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Astron. Astrophys. 331, 1130-1142 (1998)

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1. Introduction

Of fundamental importance to the propagation of astrophysical jets is the question of their stability. Jet flows are known to vary over time and certainly in the case of both jets from active galactic nuclei (AGN), e.g., the M 87 jet (Harris et al. 1997), and from young stellar objects (YSOs), e.g. the L1551 IRS 5 jet (Neckel & Staude 1987), this variability can often be traced to the source itself. Moreover, the dynamical interaction between faster jet fluid catching up with slower jet material can explain many of the features that we see for example in YSO jets (Raga et al. 1990; Stone & Norman 1993). At the same time, a number of fluid dynamic instabilities can occur even when the jet flow is, at least initially, very steady. For example, in purely hydrodynamic flows, Rayleigh-Taylor and Vishniac instabilities may be important especially at the head of YSO jets (Blondin et al. 1990). An additional concern is whether or not Kelvin-Helmholtz (KH) instabilities are important. Certainly KH instabilities are a viable explanation for some morphological features observed in AGN jets: for example the well-known sinusoidal appearance of the 3C449 jets has been successfully modeled by the non-linear growth of the helical twisting mode (Hardee et al. 1994).

Until recently, most treatments of KH instabilities in jets, both of an analytical and numerical nature (e.g. Birkinshaw 1991; Bodo et al. 1994), have ignored the effects of radiative losses. In the case of AGN jets such an approach is justified since radiative losses can, to a first approximation, be ignored. The same cannot be said of their smaller scale counterparts i.e. jets from YSOs (e.g. Edwards et al. 1993; Ray 1996) where such energy losses have major effects on the development of the flow (Blondin et al. 1990; Stone & Norman 1994 and references therein). Bührke et al. (1988) were the first to suggest that the knots seen in YSO jets might be due to the growth of KH pinching modes. Although it is now generally agreed, especially as a result of Hubble Space telescope imaging (e.g. Ray et al. 1997; Reipurth et al. 1997), that such knots are probably produced by episodic variations in the outflow from the source, it is nevertheless interesting to see what effects, if any, the growth of KH modes might have on the development of YSO jets. Effects caused by KH instabilities could be present in addition to those caused by "pulsing". Another interesting question is whether the growth of KH modes might provide a mechanism through which momentum could be transferred to the jet's surroundings and thus accelerate ambient gas. Such momentum transfer would be in addition to any acceleration provided by the so-called "prompt" entrainment of ambient material near the head of the jet (Padman et al. 1997).

Recently a number of researchers have investigated the growth of KH modes in radiatively cooled slab jets. Hardee & Stone (1997), in a linear stability analysis of such jets, solved the KH dispersion relationship numerically for a broad range of perturbation frequencies. They found that the wavelengths and growth rates of the most unstable KH modes differed considerably from the adiabatic case and that the nature of the cooling function, i.e. whether it is a steep or shallow function of temperature about its equilibrium value, along with the form of the heating function, is a crucial factor in determining initial growth rates. Their analysis was followed (Stone et al. 1997) by a time-dependent hydrodynamical simulation of a cooled jet that traced the growth of the KH instability in the non-linear regime. Stone et al. (1997) concentrated on the development of asymmetric modes. Such modes, and in particular the helical mode, might be responsible for the "wiggling" motion seen in a number of YSO jets, e.g. HH 30 (Lopéz et al. 1995) and may also transport momentum from the jet to its environment (Stone et al. 1997). Here we present the results of time-dependent hydrodynamical simulations of the growth of the pinch modes of the KH instability in a radiatively cooled flow using parameters typical of YSO jets. Rossi et al. (1997) reported the results of simulations of these axisymmetric modes of the KH instability in cylindrical jets and found that cooling appears to dampen the growth of the instability. Theory implies that the linear behaviour of these modes will be similar in both slab and cylindrical symmetry. Here we present the results of simulations in slab symmetry and we compare our results with the results of Rossi et al. (1997).

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998