Generation of radiative knots in a randomly pulsed protostellar jet

I. Dynamics and energetics

¹ Dip. Scienze Fisiche ed Astronomiche, Sez. Astronomia, Università di Palermo, P.zza del Parlamento 1, 90134 Palermo, Italy e-mail: sbonito@astropa.unipa.it
² INAF - Osservatorio Astronomico di Palermo, P.zza del Parlamento 1, 90134 Palermo, Italy
³ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁴ European Space Agency Community Coordination and Planning Office, 8-10 rue Mario Nikis, 75738 Paris Cedex 15, France

Received: 3 November 2009
Accepted: 9 December 2009

Abstract

Context. Herbig-Haro objects are characterized by a complex knotty morphology detected mainly along the axis of protostellar jets in a wide range of bands: from radio to IR to optical bands, with X-rays knots also detected in the past few years. Evidence of interactions between knots formed in different epochs have been found, suggesting that jets may result from the ejection of plasma blobs from the stellar source.

Aims. We aim at investigating the physical mechanism leading to the irregular knotty structure observed in protostellar jets in different wavelength bands and the complex interactions occurring among blobs of plasma ejected from the stellar source.

Methods. We performed 2D axisymmetric hydrodynamic numerical simulations of a randomly ejected pulsed jet. The jet consists of a train of blobs that ram with supersonic speed into the ambient medium. The initial random velocity of each blob follows an exponential distribution. We explored the ejection rate parameter to derive constraints on the physical properties of protostellar jets by comparing model results with observations. Our model takes the effects of radiative losses and thermal conduction into account.

Results. We find that the mutual interactions of blobs ejected at different epochs and with different speeds lead to a variety of plasma components not described by current models of jets. The main features characterizing the randomly pulsed jet scenario are: single high-speed knots, showing a measurable proper motion in nice agreement with optical and X-rays observations; irregular chains of knots aligned along the jet axis and possibly interacting with each other; reverse shocks interacting with outgoing knots; oblique shock patterns produced by the reflection of shocks at the cocoon surrounding the jet. All these structures work together to help determining the morphology of the jet in different wavelength bands. We also find that the thermal conduction plays a crucial role in damping out hydrodynamic instabilities that would develop within the cocoon and that contribute to the jet breaking.