Astronomy & Astrophysics

A&A 445, 1015-1029 (2006)
DOI: 10.1051/0004-6361:20053474

High angular resolution N-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument

Dusty environment spatially resolved

K. Ohnaka¹, T. Driebe¹, K.-H. Hofmann¹, Ch. Leinert², S. Morel³, F. Paresce⁴, Th. Preibisch¹, A. Richichi⁴, D. Schertl¹, M. Schöller³, L. B. F. M. Waters⁵, G. Weigelt¹ and M. Wittkowski⁴

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: kohnaka@mpifr-bonn.mpg.de
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

(Received 19 May 2005 / Accepted 12 September 2005)

Abstract
We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ~36 mas (72 $R_{\star}$ ) between 8 and 10 $\mu$ m, while it steeply increases longward of 10 $\mu$ m to reach ~53 mas (106 $R_{\star}$ ) at 13 $\mu$ m. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly - though not entirely satisfactorily - reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.

Key words: infrared: stars -- techniques: interferometric -- stars: circumstellar matter -- stars: carbon -- stars: AGB and post-AGB -- stars: individual: IRAS08002-3803

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