3. The exciting source
The radio source is located to the southwest of GGD 34 and appear projected at the edge of the ammonia condensation mapped by Torrelles et al. (1983). It has a spectral index of 0.7 0.5. Within the error, this spectral index is consistent with the value of 0.6 expected for a thermal jet (Rodríguez 1997). The confirmation of this source as a thermal jet would require a high angular resolution map to search for the elongated morphology, aligned with the outflow axis, that characterizes these sources. Unfortunately, the source is too faint for this. The radio source is most probably associated with the optical jet on two grounds. First, most background sources have negative spectral indices, while the source detected has a positive spectral index. Second, the a priori possibility of having a background 3.6 cm source with a flux density of 0.17 mJy inside a solid angle of 0.2 square arcmin is only 0.01. The 12CO(3-2) observations also show some degree of activity in this area (the south-west border) of the map in that gas, redshifted by 2.5 km s-1 with respect to the cloud emission, is detected at this location. This redshifted gas suggestively lies opposite the optical blueshifted jet and seems to be associated with the VLA source so it could be tracing the receeding part of the flow. Unfortunately the spatial resolution, and especially the coverage of our map, is not good enough to determine whether it is associated with the VLA source or with a nearby structure. Note that NGC 7129 is a very active region of star formation.
The location and properties of the radio source suggest that it may be embedded in a cloud core and excite GGD 34 to its east, outside this core, producing an optically visible object. The radio source is nearly aligned with the jet axis: it is 29 south of the axis and only 3" westwards of the cavity (GGD 34/A). Recent, H2 and K-band images of NGC 7129 show the presence of an extended nebulosity in this area which is most likely caused by scattered light from the exciting source of GGD 34 (Palacios et al. 1997). The nebulosity extends southwest from the optical cavity in the direction of the VLA source. No IRAS counterpart to the VLA source has been found in the point sources catalogue. At a distance of 1 kpc, the VLA source has a radio "luminosity" of about 0.17 mJy kpc2. This luminosity is typical of low bolometric luminosity YSOs as discussed by Anglada (1996). If the source were at the distance of Orion, its flux density would be comparable with that of HH 1-2 VLA1, the source that excites this classic Herbig-Haro system (Rodríguez 1990).
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999