Mid-infrared interferometry of the massive young stellar object NGC 3603 – IRS 9A^*

¹ Institut für Theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
² European Organisation for Astronomical Research in the Southern Hemisphere, Casilla 19001, Santiago 19, Chile
³ European Organization for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: chummel@eso.org
⁴ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁵ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
⁶ Observatoire de la Côte d'Azur, Dpt. Gemini-CNRS-UMR 6203, Avenue Copernic, 06130 Grasse, France
⁷ Institut für Theoretische Physik und Astrophysik der Christian-Albrechts-Universität zu Kiel, Leibnizstr. 15, 24118 Kiel, Germany
⁸ Steward Observatory, The University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

Received: 26 October 2009
Accepted: 7 July 2010

Abstract

Context. Very few massive young stellar objects (MYSO) have been studied in the infrared at high angular resolution due to their rarity and large associated extinction. We present observations and models for one of these MYSO candidates, NGC 3603 IRS 9A.

Aims. Our goal is to investigate with infrared interferometry the structure of IRS 9A on scales as small as 200 AU, exploiting the fact that a cluster of O and B stars has blown away much of the obscuring foreground dust and gas.

Methods. Observations in the N-band were carried out with the MIDI beam combiner attached to the VLTI, providing spatial information on scales of about 25–95 milli-arcsec (mas). Additional interferometric observations which probe the structure of IRS 9A on larger scales were performed with an aperture mask installed in the T-ReCS instrument of Gemini South. The spectral energy distribution (SED) is constrained by the MIDI N-band spectrum and by data from the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS 9A range from simple geometrical models of the brightness distribution to one- and two-dimensional radiative transfer computations.

Results. The target is resolved by T-ReCS, with an equivalent (elliptical) Gaussian width of 330 mas by 280 mas (2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component was detected by MIDI which is possibly associated with the inner regions of a flattened dust distribution. Based on our interferometric data, no sign of multiplicity was found on scales between about 200 AU and 700 AU projected separation. A geometric model consisting of a warm (1000 K) ring (400 AU diameter) and a cool (140 K) large envelope provides a good fit to the data. No single model fitting all visibility and photometric data could be found, with disk models performing better than spherical models.

Conclusions. While the data are clearly inconsistent with a spherical dust distribution they are insufficient to prove the existence of a disk but rather hint at a more complex dust distribution.