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

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4. The structure of the jet

4.1. The base

The new VLA observations indicate that the source driving GGD 34 is deeply embedded. The source illuminates a reflection nebula that surrounds the base of the jet and extends along the northeastern edge. There is a clear brightness minimum in the I-band image at the apex of the nebula through which the optical jet emerges (see Fig. 1); the jet has excavated its way through the nebula outwards from the cloud core. The nebula is brighter in H[FORMULA] than in [S II] because the star has strong H[FORMULA] emission (Gómez de Castro et al. 1993), this produces a noticeable apparent "misalignment" between the jet orientation in the H[FORMULA] and [S II] images. The [S II] emission is well aligned with the major axis of GGD 34, however the H[FORMULA] emission is [FORMULA] 17o  NE of the major axis. This "misalignment" readily disappears when the nebular contribution is subtracted out.

4.2. The body

The body of the jet is seen primarily in [S II]. The improved resolution CFHT images show a narrow jet which roughly bisects an extended faint envelope. The peak emission of condensations B and C is well aligned with the major axis however the condensations themselves are markedly non-axisymmetric. The jet shows wiggling and several abrupt changes in its apparent direction in GGD 34/B.

The width of the jet is unresolved. There are 5 main knots, shown in the H[FORMULA] panel of Fig. 1, whose coordinates are given in Table 3 in order of increasing right ascension. Two of them are associated with GGD 34/B and 3 with GGD 34/C. The size of some knots within GGD 34/C is resolved. These sizes are given in Table 3 assuming a gaussian transfer function for the optical system with FWHM=0[FORMULA]7; they are typically between 0.3 and 1.5 arcsec (300-1500 AU). These sizes are significantly larger than the inferred from recent HST images of YSOs jets ([FORMULA] AU size) suggesting that they may be constituted of finer structures unresolved in the CFHT images.


Table 3. Characteristics of the main knots in GGD 34

The ratio of H[FORMULA] to [S II] fluxes is often used as a diagnostic of shock strength with larger ratios corresponding to higher shock velocities for shock velocities smaller than 80 km s-1 (Hartigan et al. 1987). The H[FORMULA]/[S II] ratio for the knots in GGD 34 is also given in Table 3. These values are normalized to the H[FORMULA]/[S II] ratio in Knot 1. The excitation is significantly larger in the Knots 3 and 5 than in the rest of the jet suggesting larger shock velocities at these points. In particular, the H[FORMULA]/[S II] ratio is a factor of 2 larger in Knot 5 than in Knots 1, 2 or 4.

4.3. The working surface

The optical jet ends at GGD 34/C. The [S II] emission region has an arrow or bow-shaped morphology; the body of the jet is clearly distinguished as well as two back tails disposed in an approximately symmetric manner with respect to the jet axis. The H[FORMULA] emission is concentrated at the head of the jet (Knot 5); the two back tails also emit in the H[FORMULA] line but the northern (Knot 3) is significantly brighter than the southern (Knot 4) while in [S II] both have similar strengths. There is a protrusion NE off the axis which emits mainly in H[FORMULA]. These characteristics are better summarized in Fig. 5, where the H[FORMULA] -[S II] image of the head of the jet is represented. The H[FORMULA] emission dominates at the head of the jet (Knot 5), in the protrusion and in Knot 3. This high degree of excitation as well as its morphology (note the flat head of Knot 5) suggest that GGD 34/C marks the location of the working surface of the jet. In the simplest case the working surface consists of a bow shock where the ambient material is accelerated and a jet shock or Mach disk, where the jet material is decelerated. The Mach disk is the stronger shock when the jet is less dense than the ambient medium, while the bow shock is stronger for heavy jets.

[FIGURE] Fig. 3. H[FORMULA]-[S II] image of the working surface (GGD 34/C). The scale has been selected so dark grey corresponds to the maximum H[FORMULA] emission and white to the maximum [S II] emission. The scale enhances the most relevant features. North is up and East to the left.

The spectra of GGD 34 indicate that it is a beam of relatively light gas with respect to its ambient environment; the ratio between the intensities of the [S II] lines, I6717/I6730 is [FORMULA] (Goodrich 1986; Gómez de Castro et al. 1993) implying post-shock electron densities of 30 to 60 cm-3. The low excitation degree of the gas suggests small ionization fractions [FORMULA] 10% which points to post-shock jet particle densities of [FORMULA] cm-3. These values are smaller than the average density of the molecular cloud in this area which is [FORMULA] 103 cm-3 (Bechis et al. 1978). Note moreover, that there is a very steep luminosity gradient at the eastern edge of GGD 34/C (Knot 5) suggesting that the outflow has encounter an obstacle denser than the cloud environment. This makes us suspect that the brighter shock in GGD 34/C is the jet shock and that therefore Knot 5 is tracing the Mach disk. The fact that it emits strongly in H[FORMULA] is also consistent with the jet being mostly neutral. Existing spectra have not enough spatial resolution to resolve its kinematical signatures (Goodrich 1986; Gómez de Castro et al. 1993; Magakian & Movsesian 1997). It is tempting to associate the sudden decrease in radial velocity in GGD 34/C with the Mach disk. However, this deceleration is more evident at the north-east end, and it is best traced in H[FORMULA] than in [S II] indicating that it is most likely associated with the protrusion. This protrusion is probably tracing the path of the flow after a grazing collision with some obstacle; maybe some material could also be entrained by the jet at this point.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999