Other
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998iraj...25....7r&link_type=abstract
Irish Astronomical Journal, 1998, 25(1), 7
Other
15
Scientific paper
High resolution MERLIN maps of water and OH masers in the circumstellar envelopes of red supergiant stars reveal complex velocity gradients. The water masers appear to emanate from dense clouds 15 to 20 AU in diameter which are being accelerated away from the star. OH masers are mostly seen in regions of more gently increasing velocity which are likely to be less dense. Outside the dust formation zone, the stellar wind is driven by radiation pressure on grains. Chapman & Cohen (1986) found that in VX Sgr the wind continues to speed up out to a thousand or more AU from the star, and derived values for the dust absorption efficiency increasing with distance to account for this. Other authors pointed out grain growth at large distances is unlikely and suggested VX Sgr must have changed its mass loss rate in recent centuries. However this paper reports observations of continuing acceleration in all four supergiants which have now been studied. An increase in the absorption efficiency of dust is still the most likely generally applicable explanation. This paper proposes a new mechanism whereby the grain surface properties (but not the mass) change in the outflow on a timescale of many decades. Using the method of Chapman & Cohen (1986), it is found that at about 100 AU from the star, near the inner edge of the water maser shell, the dust absorption efficiency is kappaD ~ 0.2 m^2 kg^-1. kappaD increases with distance from the star, rising to >~ m^2 kg^-1 (similar to the conventional value for astronomical silicates) in thick circumstellar envelopes reaching high terminal velocities. The low values found nearer the star suggest that initially the dust grains are crystalline or have a composition high in metal oxides which renders them more transparent to the stellar IR radiation. Dust formation, and the influence of the stellar magnetic field on the grains, is also implicated in producing the small scale clumpiness and large scale axisymmetries which are seen in circumstellar envelopes.
Richards Anita M. S.
Yates Jeremy A.
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