Fresh light on stardust

Abstract

Ageing stars produce elements vital for life and disperse them into space on stellar winds. The discovery of large dust grains in the vicinity of cool giant stars sheds light on the mechanisms that drive such winds. See Letter p.220 Towards the ends of their lives, intermediate-mass stars lose much of their mass in the form of gas and dust ejected in a slow, dense wind. The underlying processes driving these outflows are poorly understood, owing in part to difficulties in observing such ejected material. Norris et al. use an innovative technique that combines interferometric imaging with high-precision differential polarimetry to observe three red giants. Their images reveal circumstellar dust shells with remarkably small radii (less than two times the radius of the star), made up of unexpectedly large dust grains approximately 300 nanometres in radius. The authors suggest that these observations support a wind-driving model based on acceleration of dust grains by the scattering, rather than absorption, of starlight.