Astron. Astrophys. 332, 1055-1063 (1998)

4. Discussion

The NIR emission of the region around the IRAS 20126+4104 source shows three main condensations (A, B and C, see figure 2), which are approximately aligned with the outflow observed by Cesaroni et al. (1997) (see figure 3). As shown by these authors, this outflow is centred quite precisely on the position of the IRAS source, and a compact molecular core is also observed at this central position. This clear source+bipolar outflow morphology argues strongly for an association of the IR emission with the outflow from the IRAS source (see Cesaroni et al. 1997).

The detailed analysis of the IR spectra carried out above shows that the situation is probably somewhat more complex. Our images and spectra show that the NW IR lobe is composed of two E-W elongated emission line condensations (B and C, see figure 2), with a star at the E end of each condensation. This morphology is somewhat curious, and might suggest a possible association of the two stars with condensations B and C.

We also find that the observed H₂ line ratios indicate that the excitation of the rotational-vibrational levels has a strong fluorescent component. Particularly interesting is the stratification of the line ratios along condensation C (see figure 2), which shows a 1-0 S(1)/2-1 S(1) line ratio indicating fluorescence towards the E end of the condensation (i. e., close to the position of the star), and a line ratio indicating collisional excitation towards the Western end of the condensation. The weighted mean ortho/para ratio for all the condensations, , is lower than the equilibrium value of 3, consistent with a mixture of fluorescent and thermal excitation of H₂. The values of the ortho/para ratios measured for condensation C are consistent with objects such as NGC 7027, S106 and NGC 2023 (Tanaka et al. 1989) and DR 21 (Fernandes et. al 1997) , which have "mixed" fluorescent plus collisional excitation, having significantly lower ratios for the transitions than for the transitions.

However, it is unclear whether or not these observations indicate an association of condensations B and C with the stars embedded at their Eastern tips. If such an association does exist, the emission of these condensations might partly be associated with radiative excitations by a UV continuum and/or Lyman- emission from the circumstellar environment of these two stars. It might even be possible that these stars are illuminating the molecular gas associated with the outflow from the IRAS 20126+4104 source. However, the fact that the Western region of condensation C shows higher values for the 1-0 S(1)/2-1 S(1) line ratio (more consistent with collisional excitation of the levels of H₂) cannot be explained in straightforward manner with this interpretation.

To summarize, from our IR spectra we find partial evidence that the H₂ emission of condensations B and C (NW of IRAS 20126+4104) is associated with the stars embedded at the Eastern end of these condensations. On the other hand, these condensations also coincide spatially with the blue lobe of the outflow from IRAS 20126+4104 (Cesaroni et al. 1997), indicating a likely association of the H₂ emission with this flow.

The nature of the IR emission might be clarified with future, high spectral resolution spectroscopy. The resulting kinematic information would help to resolve the present, somewhat complex scenario.

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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