Resolved 24.5 micron emission from massive young stellar objects

TL;DR

This study uses high-resolution 24.5 micron imaging to analyze the density structure of massive young stellar objects, revealing shallower density profiles at smaller scales and evidence of rotational support.

Contribution

It provides the first high-resolution mid-infrared observations of MYSOs, showing a change in density profiles at smaller scales compared to larger-scale studies.

Findings

Density profiles are shallower (p ≈ 1.0) at ~1000 AU scales.

Evidence of envelope flattening suggests rotational support.

Diffuse larger-scale structures are observed in several regions.

Abstract

Massive young stellar objects (MYSO) are surrounded by massive dusty envelopes. Our aim is to establish their density structure on scales of ~1000 AU, i.e. a factor 10 increase in angular resolution compared to similar studies performed in the (sub)mm. We have obtained diffraction-limited (0.6") 24.5 micron images of 14 well-known massive star formation regions with Subaru/COMICS. The images reveal the presence of discrete MYSO sources which are resolved on arcsecond scales. For many sources, radiative transfer models are capable of satisfactorily reproducing the observations. They are described by density powerlaw distributions (n(r) ~ r^(-p)) 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 MYSOs are viewed near edge-on or near face-on, respectively. The images also reveal a diffuse component tracing somewhat larger scale structures, particularly visible in the regions S140, AFGL 2136, IRAS 20126+4104, Mon R2, and Cep A. We thus find a flattening of the MYSO envelope density law going from ~10 000 AU down to scales of ~1000 AU. We propose that this may be evidence of rotational support of the envelope (abridged).