Detection of scattered light from the hot dust in HD 172555
Debris disks or belts are important signposts for the presence of colliding planetesimals and, therefore, for ongoing planet formation and evolution processes in young planetary systems. Imaging of debris material at small separations from the star is very challenging but provides valuable insights into the spatial distribution of so-called hot dust produced by solid bodies located in or near the habitable zone. We report the first detection of scattered light from the hot dust around the nearby (d = 28.33 pc) A star HD 172555. We want to constrain the geometric structure of the detected debris disk using polarimetric differential Imaging (PDI) with a spatial resolution of 25 mas and an inner working angle of about 0.1$''$. We measured the polarized light of HD 172555, with SPHERE-ZIMPOL, in the very broad band (VBB; $λ=735$ nm) filter for the projected separations between 0.08$''$ (2.3 au) and 0.77$''$ (22 au). We constrained the disk parameters by fitting models for scattering of an optically thin dust disk taking the limited spatial resolution and coronagraphic attenuation of our data into account. The geometric structure of the disk in polarized light shows roughly the same orientation and outer extent as obtained from thermal emission at 18 $μ$m. Our image indicates the presence of a strongly inclined ($i\sim 103.5^\circ$), roughly axisymmetric dust belt with an outer radius in the range between 0.3$''$ (8.5 au) and 0.4$''$ (11.3 au). We derive a lower limit for the polarized flux contrast ratio for the disk of $(F_{\rm pol})_{\rm disk}/F_{\rm \ast}> (6.2 \pm 0.6)\cdot 10^{-5}$ in the VBB filter. This ratio is small, only 9 %, when compared to the fractional infrared flux excess ($\approx 7.2\cdot 10^{-4}$). The model simulations show that more polarized light could be produced by the dust located inside 2 au, which cannot be detected with the instrument configuration used.