Astronomy & Astrophysics

Free access article

A&A 464, 81-86 (2007)
DOI: 10.1051/0004-6361:20054134

AMBER/VLTI and MIDI/VLTI spectro-interferometric observations of the B[e] supergiant CPD $-57\mathrm{^\circ}\,2874$

Size and geometry of the circumstellar envelope in the near- and mid-IR

A. Domiciano de Souza^{1, 2}, T. Driebe¹, O. Chesneau³, K.-H. Hofmann¹, S. Kraus¹, A.S. Miroshnichenko^{1, 4}, K. Ohnaka¹, R.G. Petrov², T. Preisbisch¹, P. Stee³, G. Weigelt¹, F. Lisi⁵, F. Malbet⁶, and A. Richichi⁷

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Laboratoire Universitaire d'Astrophysique de Nice, UMR 6525 Université de Nice - Sophia Antipolis/CNRS, Parc Valrose, 06108 Nice Cedex 2, France
³ Laboratoire Gemini, UMR 6203 Observatoire de la Côte d'Azur/CNRS, BP 4229, 06304 Nice Cedex 4, France
⁴ Dept. of Physics and Astronomy, PO Box 26170, University of North Carolina at Greensboro, Greensboro, NC 27402 6170, USA
⁵ INAF-Osservatorio Astrofisico di Arcetri, Istituto Nazionale di Astrofisica, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ Laboratoire d'Astrophysique de Grenoble, UMR 5571 Université Joseph Fourier/CNRS, BP 53, 38041 Grenoble Cedex 9, France
⁷ European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching, Germany

(Received 31 August 2005 / Accepted 17 October 2005 )

Abstract
We present the first high spatial and spectral resolution observations of the circumstellar envelope (CSE) of a B[e] supergiant (CPD $-57{^\circ}\,2874$ ), performed with the Very Large Telescope Interferometer (VLTI). Spectra, visibilities and closure phase were obtained using the beam-combiner instruments AMBER (near-IR interferometry with three 8.3 m Unit Telescopes or UTs) and MIDI (mid-IR interferometry with two UTs). The interferometric observations of the CSE are well fitted by an elliptical Gaussian model with FWHM diameters varying linearly with wavelength. Typical diameters measured are $\simeq$ $1.8\times3.4$ mas or $\simeq$ $4.5\times8.5$ AU (adopting a distance of 2.5 kpc) at $2.2\,\mu{\rm m}$ , and $\simeq$ $12\times15$ mas or $\simeq$ $30\times38$ AU at $12\,\mu{\rm m}$ . The size of the region emitting the Br $\gamma$ flux is $\simeq$ $2.8\times5.2$ mas or $\simeq$ $7.0\times13.0$ AU. The major-axis position angle of the elongated CSE in the mid-IR ( $\simeq$ $144{^\circ}$ ) agrees well with previous polarimetric data, hinting that the hot-dust emission originates in a disk-like structure. In addition to the interferometric observations we also present new optical ( $UBVR_{\rm c}I_{\rm c}$ ) and near-IR ( $\it JHKL$ ) broadband photometric observations of CPD $-57{^\circ}\,2874$ . Our spectro-interferometric VLTI observations and data analysis support the non-spherical CSE paradigm for B[e] supergiants.

Key words: techniques: high angular resolution -- techniques: interferometric -- infrared: stars -- stars: early-type -- stars: emission-line, Be -- stars: mass-loss