In recent years, shocked molecular hydrogen (H2) emission has been found in association with Herbig-Haro objects and/or with molecular outflows, suggesting that these manifestations of the outflow phenomenon are closely related.
At the present time, the relationship between the highly supersonic collimated gas (optical and/or NIR jet) and the slower, denser and less collimated molecular outflow seen mainly in CO, SiO and , as well as the nature of the excitation of the molecular emission observed in the NIR are not clear. The scenarios which have been proposed to explain this relationship fall into two main groups: two-wind models, and unified models. In the former, the supersonic jet arises from a stellar wind, while the slower and less collimated molecular outflow arises from an accretion disk. In the latter, the jet provides the necessary momentum to drive the molecular outflow, either by displacing jet gas sideways in a `wake' (Raga and Cabrit 1993) or by sweeping up molecular gas in `shells' (Chernin & Masson 1993).
Direct imaging of molecular outflows has revealed a variety of morphologies in the H2 line emission, including compact structures, filamentary jet-like structures or sweeping bow shocks; and in several cases diffuse, extended structures. Spectroscopic studies of the near infrared (NIR) H2 emission in molecular outflows show that the molecular hydrogen is thermally excited, probably in molecular shock waves (e.g., Gredel 1994; Schwartz et al. 1995). However, in some cases the H2 upper vibrational levels appear to be excited by fluorescence (e.g., Fernandes & Brand 1995; Fernandes, Brand & Burton 1996). The molecular lines produced by collisionally excited H2 molecules are typically characterized by an excitation temperature Texc 2000-3000 K, while in the cases where the upper vibrational levels are contaminated by fluorescence, the observed lines cannot be explained with a single excitation temperature.
IRAS 20126+4104 is situated in a dark globule in the Cygnus-X -region at a distance of 1.7 kpc. This source is associated with a high-velocity molecular outflow roughly oriented in the North-South direction (Wilking, Blackwell & Mundy, 1990). Based on its far-infrared colour characteristics, this IRAS source has been classified as an ultracompact (UC) HII region (Bronfman et al. 1996, Molinari et al. 1996). In the near-infrared, this region has been observed in the -band by Hodapp (1994), who found a bipolar nebulosity close to the center of the outflow, which is embedded in a more extended, diffuse nebulosity. More recently, Cesaroni et al. (1997) found a compact, bipolar molecular outflow in and CS in the SE-NW direction, as well as extended NIR H2 line emission and continuum structures, nearly aligned with this outflow. They also found a compact 3-mm continuum source located close to its center. This radio continuum source was also detected at centimeter wavelengths by Martí and Rodríguez (1997), showing that its spectral index is consistent with optically thick free-free emission at centimeter wavelengths, and with optically thin dust emission at millimeter wavelengths (see also Cesaroni et al. 1997). This radio source is deeply embedded in a dense molecular core observed in 13 CO, CS, CH3 CN and CH3 OH, and has a number of H2 O maser spots coinciding with it (Cesaroni et al. 1997). These characteristics indicate that the powering source of this bipolar outflow is associated with a very young B2.5-B0.5 star.
We present narrow-band H2 and broad-band images with high spatial resolution around this IRAS source. These images show two bright nebulae with cometary morphology, apparently associated with embedded stars. We discuss the origin of the excitation of the H2 line emission based on low resolution spectra taken along these extended structures.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998