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Astron. Astrophys. 345, 949-964 (1999)

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1. Introduction

The study of the formation of massive stars is directly related to the investigation of sources believed to be the signposts of embedded luminous young stellar objects (YSOs). Such signposts are mostly ultracompact (UC) HII regions, H2O masers, and IRAS sources selected on the basis of their far-infrared (FIR) colours (see Kurtz et al. 1999and references therein). In recent years, the attention has gradually shifted towards regions with higher density and temperature, which should represent the environment immediately facing the embedded YSO. In particular, an ever increasing number of dense, hot molecular condensations have been found close to (but not necessarily associated with) UC HII regions and coincident with H2O masers: these are named "hot cores". The interest in such objects is justified by the fact that very likely they represent the site where high-mass stars are born (Kurtz et al. 1999).

The relevance of the hot core fine structure in the study of massive star formation is one of the reasons which led Cesaroni et al. (1997) (hereafter CFTWO) to perform high angular resolution observations of the molecular clump associated with the luminous source IRAS 20126+4104, situated at a distance of 1.7 kpc. By mapping the CH3CN and HCO+ emission with the Plateau de Bure interferometer (PdBI), they were able to resolve the inner part of the bipolar outflow and detect a hot core at the geometric centre of the flow. Both the core and the outflow are centred at the nominal position of the IRAS source. Interestingly, the hot core is affected by a velocity gradient perpendicular to the axis of the flow, which is suggestive of rotation. Also, H2 emission at 2.122 µm is seen along the axis of the flow, which is likely arising from shocked gas (see Ayala et al. 1998) in a jet propagating from the embedded YSO. CFTWO conclude that IRAS 20126+4104 is very likely a disk-outflow system around a young massive (proto)star.

In this work we present a follow-up of the study of CFTWO. The main goals of our project were: to map the hot core with higher angular resolution in the CH3CN lines, in order to get a better picture of the velocity gradient and confirm the rotating disk hypothesis; to study structure and kinematics of the jet by means of the SiO emission, which is known to arise from shocked molecular gas; to obtain high angular resolution images of the continuum emission in IRAS 20126+4104, from the millimeter to the mid-infrared (MIR). The latter results are important to assess which fraction of the FIR emission measured at low angular resolution by IRAS is coming from the hot core, and hence give a more accurate estimate of its bolometric luminosity. In order to attain these goals, we have performed a whole set of observations using a variety of instruments from the millimeter (with the PdBI), to the sub-millimeter (with the James Clerk Maxwell telescope; hereafter JCMT), to the MIR (with the United Kingdom infrared telescope; hereafter UKIRT). The technical details of these observations are given in Sect. 2, while the results are illustrated in Sect. 3 and discussed in Sect. 4. Finally, the conclusions are drawn in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999