Resolved 24.5 micron emission from massive young stellar objects

Abstract

Context. Massive young stellar objects (MYSO) are surrounded by massive dusty envelopes, whose physical structure and geometry are determined by the star formation process. Aims. Our principal aim is to establish the density structure of MYSO envelopes on scales of ∼1000 AU. This constitutes an increase of a factor ∼10 in angular resolution compared to similar studies performed in the (sub)mm. Methods. We have obtained diffraction-limited (0.6 �� ) 24.5 μm images (field of view of 40 �� × 30 �� ) of 14 well-known massive star formation regions with the COMICS instrument mounted on the 8.2 m Subaru telescope. We construct azimuthally averaged intensity profiles of the resolved MYSO envelopes and build spectral energy distributions (SEDs) from archival data and the COMICS 24.5 μm flux density. The SEDs range from near-infrared to millimeter wavelengths. Self-consistent 1-D radiative transfer models described by a density dependence of the form n(r) ∝ r −p are used to simultaneously compare the intensity profiles and SEDs to model predictions. Results. The images reveal the presence of discrete MYSO sources which are resolved on arcsecond scales, and, to first-order, the observed emission is circular on the sky. For many sources, the spherical models are capable of satisfactorily reproducing the 24.5 μm intensity profile, the 24.5 μm flux density, the 9.7 μm silicate absorption feature, and the submm emission. They are described by density distributions with p = 1.0 ± 0.25. Such distributions are shallower than those found on larger scales probed with single-dish (sub)mm studies. Other sources have density laws that are shallower/steeper than p = 1.0 and there is evidence that these are viewed near edge-on or near face-on respectively. In these cases spherical models fail to provide good fits to the data. The images also reveal a diffuse component tracing somewhat larger scale structures, particularly visible in the regions S 140, AFGL 2136, IRAS 20126+4104, Mon R2, and Cep A. Conclusions. We find a flattening of the MYSO density law going from scales probed with single-dish submm observations down to scales of ∼1000 AU probed with the observations presented here. We propose that this may be evidence of rotational support of the envelope. This finding will be explored further in a future paper using 2-D axisymmetric radiative transfer models.