Resolved 24.5 micron emission from massive young stellar objects

Issue		A&A Volume 494, Number 1, January IV 2009


Page(s)		157 - 178
Section		Interstellar and circumstellar matter
DOI		10.1051/0004-6361:200810771
Published online		04 December 2008

A&A 494, 157-178 (2009)
DOI: 10.1051/0004-6361:200810771

Resolved 24.5 micron emission from massive young stellar objects

W. J. de Wit¹, M. G. Hoare¹, T. Fujiyoshi², R. D. Oudmaijer¹, M. Honda³, H. Kataza⁴, T. Miyata⁵, Y. K. Okamoto⁶, T. Onaka⁷, S. Sako⁵, and T. Yamashita²

¹ School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
e-mail: w.j.m.dewit@leeds.ac.uk
² Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 650 North A'ohoku Place, Hilo, HI 96720, USA
³ Department of Information Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa, 259-1293, Japan
⁴ Department of Infrared Astrophysics, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 229-8510, Japan
⁵ Institute of Astronomy, University of Tokyo, Osawa 2-21-1, Mitaka, Tokyo 181-0015, Japan
⁶ Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
⁷ Department of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0022, Japan

Received 8 August 2008 / Accepted 24 November 2008

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 $\mu$ m images (field of view of $40\arcsec\times30\arcsec$ ) 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 $\mu$ m 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 $n(r) \propto r^{-p}$ 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 $\mu$ m intensity profile, the 24.5 $\mu$ m flux density, the 9.7 $\mu$ m silicate absorption feature, and the submm emission. They are described by density distributions with $p =1.0\pm0.25$ . 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.

Key words: stars: formation -- stars: imaging -- stars: early-type -- infrared: stars -- ISM: clouds

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.