The origin of mid-infrared emission in massive young stellar objects: multi-baseline VLTI observations of W33A

TL;DR

This study uses VLTI observations and radiative transfer models to analyze the circumstellar structure of W33A, revealing the dominance of outflow cavity walls in mid-infrared emission and constraining the presence of accretion disks.

Contribution

It provides the first detailed interferometric modeling of W33A's inner structure, integrating multi-baseline data with advanced 2D models to constrain disk and envelope properties.

Findings

Mid-infrared emission is dominated by outflow cavity walls.

Dust disks with masses over 0.01 Msun are ruled out.

Optically thick accretion disks are consistent with observations.

Abstract

The circumstellar structure on 100 AU scales of the massive young stellar object W33A is probed using the VLTI and the MIDI instrument. N-band visibilities on 4 baselines are presented which are inconsistent with a spherically symmetric geometry. The visibility spectra and SED are simultaneously compared to 2D axi-symmetric dust radiative transfer models with a geometry including a rotationally flattened envelope and outflow cavities. We assume an O7.5 ZAMS star as the central source, consistent with the observed bolometric luminosity. The observations are also compared to models with and without (dusty and gaseous) accretion disks. A satisfactory model is constructed which reproduces the visibility spectra for each (u,v) point. It fits the silicate absorption, the mid-IR slope, the far-infrared peak, and the (sub)mm of the SED. It produces a 350 micron morphology consistent with observations. The 10 micron emission on 100 AU scales is dominated by the irradiated walls of the cavity sculpted by the outflow. The visibilities rule out the presence of dust disks with total (gas and dust) masses more than 0.01 Msun. However, optically thick accretion disks, interior to the dust sublimation radius, are allowed to accrete at rates equalling the envelope's mass infall rate (up to 10^(-3) Msun/yr) without substantially affecting the visibilities due to the extinction by the extremely massive envelope of W33A.