Compact dust shell in the symbiotic system HM Sagittae

We present high spatial resolution observations of the mid-infrared core of the dusty symbiotic system HM Sge. The MIDI interferometer was used with the VLT Unit Telescopes and Auxiliary Telescopes providing baselines oriented from PA=42 degrees to 105 degrees. The MIDI visibilities are compared with the ones predicted in the frame of various spherical dust shells published in the literature involving single or double dusty shells intended to account for the influence of the hot White Dwarf. The mid-IR environment is unresolved by a 8m telescope and the MIDI spectrum exhibits a level similar to the ISO spectra recorded 10yr ago. The discrepancies between the HWHM at different angle orientations suggest an increasing level of asymmetry from 13 to 8 mu m. The observations are surprisingly well fitted by the densest (optically thick in the N band) and smallest spherical model published in the literature based on the ISO data, although such a model does not account for the variations of near-IR photometry due to the Mira pulsation cycle suggesting a much smaller optical thickness. These observations also discard the two shells models, developed in an attempt to take into account the effect of the White Dwarf illumination onto the dusty wind of the Mira. These models are too extended, and lead to a level of asymmetry of the dusty environment tightly constrained by the MIDI visibilities. These observations show that a high rate of dust formation is occurring in the vicinity of the Mira which seems to be not highly perturbed by the hot companion.