The circumbinary dusty disk around the hydrogen-deficient binary star  Sagittarii ^*,**

¹ Astronomical Institute of the Academy of Sciences, 25165 Ondřejov, Czech Republic e-mail: netol@physics.muni.cz
² Masaryk University, Faculty of Science, Department of Theoretical Physics and Astrophysics, Kotlářská 2, 61137 Brno, Czech Republic
³ UMR 6525 H. Fizeau, Univ. Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, Av. Copernic, 06130 Grasse, France
⁴ Astronomical Institute of the Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 18000 Praha 8, Czech Republic

Accepted: 2 December 2008

Abstract

Aims. The aim of this paper is to determine the properties of the dusty environment of the hydrogen-deficient binary system υ Sgr, whose binary properties and other characteristics are poorly known.

Methods. We obtained the first mid-IR interferometric observations of υ Sgr using the instrument MIDI of the VLTI used with different pairs of 1.8 m and 8 m telescopes. The calibrated visibilities, the N band spectrum, and the SED were compared with disk models computed with the MC3D code to determine the geometry and chemical composition of the envelope.

Results. υ Sgr is unresolved with an 8 m telescope at 8.7 μm. We propose a disk model that agrees with the measured visibilities and the SED, consisting of a geometrically thin disk with an inner radius  AU and a scale height  AU. The chemical composition of the dust is approximately 60% of carbon dust and 40% of silicate dust, as a consequence of several episodes of mass transfers, whose chemistry was imprinted in the dust composition. We also constrain the inclination of the disk and its orientation position angle PA = .

Conclusions. The mid-infrared interferometric observations of the binary star υ Sgr allowed us to constrain the geometry of the circumbinary dusty envelope. By defining the inclination and PA of the system with better accuracy than before, these observations restrict the parameter space for the orbital parameters and thus the nature of the stars orbiting in this system.