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



An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind1,2,3. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust1, which is then accelerated by radiation pressure, entraining the gas and driving the wind2,4,5. 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 wind6,7, the radiation field can accelerate these large grains through photon scattering rather than absorption8—a plausible mass loss mechanism for lower-amplitude pulsating stars.

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Figure 1: Polarimetric interferometry of W Hya at 1.24 μm.
Figure 2: Wavelength dependence of scattering for W Hya and R Dor.
Figure 3: Model image for W Hya with circumstellar shell viewed in horizontally and vertically polarized light.
Figure 4: Grain size measurement.


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This work was based on observations collected with the NACO instrument at the European Southern Observatory, Chile.

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Authors and Affiliations



P.G.T., M.J.I. and S.L. commissioned the SAMPol observing mode and conducted the initial observations, and developed the standard aperture-masking data reduction pipeline. B.R.M.N. developed the polarimetric data reduction and modelling/fitting procedures, analysed the data and conducted later observations. T.M.E. and P.S. contributed to the data analysis procedures. T.R.B., A.A.Z. and F.L. contributed to both the observing programme and the interpretation of the findings.

Corresponding author

Correspondence to Barnaby R. M. Norris.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text, Supplementary Figures 1-2 and additional references. The Supplementary Text contains a detailed description of the modelling process used in these investigations and additional discussion regarding other dust species. This section also refers to Supplementary Figure 1, a diagram explaining the geometry of the model used. Supplementary Figure 2 shows a diagram demonstrating how starlight becomes polarised by a spherical scattering shell. (PDF 767 kb)

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Norris, B., Tuthill, P., Ireland, M. et al. A close halo of large transparent grains around extreme red giant stars. Nature 484, 220–222 (2012).

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