The close circumstellar environment of the semi-regular S-type star π¹ Gruis^*,**

¹ Observatoire de la Côte d'Azur, Dpt. Gemini-CNRS-UMR 6203, Avenue Copernic, 06130 Grasse, France
² Department of Astronomy, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria e-mail: stephane.sacuto@obs-azur.fr; stephane.sacuto@univie.ac.at
³ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, Campus Plaine CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ ZAH Landessternwarte, Koenigstuhl 12, 69117 Heidelberg, Germany

Received: 18 July 2007
Accepted: 30 January 2008

Abstract

Aims. We study the close circumstellar environment of the nearby S-type star π¹ Gruis using high spatial-resolution, mid-infrared observations from the ESO/VLTI.

Methods. Spectra and visibilities were obtained with the MIDI interferometer on the VLT Auxiliary Telescopes. The cool M5III giant β Gruis was used as bright primary calibrator, and a dedicated spectro-interferometric study was undertaken to determine its angular diameter accurately. The MIDI measurements were fitted with the 1D numerical radiative transfer code DUSTY to determine the dust shell parameters of π¹ Gruis. Taking into account the low spatial extension of the model in the 8–9 μm spectral band for the smallest projected baselines, we consider the possibility of a supplementary molecular shell.

Results. The MIDI visibility and phase data are mostly dominated by the spherical 21 mas (694 ) central star, while the extended dusty environment is over-resolved even with the shortest baselines. No obvious departure from spherical symmetry is found on the milliarcsecond scale. The spectro-interferometric observations are well-fitted by an optically thin ( in the N band) dust shell that is located at about 14 stellar radii with a typical temperature of 700 K and composed of 70% silicate and 30% of amorphous alumina grains. An optically thin ( in the N band) H₂O + SiO molecular shell extending from the photosphere of the star up to 4.4 stellar radii with a typical temperature of 1000 K is added to the model to improve the fit in the 8–9 μm spectral band. We discuss the probable binary origin of asymmetries as revealed by millimetric observations.