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Astron. Astrophys. 333, 1001-1006 (1998)

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1. Introduction

High spatial resolution observations of jets from young stellar objects (YSOs), performed by ground based telescopes and recently by the WFPC2 on-board of the Hubble Space Telescope (see Ray 1996), have revealed the finest details of the complex morphological structures that characterize these objects and, in particular, of the emission knots located along the jets. The origin of these knots, that are clearly visible in many objects such as HH34 (Bührke, Mundt & Ray 1988; Raga & Mateo 1988; Reipurth & Heathcote 1992; Eislöffel & Mundt 1992), and HH111 (Reipurth 1989; Reipurth, Raga & Heathcote 1992; Morse et al. 1993a) is an important issue because determining the mechanism that leads to their formation would allow a leap forward in the understanding of the nature of YSO jets, since the physical parameters of the flow could be constrained opening a window on the hidden region where the jet origin takes place actually.

Concerning the problem of the origin of the emission knots, it has been subject of many investigations during the last years. Two different interpretations have been proposed, namely: Bührke et al. (1988) suggested nonlinearly evolved Kelvin-Helmholtz instabilities as responsible for the formation and growth of perturbations into shocks, as has been later confirmed by the numerical calculation of Bodo et al. (1994, 1995), Paper I and Paper II); an alternative explanation was proposed by Raga et al. 1990 in terms of internal working surfaces that form in the jet as a consequence of variations in the jet ejection velocity (see also Kofman & Raga 1992; Raga & Kofman 1992; Raga & Cantò 1995; Hartigan & Raymond 1993; Biro & Raga 1994; Stone & Norman 1993a, 1993b).

Herbig-Haro jets therefore pose two main problems: i) they survive instabilities for distances as large as a thousand jet radii, and ii) the displayed emission knots require interpretation. The former problem was discussed, in the 2-D cylindrically symmetric limit, by Bodo et al. (1994, 1995) and Rossi et al. (1997) (see also the companion Paper III, Micono et al. 1997) who have shown that radiative losses may drastically reduce the nonlinear growth of Kelvin-Helmholtz instabilities as far as jet momentum transfer from the jet to the ambient medium is concerned, and therefore increase the jet capability to survive instabilities. Stone, Xu & Hardee (1997) have recently addressed the same problem for asymmetric modes in a slab jet and discussed the effects of different cooling and heating functions.

In this paper we study the spatial evolution of symmetric perturbations in a two-dimensional cylindrical geometry and in the presence of non-equilibrium radiative losses, and we then apply the results to stellar jets in order to interpret the origin of their emission knots, pointing out the main differences between the present scheme and the alternative internal working surfaces (IWSs) picture. We discuss also how observations can help to discriminate between these two interpretations.

The plan of the paper is the following: In the next section (Sect. 2), we summarize the observational constraints, Sect. 3 is devoted to the computational results; astrophysical applications and the conclusions are given in Sect. 4.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998