A close halo of large transparent grains around extreme red giant stars

The dust shells of three intermediate-mass stars are observed to lie remarkably close to the photosphere and to be composed of unexpectedly large grains, consistent with mass loss from such stars occurring by means of ejection of this dust by photon scattering rather than as a result of radiation pressure. Last gasp of a red giant 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. An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind 1 , 2 , 3 . Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust 1 , which is then accelerated by radiation pressure, entraining the gas and driving the wind 2 , 4 , 5 . Explaining the amount of mass loss, however, has been a problem because of the difficulty of observing tenuous gas and dust only tens of milliarcseconds from the star. For this reason, there is no consensus on the way sufficient momentum is transferred from the light from the star to the outflow. Here we report spatially resolved, multiwavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the Hertzsprung–Russell diagram. When imaged in scattered light, dust shells were found at remarkably small radii (less than about two stellar radii) and with unexpectedly large grains (about 300 nanometres in radius). This proximity to the photosphere argues for dust species that are transparent to the light from the star and, therefore, resistant to sublimation by the intense radiation field. Although transparency usually implies insufficient radiative pressure to drive a wind 6 , 7 , the radiation field can accelerate these large grains through photon scattering rather than absorption 8 —a plausibl