Grain formation in the expanding gas flow around cool luminous stars |
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| Authors: | Takashi Kozasa Hiroichi Hasegawa Junji Seki |
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| Institution: | (1) Department of Physics, Kyoto University, Kyoto, Japan;(2) National Cardiovascular Center Research Institute, Osaka, Japan |
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| Abstract: | We studied grain formation process and flow structure around cool luminous mass-loss stars. The nucleation and growth theory of Yamamoto and Hasegawa was extended to the case of expanding gas flow.The envelope was assumed to be steady, spherically symmetric, in thermal and radiative equilibrium, optically thin, and driven by radiation pressure on grains. For oxygen rich stars, Mg-silicate was found to be the first condensate which can drive the gas effectively. The following stellar parameters were chosen; stellar massM
*=1M
 , effective temperatureT
*=3000K, stellar luminosityL
* from 7.5×103 to 2.0×104
L
, and mass-loss rate |M| from 1.0×10–6 to 1.0×10–4
M
yr–1.Main results of calculations are as follows; (1) grain condensation temperatureT
c 980 1080 K; (2) total gas pressure at the condensation pointP
t6×10–116×10–9 atm; (3) scale parameterA
c1036×104; and (4) final grain sizer
f=400Å1 m. For the smaller |M| or the largerL
*, these values are the smaller. We recognized two types of flow solutions (1) Dust driven flow for large |M|, which reaches the sonic point near the condensation point; and (2) Modified Parker flow for small |M| for which grain sizer
f is almost independent of |M|.A comparison with observational results ofL
* and gas terminal velocityV
 suggests that Mg-silicate grains are of submicron size, which are effective for interstellar extinction in visible and infrared. Fe-grains condense in the rarefied outflow with a size probably smaller than 100Å, which may contribute for interstellar ultraviolet extinction. The envelope has three-layer structure inner dense region with small outflow velocity, grain formation layer and outer rarefied region with fast outflow velocity.No flow solutions exist forM
* greater than a critical stellar massM
*cr for a given stellar luminosityL
* and mass-loss rate |M|.For example, critical stellar massM
*cr=1.8M
forL
*=104
L
,T
*=3000 K, and |M|=10-5
M
yr-1. |
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