3.1. The morphology
The multi-wavelength emission around the IRAS 20126+4104 source shows a very complex structure. Wilking et al. (1990) have detected a N-S bipolar CO outflow, with a total angular extent of . This outflow appears to be centred on a position to the SW of the IRAS source. Fig. 1 (top) shows this outflow overlaid on an H2 image, where the Northern lobe corresponds to the blue-shifted gas and the Southern lobe to the red-shifted gas.
Cesaroni et al. (1997) have detected an outflow in the same region, oriented in a NE-SW direction, with a total angular extent of . We show an overlay of this outflow on our H2 1-0 S(1) high resolution image in Fig. 3. This outflow is centred on the nominal position of the IRAS source, which also coincides with a compact molecular core and a compact radio continuum source detected at mm/cm wavelengths (Cesaroni et al., 1997; Martí & Rodríguez, 1997).
On the , H2 1-0 S(1) line (+ continuum) narrow-band image shown in Fig. 1, there are three bright nebulous objects located very near to the IRAS 20126+4104 position, which have previously been reported (Hodapp 1994; Cesaroni et al. 1997; Ayala et al. 1997). The brightest object (labeled C in figure 2) has an integrated flux in the H2 1-0 S(1) line of in an aperture of , and its surface brightness is . This object appears to be brighter than HH objects such as HH43, which has a flux in the same line of in an aperture of and has a surface brightness of (Gredel, 1994). Condensation C has a cometary morphology, and appears to be connected with the star located at the Eastern end of the object; the flux presented above for condensation C does not include the stellar emission. Condensation B is smaller and fainter than C, with a cometary morphology and a weak embedded star at its Eastern edge. Its surface brightness in the H2 1-0 S(1) line is .
Two more interesting objects appear in our H2 1-0 S(1) image. An object with compact morphology located Northwest of the IRAS source (20:12:39.8 +41:04:42, 1950; labeled D in figure 1), which is not detected in our cK image, indicating that it is an emission line object with low surface brightness ( within a circular aperture with radius of ). The other outstanding feature in this region is the jet-like object located to the Southeast of the IRAS source (20:12:42.8 +41:03:43, 1950; labeled "jet" in figure 1), observed by Hodapp (1994) in a -band image. This object looks like an almost North-South oriented bipolar jet (length and PA= ), which is still visible on a continuum subtracted H2 image. This emission apparently connects a central star with knot-like nebulosities at the two ends of the bipolar structure, having a surface brightness of about within a circular aperture of radius (see Fig. 1, bottom panel).
The brighter NIR emission detected in our images (knots A, B and C in figure 2) has two main maxima, which approximately coincide with the lobes of the outflow, as shown in Fig. 3. This coincidence between the IR and emission was also noted by Cesaroni et al. (1997), who concluded that the IR emission is directly associated with the two lobes of the outflow.
Also quite remarkable is the fact that the IR emission NW of the IRAS source appears to be divided into two H2 -emitting, E-W directed ridges (condensations B and C of figure 2). Interestingly, a star is located at the Eastern end of each of these ridges. This can be appreciated by comparing the three panels of Fig. 2, and is confirmed by the spectroscopic data (described in the following section), which clearly show the stellar continua. In the region SE of the IRAS source (condensation A in figure 2), we detect a structure with two maxima, only one of which still remains in the continuum subtracted image (see figure 2), with a surface brightness of in the H line.
3.2. Spectroscopic characteristics
From the three slit positions (1, and ) shown in Fig. 2(a), we can construct spatially integrated spectra for condensations B and C (NW of the IRAS source, see figure 2), and condensation A (SE of the IRAS source). These spectra are shown in Figs. 4 and 5. The integrated fluxes of the detected lines were extracted by fitting Gaussian profiles to each line, all of them required to have the same width (corresponding to that of the H2 1-0 S(1) line). Table 2 lists the wavelength (Black & van Dishoeck, 1987), the energy of the upper level (Dabrowski 1984) and the measured fluxes of the detected H2 lines for each object. The portions of the slit integrated for each object are shown in the upper panels of Figs. 4 and 5. For object C we have constructed a one-dimensional spectrum integrating over along slit 1 (see the upper panel in figure 4). The aperture includes the weak star to the E, indicated in Fig. 4, which contributes approximately 10% of the total emission at . From the spectrum constructed with slits 2a+2b (see the upper panel of figure 5) we extracted spectra for condensations A and B, using and apertures, respectively.
Table 2. Integrated H2 line fluxes for objects A, B and .
In condensation C, we have detected 6 low excitation H2 lines (including both 1-0 and 2-1 vibrational transitions), and only 3 lines in the weaker condensations A and B. The measured 1-0 S(1)/2-1 S(1) line ratio for condensation C is , which is comparable with the value of 10 expected for collisionally excited H2 levels (see, e. g., Gredel et al. 1992; Gredel 1994). However, if we determine an upper limit for the 2-1 S(1) line flux over the spectra of the knots A and B, the estimated value of the 1-0 S(1)/2-1 S(1) line ratio is about 5, which would be somewhat low compared with the value for collisionally excited H2 mentioned above.
