Modeling the NIR-silhouette massive disk candidate in M 17

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [stein;semenov]@mpia.de
² Astronomisches Rechen-Institut am Zentrum für Astronomie Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
³ Astronomisches Institut, Ruhr-Universität Bochum, 44780 Bochum, Germany e-mail: [chini;nielbock;hoffmeister]@astro.rub.de
⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: dnuernbe@eso.org
⁵ ERA/L3AB/Observatoire Aquitain des Sciences de l'Univers, 2 rue de l'Observatoire, 33270 Floirac, France e-mail: jean-marc.hure@obs.u-bordeaux1.fr
⁶ Université Bordeaux 1, 351 cours de la Libération, 33405 Talence, France
⁷ LUTh/Observatoire de Paris-Meudon (research associate), Place Jules Janssen, 92195 Meudon Cedex, France

Received: 6 October 2005
Accepted: 23 May 2006

Abstract

Aims.The physical properties of the massive disk candidate in the star-forming region M 17 are analyzed.

Methods.Making use of the rare configuration in which the gas and dust structure is seen in silhouette against the background radiation at m, we determine the column density distribution from a high-resolution NAOS/CONICA image. The influence of scattered light on the mass determination is analyzed using 3D radiative transfer calculations. Further upper flux limits derived from observations with the Spitzer telescope at MIR wavelengths are used together with the NACO image to estimate the flux from the central object. For a range of stellar radii, stellar surface temperatures, and dust grain sizes, we apply three different models to account for the observed fluxes. The stability of the disk against self-gravitational forces is analyzed calculating the ratio of the gravitational acceleration by the central object and the disk, and the deviations from a Keplerian profile.

Results.We find that the column density is consistent with a central source surrounded by a rotationally symmetric distribution of gas and dust. The extent of the symmetric disk part is about 3000 AU, with a warped point-symmetrical extension beyond that radius, and therefore larger than any circumstellar disk yet detected. The modeling yields a radial density powerlaw exponent of -1.1 indicating a flat radial density distribution, and a large e-folding scale height ratio  of about 0.5. The mass of the entire disk estimated from the column density is discussed depending on the assumed distance and the dust model and ranges between 0.02 and 5 . We conclude that unless a star is located close to the disk in the foreground, scattered light will have little influence on the mass determination. We present a Spitzer image taken at m with the disk seen in emission and identify polycyclic aromatic hydrocarbon (PAH) emission on the disk surface excited by the nearby massive stars as a possible source. Our 3D radiative transfer calculations for the scattered light image of the central source through an edge-on disk indicate that the elliptical shape seen in the NACO image does not require the assumption of a binary system and that it is consistent with a single object. We derive stellar main sequence masses of several , 50 , or 10 , depending on our assumptions that the extinction of the stellar flux is dominated (i) by the outer disk, (ii) by an inner disk comparable to the disks around intermediate-mass stars, or (iii) by an inner disk with dominating hot dust emission. We find that even for a star-disk mass ratio of 1, only the outer parts of the circumstellar disk may be influenced by self-gravity effects due to the large e-folding scale height ratio.