A multi-scale exploration of a massive young stellar object - a
  transition disk around G305.20+0.21?
  The rarity of young massive stars combined with the fact that they are often
deeply embedded has limited the understanding of their formation. Ground based
mid-infrared (IR) interferometry is one way of securing the spatial resolution
required to study massive young stellar objects (MYSOs) and as the
spatial-frequency coverage of such observations is often incomplete,
direct-imaging can be supplementary to such a dataset. By consolidating these
observations with modelling, the features of a massive protostellar environment
can be constrained. This work simultaneously fits the aforementioned
observations and a spectral energy distribution (SED) with a 2.5D radiative
transfer model, providing an extensive view of the physical characteristics of
the accreting regions of the MYSO G305.20+0.21. The high-resolution
observations were obtained using the Very Large Telescope's MIDI and VISIR
instruments, producing visibilities in the N-band and near-diffraction-limited
imaging in the Q-band respectively. A model including a central protostar with
a luminosity of $\sim$5$\times$10$^{4}$L$_{\odot}$ surrounded by a low-density
bipolar cavity, a flared 1M$_{\odot}$ disk and an envelope provides a
sufficient fit all three types of observation. The need to include a disk in
the model implies that this MYSO follows a scaled-up version of the low-mass
star formation process. The weak silicate absorption feature within the SED
requires low-density envelope cavities to be successfully fit and is an
atypical characteristic in comparison to previously studied MYSOs.
Additionally, the inner radius of the disk must be three times the dust
sublimation radius to satisfy the MIDI visibilities. The low density, low
extinction environment implies the object is a more evolved MYSO and this
combined with large inner radius of the disk suggests that it could be an
example of a transitional disk around an MYSO.
