INFRARED-EMISSION AND DYNAMICS OF OUTFLOWS IN LATE-TYPE STARS

The dynamical structure and infrared emission of winds around late-type stars are studied in a self-consistent model that couples the equations of motion and radiative transfer. Thanks to its scaling properties, both the dynamics and IR spectrum of the solution are fully characterized by tau(F), the flux-averaged optical depth of the wind. Five types of dust grains are considered: astronomical silicate, crystalline olivine, graphite, amorphous carbon and SiC, as well as mixtures. Analysis of infrared signatures provides constraints on the grain chemical composition and indications for the simultaneous existence of silicate and carbon grains. The abundances of crystalline olivine in Si-dominated grains and of SiC in C-dominated grains are found to be limited to less than or similar to 20%-30%. Furthermore, in carbonaceous grains carbon is predominantly in amorphous form, rather than graphite. In mixtures, carbonaceous grains tend to dominate the dynamic behavior while silicate and SiC grains dominate the IR signature. The region of parameter space where radiation pressure can support a given mass-loss rate is identified, replacing the common misconception Mv less than or equal to L*/c, and it shows that radiatively driven winds explain the highest mass-loss rates observed to date. A new method to derive mass-loss rates from IR data is presented, and its results agree with other determinations. The theoretical spectra and colors are in good agreement with observations. IRAS Low Resolution Spectrometer classes are associated with tau(F), for various grain materials and the regions of color-color diagrams expected to be populated by late-type stars are identified. For a given grain composition, location in the color-color diagram follows a track with position along the track determined by tau(F). We show that cirrus emission can severely affect point source measurements to the extent that their listed IRAS long-wavelength fluxes are unreliable. Whenever the listed IRAS flag cirr3 exceeds the listed 60 mu m flux by more than a factor of 2, the 60 and 100 mu m fluxes are no longer indicative of the underlying point source. After accounting of cirrus contamination, essentially all IRAS point sources (95%) located in the relevant regions of the color-color diagrams can be explained as late-type stars. There is no need to invoke time dependent effects, such as detached shells, for example, to explain either the colors or mass-loss rates of these sources. Although various indications of time varying mass-loss rates exist in numerous sources, the infrared properties of this class of stars are well explained as a whole with steady state shows.