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

TL;DR

This study reveals that large, transparent dust grains close to red giant stars can be accelerated by photon scattering, providing a new mechanism for stellar mass loss in evolved stars.

Contribution

The paper presents spatially-resolved observations of dust shells with large grains near red giant stars, suggesting photon scattering as a mass-loss mechanism.

Findings

Dust shells are found within 2 stellar radii of the stars.

Large grains (~300 nm) are detected close to the photosphere.

Transparent dust grains can be accelerated by photon scattering, aiding mass loss.

Abstract

Intermediate-mass stars end their lives by ejecting the bulk of their envelope via a slow dense wind back into the interstellar medium, to form the next generation of stars and planets. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure from starlight, entraining the gas and driving the wind. However accounting for the mass loss has been a problem due to the difficulty in observing tenuous gas and dust tens of milliarcseconds from the star, and there is accordingly no consensus on the way sufficient momentum is transferred from the starlight to the outflow. Here, we present spatially-resolved, multi-wavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the HR diagram. When imaged in scattered light, dust shells were found at remarkably small radii (<~ 2 stellar radii) and with unexpectedly large grains (~300 nm radius). This proximity to the photosphere argues for dust species that are transparent to starlight and therefore resistant to sublimation by the intense radiation field. While transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains via photon scattering rather than absorption - a plausible mass-loss mechanism for lower-amplitude pulsating stars.