Molecular jets driven by high-mass protostars:: a detailed study of the IRAS20126+4104 jet

Context. Protostellar jets from intermediate- and high-mass protostars provide an excellent opportunity to understand the mechanisms responsible for intermediate- and high-mass star-formation. A crucial question is if they are scaled-up versions of their low-mass counterparts. Such high-mass jets are relatively rare and, usually, they are distant and highly embedded in their parental clouds. The IRAS 20126+4104 molecular jet, driven by a 10(4) L-circle dot protostar, represents a suitable target to investigate. Aims. We present here an extensive analysis of this protostellar jet, deriving the kinematical, dynamical, and physical conditions of the H-2 gas along the flow. Methods. The jet was investigated by means of near-IR H-2 and [Fe II] narrow-band imaging, high-resolution spectroscopy of the 1-0 S(1) line (2.12 mu m), NIR (0.9-2.5 mu m) low-resolution spectroscopy, along with ISO-SWS and LWS spectra (from 2.4 to 200 mu m). Results. The flow shows a complex morphology. In addition to the large-scale jet precession presented in previous studies, we detect a small-scale wiggling close to the source, which may indicate the presence of a multiple system. The peak radial velocities of the H-2 knots range from -42 to -14 km s(-1) in the blue lobe, and from -8 to 47 kms(-1) in the red lobe. The low-resolution spectra are rich in H2 emission, and relatively faint [Fe II] (NIR), [O I] and [C II] (FIR) emission is observed in the region close to the source. A warm H-2 gas component has an average excitation temperature that ranges between 2000 K and 2500 K. Additionally, the ISO- SWS spectrum reveals a cold component (520 K) that strongly contributes to the radiative cooling of the flow and plays a major role in the dynamics of the flow. The estimated L-H2 of the jet is 8.2 +/- 0.7 L-circle dot, suggesting that IRAS 20126+4104 has a significantly increased accretion rate compared to low-mass YSOs. This is also supported by the derived mass flux rate from the H-2 lines ((M) over dot out(H-2) similar to 7.5 x 10(-4) M-circle plus yr(-1)). The comparison between the H-2 and the outflow parameters strongly indicates that the jet is driving the outflow, at least partially. As already found for low-mass protostellar jets, the measured H-2 outflow luminosity is tightly related to the source bolometric luminosity. Conclusions. As for a few other intermediate- and high-mass protostellar jets in the literature, we conclude that IRAS 20126+4104 jet is a scaled-up version of low-mass protostellar counterparts.