Dust Emission from IRC+10216

Abstract

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 μm, and visibility and array observations. Previous conclusions about two dust shells, derived from modelling the data with a few single-temperature components of different radii, are not supported by our results. The IR properties vary with the stellar phase, reflecting changes in both the dust condensation radius r1 and the overall optical depth τ - as the luminosity increases from minimum to maximum, r1 increases while τ decreases. We find that the angular size of the dust condensation zone varies from 0.″3 at minimum light to 0.″5 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 ∼ 0.1 μm, 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.