Abstract

Aims. We study the close circumstellar environment of the nearby S-type star pi(1) 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 beta 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 p1 Gruis. Taking into account the low spatial extension of the model in the 8-9 mu m 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 R (circle dot)) 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 ( tau(dust) 0.01 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 ( tau(mol) 0.1 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