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We study the close circumstellar environment of the nearby S-type star Pi^1Gruis using high spatial-resolution, mid-infrared observations from theESO/VLTI. Spectra and visibilities were obtained with the MIDI interferometeron the VLT Auxiliary Telescopes. The cool M5III giant Beta Gruis was used asbright primary calibrator, and a dedicated spectro-interferometric study wasundertaken to determine its angular diameter accurately. The MIDI measurementswere fitted with the 1D numerical radiative transfer code DUSTY to determinethe dust shell parameters of Pi^1 Gruis. Taking into account the low spatialextension of the model in the 8-9 $\mu$m spectral band for the smallestprojected baselines, we consider the possibility of a supplementary molecularshell. The MIDI visibility and phase data are mostly dominated by the spherical21 mas (694 Rsol) central star, while the extended dusty environment isover-resolved even with the shortest baselines. No obvious departure fromspherical symmetry is found on the milliarcsecond scale. Thespectro-interferometric observations are well-fitted by an optically thin(tau(dust)<0.01 in the band) dust shell that is located at about 14 stellarradii 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 4.4stellar radii with a typical temperature of 1000 K is added to the model toimprove the fit in the 8-9 $\mu$m spectral band. We discuss the probable binaryorigin of asymmetries as revealed by millimetric observations.

The close circumstellar environment of the semi-regular S-type star Pi^1 Gruis