An oxygen-rich dust disk surrounding an evolved star in the Red Rectangle

The Red Rectangle 1 is the prototype of a class of carbon-rich reflection nebulae surrounding low-mass stars in the final stages of evolution. The central star of this nebula has ejected most of its layers (during the red-giant phase), which now form the surrounding cloud, and is rapidly evolving to a white dwarf. This star is also a member of a wide binary system 2 , which is surrounded by a thick, dusty disk of material 3 , 4 . Here we report infrared observations of the Red Rectangle that reveal the presence of oxygen-rich material: prominent emission bands from crystalline silicates, and absorption lines arising from carbon dioxide. The oxygen-rich material is located in the circumbinary disk, in contrast to the previously known carbon-rich dust, which is found mainly in the extended nebula 5 , 6 . The properties of the oxygen-rich dust are similar to those of dusty disks surrounding young stars 7 , which are believed to be the sites of planet formation. Grain processing, and perhaps even planet formation, may therefore also be occurring in the circumbinary disk of this evolved star.