VLTI/MIDI 10 micron interferometry of the forming massive star W33A

TL;DR

This study uses VLTI/MIDI interferometry to measure the size and structure of the massive young stellar object W33A, revealing larger-than-expected sizes and suggesting a lower luminosity than previously assumed, with implications for dust distribution models.

Contribution

First interferometric measurements of W33A at 8-13 microns reveal size scales and dust properties, challenging previous luminosity estimates and informing dust density models.

Findings

W33A has a size of ~120AU at 8 microns, increasing to 240AU at 13 microns.

A reduced luminosity of one-third of previous estimates fits the interferometric data.

Shallow dust density distributions and specific dust compositions best match observations.

Abstract

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 45m over the wavelength range 8-13 micron. The visibilities indicate that W33A has a FWHM size of approximately 120AU (0.030'') at 8 micron which increases to 240AU at 13 micron, scales previously unexplored among MYSOs. This observed trend is consistent with the temperature falling off with distance. 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 powerlaw 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 micron 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 ISM ratio of graphite to silicates and using optical grain properties by Ossenkopf et al. (1992).