Abstract
Context. While most sources above 10 5 $L_{\odot}$ have already formed an Ultracompact Hii region (UCHii), this is not necessarily the case for sources of lower luminosity. Characterizing sources in the transition phase, i.e., very luminous objects without any detectable free-free emission, is important for a general understanding of massive star formation. Aims. We Characterize one of the most luminous High-Mass Protostellar Objects (HMPO) that has not yet formed any detectable UCHii region. Methods. The region was observed with the Submillimeter Array in three different array configurations at 875 μ m in the submm continuum and spectral line emission at sub-arcsecond resolution. Results. The 875 μ m submm continuum emission has been resolved into at least two condensations. The previously believed driving source of one of the outflows, the infrared source IRS1, is ~ 0.9” offset from the main submm peak. The data do not allow to differentiate whether this offset is real, either caused by different sources or a shift of the photo-center due to scattering, or whether it is only due to poor astrometry of the infrared data. Over the entire 4 GHz bandwidth we detect an intermediate dense spectral line forest with 27 lines from 8 different species, isotopologues or vibrationally-torsionally excited states. Temperature estimates based on the CH 3 OH line series result in values of T (Peak1) ~ 150 ± 50 K and T (Peak2) ~ 80 ± 30 K for the two submm peak positions, respectively. The SiO(8–7) red- and blue-shifted line maps indicate the presence of two molecular outflows. In contrast, the vibrationally-torsionally excited CH 3 OH line exhibits a velocity gradient approximately perpendicular to one of the outflows. With a size of approximately 5000 AU and no Keplerian rotation signature, this structure does not resemble a genuine accretion disk but rather a larger-scale rotating toroid that may harbor a more common accretion disk at its so far unresolved center.
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