2007ApJ...671L.169D

We report on resolved interferometric observations with VLTI/MIDI of the massive young stellar object (MYSO) W33A. The MIDI observations deliver spectrally dispersed visibilities with values between 0.03 and 0.06, for a baseline of 45 m over the wavelength range 8-13 µm. The visibilities indicate that W33A has a FWHM size of approximately 120 AU (0.030") at 8 µm that increases to 240 AU at 13 µm, scales previously unexplored among MYSOs. This observed trend is consistent with the temperature falling off with distance. One-dimensional (1D) dust radiative transfer models are simultaneously fit to the visibility spectrum, the strong silicate feature, and the shape of the mid-infrared spectral energy distribution (SED). For any power-law density distribution, we find that the sizes (as implied by the visibilities) and the stellar luminosity are incompatible. A reduction to a third of W33A's previously adopted luminosity is required to match the visibilities; such a reduction is consistent with new high-resolution 70 µm data from Spitzer's MIPSGAL survey. We obtain best fits for models with shallow dust density distributions of r^–0.5 and r^–1.0 and for increased optical depth in the silicate feature produced by decreasing the interstellar medium (ISM) ratio of graphite to silicates and using optical grain properties of Ossenkopf et al.