Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.313r&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Other
Scientific paper
Masers provide the highest-detail studies of mass loss from evolved stars, showing that oxygen-rich stars produce water maser clouds of sizes proportional to the stellar mass. Recent work shows that maser beaming can be used to distinguish between approximately spherical clouds (typically a few AU in diameter), which produce tightly beamed masers, and flattened clumps, which can produce bright, extended emission. This provides a diagnostic for the presence of shocks in the water maser shell, at distances of five to several tens of stellar radii. Hydroxyl masers at 1665 and 1667 MHz interleave the outer water maser clouds, showing that the clumps must be embedded in cooler, less dense gas. However, exponential maser amplification exaggerates small differences in the underlying conditions, making it hard to measure precisely properties such as abundances or density. This is about to change, as ALMA will image multiple excited water maser transitions, some probably within the dust formation zone (overlapping SiO masers) and others possibly extending further out. Milli-arcsec resolution will show whether different transitions come from the same gas, closely constraining the conditions. Thermal lines and dust around the closer stars will be imaged at comparable resolution. In concert with e-MERLIN, the EVLA and VLBI, we will be able to trace disturbances from within the radio photosphere. This will show whether the clumping scale of the wind is set within the star, such as by starspots or convection cells, opening up the intriguing possibility that the dense clouds might be differentiated by intrinsic chemical composition as well as reactive chemistry.
Assaf K.
Elitzur Moshe
Etoka Sandra
Gray Malcolm
Harper Graham
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