In this work, a model of a wind driven by radiation pressure on dust was developed. The wind was composed of gas and dust and was treated as a two-component fluid. The equations of motion for the gas and dust were coupled through a collision term which accounts for the transfer of momentum between the two fluids. The dust was assumed to form instantaneously near the sonic point of the flow with either a power law or log-normal size distribution. As the dust was blown outward in the wind, it interacted with the gas and was modified by the gas in the following two ways: (i)physical sputtering if the dust-gas relative velocity exceeded a velocity threshold that depended upon the surface binding energy of the dust material as well as the mass of the impinging gas atoms; and (ii)chemical sputtering if the temperature of the dust grains were within the range of temperatures that allows any molecules bound to the surface of the dust grain to desorb. To study the effects of physical and chemical sputtering, the size distribution of dust grains was calculated at every point in the flow. It was found that physical sputtering dominates when the stellar luminosity and effective temperature were high. This created the large dust-gas drift velocity needed to produce collisions energetic enough to physically sputter the grains. Chemical sputtering was found to be important when the stellar luminosity and temperature were low. This allowed the grains to remain for a longer time in the region where the atomic hydrogen density was high and also where the grain temperature was in the range of 400 800 K, as this is the temperature range where the chemical sputtering rate peaks. The model developed in this work was used to estimate the amount of carbon dust injected into the galaxy by carbon- rich Asymptotic Giant Branch (AGB) stars. It was found that these stars inject 1.48 × 10-5 M☉ /yr of carbon dust into the galaxy; this value is a factor of 60 less than other current estimates. However, only about 50% of the carbon-rich AGB stars were included in this calculation. This was due to a limitation of the current work which is only reliable for optically-thin stellar atmospheres. Estimates were made for the optically thick stars that raised the amount of dust mass injected into the galaxy to 5.1 × 10-5 M☉ /yr.
- Pub Date:
- August 2002
- Physics: Astronomy and Astrophysics