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Session 72 - Late Type Stars and Planetary Nebulae.
Display session, Thursday, June 13
Tripp Commons,
Infrared emission from the dust shell around IRC+10216 is analysed in detail, employing a self-consistent model for radiatively driven winds around late-type stars that couples the equations of motion and radiative transfer in the dust. The resulting model provides agreement with the wealth of available data, including the spectral energy distribution in the range 0.5--1000 \mum, and visibility and array observations. Previous conclusions about two dust shells, derived from modeling the data with a few single-temperature components of different radii, are not supported by our results. The extended, continuous temperature and density distributions derived from our model obviate the need for such discrete shells. The IR properties vary with the stellar phase, reflecting changes in both the dust condensation radius r_1 and the overall optical depth \tau -- as the luminosity increases from minimum to maximum, r_1 increases while \tau decreases. We find that the angular size of the dust condensation zone varies from 0.3 arcsec at minimum light to 0.5 arcsec at maximum. The shortage of flux at short wavelengths encountered in previous studies is resolved by employing a grain size distribution that includes grains larger than \sim 0.1 \mum, required also for the visibility fits. This distribution is in agreement with the one recently proposed by Jura in a study that probed the outer regions of the envelope. Since our constraints on the size distribution mostly reflect the envelope's inner regions, the agreement of these independent studies is evidence against significant changes in grain sizes through effects like sputtering or grain growth after the initial formation at the dust condensation zone.