We have also analyzed the spatially resolved line intensities, measured along the slit positions shown in Fig. 2(a). We computed the column densities assuming that the lines are optically thin and using the spontaneous radiative decay probabilities from Turner et al. (1977). Figs. 6 (slit 1) and 7 ( slit) show the column densities as a function of the energy of the upper level for the transition, deduced from the spatially integrated spectra. The successive panels correspond to spectra extracted in discrete positions along the spectrograph slits, integrating spatially over . Where the lines were not clearly detected, we estimated an upper limit for the intensities and a column density was computed using these values. The slope obtained by fitting the points in a plot of versus excitation energy is inversely proportional to the excitation temperature, where g is the corresponding level degeneracy. From these Figures, it is clear that we detect a rather strong spatial variability for the measured column densities. The derived values for different vibrational levels do not lie on a single smooth curve, as would be expected for emission produced by a shock (Burton et al. 1990).
By fitting straight lines to the column density versus excitation energy diagrams, we determine two excitation temperatures: (obtained from fitting all of the observed lines) and (obtained from a fit to the 1-0 transitions only). These temperatures are plotted as a function of position along the corresponding spectrograph slits in Figs. 8 (slit 1, covering condensation C) and 9 (slit + , covering condensations A and B).
Along slit 1 (figure 8, panels (b) and (c)), we systematically have . This discrepancy between the two temperatures indicates that the excitation of the vibrational level could have an important fluorescent component. This is also seen from the 1-0 S(1)/2-1 S(1) line ratio (figure 8(e)). We see two main regions, one where the line ratio has a mean value of (close to the star), and the other with a value , and with a general tendency of decreasing line ratios towards the star. The behavior of this ratio close to the star is suspicious, because in this region the intensity of the lines decreases. In order check this behavior, we calculate the ratio over spectra integrated in only two bigger apertures which do not include the star. Using apertures of (centred on a position to W from the star) and (centred to W from the star). We find 1-0 S(1)/2-1 S(1) line ratios of and , respectively. We therefore find that the tendency presented in Fig. 8 (e) for the 1-0 S(1)/2-1 S(1) line ratio remains. In the case of the vibrational temperature , the region closer to the star has a mean temperature of about 1000 K, and it decreases slightly towards the W along condensation C (over an angular scale of ). On the other hand, the excitation temperature, , has an approximately constant value of 2400 K over the same angular scale. This value is somewhat low compared with the values observed in collisionally and fluorescent excited objects such as DR 2, where 3000 K using only 1-0 and 2-1 s(1) lines (Fernandes et. al 1997), and is also consistent with collisionally excited objects like HH 43, HH 120 and HH 99A in which 2200 K (Gredel 1994).
In all of the objects analyzed in this paper, the transitions between and are consistent with what is expected for levels populated in a thermalized gas at K. This temperature is lower than the ones found in the considered collisionally excited objects, where the excitation temperature estimated for the lowest transitions is of K. The larger difference between and found near the star at the E end of condensation C together with the lower 1-0 S(1)/2-1 S(1) line ratio found in this region indicate that the H2 molecules are excited by other processes besides collisions, like in DR 21 (Fernandes et. al 1997). A possible explanation for our results is that the star at the E of this condensation produces an UV field that significantly contributes to the excitation of the hydrogen molecules.
From the spectrum obtained through slits + (figure 9) we see that condensations A and B, K, which is substantially smaller than a K estimated using an flux upper limit for 2-1 lines determinate over each spectra. The value is consistent with the mean temperature value founded for object C.
We also computed empirical values of the ratio of ortho-hydrogen to para-hydrogen, from ratios of available lines of even and odd J values in both vibrational levels. In panel (d) of Fig. 8 we plot the ortho/para ratio for condensation C, derived from the 1-0 S(1) and S(2) transitions (open squares) and the corresponding ratio from the 2-1 S(1) and S(2) transitions (open triangles). Along the slit the ortho/para ratio for 1-0 is near the LTE value of 3 (within error), as expected for collisional excitation. For the 2-1 transitions, we have lower values () for this ratio than the equilibrium value, which is characteristic of objects where there is a contribution by fluorescent excitation. For comparison, the ortho/para ratio estimated for the spectrum of the planetary nebula Hubble 12, where the H2 lines is mainly excited by fluorescence, is (Ramsay et al. 1993). In condensation B the values of the ortho/para ratio estimated from the 1-0 transitions are about 3.1, which is consistent with the equilibrium value. In the case of condensation A we can only estimate lower limits for this ratio ().
From these results, we conclude that the H2 excitation in condensation C has a strong fluorescent component in the Eastern region, close to the star that is observed at the E end of this condensation. Towards the W, the H2 excitation along condensation C appears to be dominated by collisions (as supported by the high 1-0 S(1)/2-1 S(1) line ratio). On the other hand, the available data for the weak condensations A and B are not sufficient in order to propose the mechanism of molecular excitation more suitable in each case.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998