A compact dust shell in the symbiotic system HM Sge

Abstract

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 UTs and ATs providing baselines oriented from PA=42° to 105°. 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. The mid-IR environment is unresolved by a 8m telescope and the MIDI spectrum exhibits a level similar to the ISO spectra recorded 10 yr ago. The estimated Gaussian HWHM of the shell of 12AU in the 8-9$μ$m range, and 18AU in the 11-12$μ$m range, are much smaller than the angular separation between the Mira and the White Dwarf of 60AU. The discrepancies between the HWHM at different angle orientations suggest an increasing level of asymmetry from 13 to 8$μ$m. The observations are well fitted by the densest and smallest 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 to take into account the effect of the WD illumination onto the dusty wind of the Mira. 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.