 2-1 and SO N_J=5_4-4_3 rotational
transitions with NOEMA, towards the intermediate-mass Class 0 protostellar
system Cep E. The CO high-velocity jet emission reveals a central component
associated with high-velocity molecular knots, also detected in SO, surrounded
by a collimated layer of entrained gas. The gas layer appears to accelerate
along the main axis over a length scale delta_0 ~700 au, while its diameter
gradually increases up to several 1000au at 2000au from the protostar. The jet
is fragmented into 18 knots of mass ~10^-3 Msun, unevenly distributed between
the northern and southern lobes, with velocity variations up to 15 km/s close
to the protostar, well below the jet terminal velocities. The knot interval
distribution is approximately bimodal with a scale of ~50-80yr close to the
protostar and ~150-200yr at larger distances >12". The mass-loss rates derived
from knot masses are overall steady, with values of 2.7x10^-5 Msun/yr
(8.9x10^-6 Msun/yr) in the northern (southern) lobe. The interaction of the
ambient protostellar material with high-velocity knots drives the formation of
a molecular layer around the jet, which accounts for the higher mass-loss rate
in the north. The jet dynamics are well accounted for by a simple precession
model with a period of 2000yr and a mass-ejection period of 55yr.
