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Astron. Astrophys. 344, 632-638 (1999)

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1. Introduction

Optical jets from young stellar objects (YSOs) often display wiggles or small departures from strict cylindrical symmetry when looked at with high enough spatial resolution (see e.g. HH 46/47 (Heathcote et al. 1996)). These distortions have been accounted for two main physical mechanisms namely, variations in the direction of the outflow axis (Raga et al. 1993; Biro et al. 1995) or jet instabilities (Camenzind 1997; Stone 1997; Massaglia et al. 1997). Both mechanisms may be acting in one way or another in YSOs jets.

GGD 34 is a unique protostellar jet for this type of study because its morphology and radial velocity field seem to be well correlated as shown by Gómez de Castro et al. (1993). GGD 34 is located in NGC 7129, a well known star forming region at 1 kpc from the Sun (Racine 1968). The object consist of three major condensations aligned over 0.17 pc which are embedded in a weak envelope emitting primarily in [S II] (Hartigan & Lada 1985; Strom et al. 1986; Ray 1987; Gómez de Castro 1989; Eiroa et al. 1992; Gómez de Castro et al. 1993). The westernmost condensation has a significant reflection component that most likely traces the bow-shaped borders of the cavity illuminated by the exciting star. The source of the flow has not yet been identified. Spectra of GGD 34 have been published by Goodrich (1986), Movsessian (1992) and Gómez de Castro et al. (1993). The spectra show emission lines of low ionization or neutral species such as [O I] or [S II]; the post-shock electron density is very low: [FORMULA] cm-3 (Goodrich 1986; Movsessian 1992; Gómez de Castro et al. 1993). The radial velocity field along the jet can be well fitted to a sine curve with mean velocity [FORMULA] km s-1 and maximum of -240 km s-1 (Gómez de Castro et al. 1993). Recent independent studies confirm this behaviour (Magakian & Movsesian 1997). The correlation between the morphology and the radial velocity field, as well as the lack of a correlation between the velocity field and the excitation conditions of the gas, led Gómez de Castro et al. (1993) to suggest that the variations are associated with geometric distortions that could be induced by the onset of an instability in the jet.

In this work we present high resolution images of GGD 34 obtained with the CFHT and the High Resolution Camera (HRCam). The comparison between these new images with previous lower spatial resolution images (Ray 1987; Eiroa et al. 1992; Gómez de Castro et al. 1993) suggests that the large scale wiggles are most likely associated with a low-surface-luminosity nebula emitting primarily in [S II]; the jet itself being well collimated. Nevertheless, we emphasize that the jet does wiggle from side to side although the amplitudes of its oscillations are much smaller than in the case of the nebula. Note also that the huge amount of energy needed to feed distortions in the jet of 10% of its length makes them very improbable anyway and that such distortions have never been seen before in much more closer jets (e.g. the distance to NGC 7129 is 7 times the distance to Taurus i.e. 140 pc (Elias 1978)).

We also present radiocontinuum (3.6 and 6 cm) VLA observations intended to search for the source of the outflow. 12CO(3-2) line observations have also been obtained to study the perturbations produced by the jet in the heavily stirred environment of the NGC 7129 molecular cloud. The observations are described in Sect. 2. The VLA data are used in Sect. 3 to identify the source of jet. In Sect. 4 we study the structure of GGD 34 in view of the new optical images. The variability of the jet is analyzed in Sect. 5. A brief discussion on the nature of the envelope is given in Sect. 6 and the main conclusions of the work are summarized in Sect. 7.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999