Dust in Sauron’s eye

Around the star HR 4796, located approximately 71 parsecs away, a narrow ring of dust scatters the star’s light with strong polarisation. Observed with the VLT’s SPHERE instrument in three filters (R’ at 0.63 µm, I’ at 0.79 µm and J at 1.25 µm), this debris disc appears as a cosmic ‘Eye of Sauron’ : a highly inclined ring, situated approximately 80 astronomical units from its star. Polarimetric analysis indicates that the light is scattered by large, absorbing grains, likely rich in opaque minerals such as iron sulphides, which are more akin to evolved asteroidal dust than to primitive cometary dust. These results, to which our colleague Robin Sultana from LIRA has contributed, shed light on the composition and evolution of dust in young planetary systems.