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HH objects are characterized by a complex knotty morphology detected mainlyalong the axis of protostellar jets in a wide range of bands. Evidence ofinteractions between knots formed in different epochs have been found,suggesting that jets may result from the ejection of plasma blobs from thesource. We aim at investigating the physical mechanism leading to the irregularknotty structure observed in jets in different bands and the complexinteractions occurring among blobs of plasma ejected from the stellar source.We perform 2D axisymmetric HD simulations of a randomly ejected pulsed jet. Thejet consists of a train of blobs which ram with supersonic speed into theambient medium. The initial random velocity of each blob follows an exponentialdistribution. We explore the ejection rate parameter to derive constraints onthe physical properties of protostellar jets by comparison of model resultswith observations. Our model takes into account radiative losses and thermalconduction. We find that the mutual interactions of blobs ejected at differentepochs and with different speed lead to a variety of plasma components notdescribed by current models. The main features characterizing the random pulsedjet scenario are: single high speed knots, showing a measurable proper motionin nice agreement with observations; irregular chains of knots aligned alongthe jet axis and possibly interacting with each other; reverse shocksinteracting with outgoing knots; oblique shocks produced by the reflection ofshocks at the jet cocoon. All these structures concur to determine themorphology of the jet in different bands. We also find that the thermalconduction plays a crucial role in damping out HD instabilities that woulddevelop within the cocoon and that contribute to the jet breaking.

Generation of radiative knots in a randomly pulsed protostellar jet I. Dynamics and energetics