Letters to the Editor

Mid-infrared interferometry towards the massive young stellar object CRL 2136: inside the dust rim^⋆

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
e-mail: wdewit@eso.org
² School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

Received: 3 November 2010
Accepted: 15 November 2010

Abstract

Context. Establishing the importance of circumstellar disks and their properties is crucial to fully understand massive star formation.

Aims. We aim to spatially resolve the various components that make-up the accretion environment of a massive young stellar object (≲ 100 AU), and reproduce the emission from near-infrared to millimeter wavelengths using radiative transfer codes.

Methods. We apply mid-infrared spectro-interferometry to the massive young stellar object CRL 2136. The observations were performed with the Very Large Telescope Interferometer and the MIDI instrument at a 42 m baseline probing angular scales of 50 milli-arcseconds. We model the observed visibilities in parallel with diffraction-limited images at both 24.5 μm and in the N-band (with resolutions of 0.6″and 0.3″, respectively), as well as the spectral energy distribution.

Results. The arcsec-scale spatial information reveals the well-resolved emission from the dusty envelope. By simultaneously modelling the spatial and spectral data, we find that the bulk of the dust emission occurs at several dust sublimation radii (approximately 170 AU). This reproduces the high mid-infrared fluxes and at the same time the low visibilities observed in the MIDI data for wavelengths longward of 8.5 μm. However, shortward of this wavelength the visibility data show a sharp up-turn indicative of compact emission. We discuss various potential sources of this emission. We exclude a dust disk being responsible for the observed spectral imprint on the visibilities. A cool supergiant star and an accretion disk are considered and both shown to be viable origins of the compact mid-infrared emission.

Conclusions. We propose that CRL 2136 is embedded in a dusty envelope, which truncates at several times the dust sublimation radius. A dust torus is manifest in the equatorial region. We find that the spectro-interferometric N-band signal can be reproduced by either a gaseous disk or a bloated central star. If the disk extends to the stellar surface, it accretes at a rate of 3.0 × 10^-3 M_⊙ yr^-1.