The circumstellar matter distribution of massive young stellar objects

A multiwavelength study of the circumstellar matter distribution of massive young stellar objects (MYSOs) was conducted. First, the potential of the new Herschel 70 micron data to resolve MYSOs in the Hi-GAL survey was analysed. These data have the highest resolution achieved at far infrared wavelengths where the spectral energy distribution of MYSOs peaks. These data showed that relatively isolated sources with high L^0.5/d, where L is the luminosity and d the distance of the source, are resolved at 70 micron. The analysis of these data and 1-D spherically symmetric radiative transfer modelling of three sources in the l=30 deg and 59 deg fields showed that they have a shallower density power law index than expected for infalling material. This suggests that the far-IR emission may be dominated by warm dust from the outflow cavity walls rather than rotational flattening as suggested by earlier studies. In order to explain the 70 micron observations, the circumstellar matter of the proto-typical MYSO AFGL 2591 was studied by utilising and modelling full resolution Herschel data from the HOBYS survey and other multi-wavelength dust continuum observations including high-resolution near-IR and mm interferometric data. A 2-D axi-symmetric radiative transfer model was used to find the density and temperature distributions that better reproduce the observations. The model that best fits the continuum observations has a rotationally flattened envelope, paraboloidal outflow cavities and a flared disc with a mass of 1 solar mass. As a result it was found that the extended emission observed at 70 micron can be explained in part by dust emission from the envelope outflow cavity walls. The modelling was able to reproduce most of the other multi-wavelength observations. Finally, the velocity structure of gas in the envelope of AFGL 2591 was studied by modelling methyl cyanide observations in the CH3CN J=12-11 transition at 1.3 mm. The transition K-ladder was fitted assuming a constant density and isothermal distribution of gas, and an excitation temperature ranging between 100-300 K was found. In addition, the first moment (velocity) maps are consistent with rotation of the inner envelope, and its linear velocity gradient is slower than the one observed at smaller scales. The radiative transfer modelling of the methyl cyanide data with a velocity structure of a rotating and infalling envelope suggests that rotation is faster than predicted by the model. This may be solved by magnetic fields transporting angular momentum from the accretion disc.