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Astron. Astrophys. 364, 763-768 (2000)

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1. Introduction

The Serpens triple radio source (Rodríguez et al. 1980; Snell & Bally 1986) coincides with the FIRS 1 (IRAS 18273+0113) far infrared source (Harvey et al. 1984), but has not been detected at near infrared nor optical wavelengths (Zeiner & Eislöffel 1999). The region around this source has been mapped in lines of the NH3 molecule, and has peculiar kinematics which are probably a result of the interaction of the triple source with the surrounding environment (Torrelles et al. 1992; Curiel et al. 1996). Single-dish and interferometric line observations of other molecules have also shown a dense molecular core around this radio continuum source, as well as widespread outflow activity (e.g., McMullin et al. 1994; White et al. 1995).

The triple radio continuum source itself has most interesting properties. One of the notable features is that some regions of this object appear to show a partial contribution from a non-thermal component (as evidenced by a negative spectral index, Rodríguez et al. 1989). This result has prompted theoretical studies of the acceleration of relativistic electrons in shocks related to outflows from young stars (Crusius-Wätzel 1990; Henriksen et al. 1991). In more recent observational work, Curiel et al. (1993) concluded that even though there appears to be an extended non-thermal component, most of the emission that one observes from the knots in the triple radio source appears to correspond to a thermal, free-free radio continuum.

The most remarkable property of the triple source are the large proper motions of its outer components. While the central component does not show a detectable proper motion, the main two outer components move away from the central source with velocities of [FORMULA] km s-1 (Rodríguez et al. 1989; Curiel et al. 1993, 1996). This result leads to a possible interpretation of this object as a young star (coinciding with the position of the central component) ejecting a bipolar jet system. Far infrared, sub-millimiter and millimiter observations have revealed that this object is very young, having characteristics of a class 0 protostar (e.g., Casali et al. 1993; Hurt & Barsony 1996). More recent radio continuum observations (Curiel et al. 1993) show that the NW lobe of the Serpens jet actually has a chain of four or five aligned knots, resulting in a morphology that closely resembles optically detected Herbig-Haro (HH) jets (such as HH 34 or HH 111, see, e. g., Reipurth et al. 1997), though having a spatial extent which is smaller by about an order of magnitude.

Curiel et al. (1993) interpreted the structure of the Serpens triple radio continuum source as a bipolar jet from a precessing source. In the present paper, we explore this possibility by first computing the parameters necessary for explaining the observed structure with a model of a precessing, variable ejection velocity jet (Sect. 2). We then compute a 3D numerical simulation with these parameters, from which we obtain predicted radio continuum maps that can be directly compared with the observations (Sect. 3). Finally, we discuss the relative success of this model at reproducing the radio continuum observations of the Serpens triple radio source (Sect. 4).

We should note that both analytic (Raga et al. 1993) and numerical models (Cox et al. 1991; de Gouveia dal Pino & Benz 1993; Biro et al. 1995; Cliffe et al. 1996) of HH jets from precessing sources have been studied in the past. Also, the problem of a jet with a general velocity+direction ejection variability has been studied (analytic considerations and 2D numerical models are presented by Raga & Biro 1993). Finally, a number of 3D numerical simulations of HH jets from variable sources have been carried out (e.g., Stone & Norman 1994; Suttner et al. 1997), and some of them include a small precession of the outflow in order to break the axisymmetry of the jet (de Gouveia dal Pino et al. 1996; Völker et al. 1999).

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001