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

The dynamical structure and infrared emission of winds around late-typ e stars are studied in a self-consistent model that couples the equati ons of motion and radiative transfer. Thanks to its scaling properties , both the dynamics and IR spectrum of the solution are fully characte rized by tau(F), the flux-averaged optical depth of the wind. Five typ es of dust grains are considered: astronomical silicate, crystalline o livine, graphite, amorphous carbon and SiC, as well as mixtures. Analy sis 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-domina ted 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 mi xtures, carbonaceous grains tend to dominate the dynamic behavior whil e silicate and SiC grains dominate the IR signature. The region of par ameter space where radiation pressure can support a given mass-loss ra te is identified, replacing the common misconception Mv less than or e qual to L/c, and it shows that radiatively driven winds explain the h ighest mass-loss rates observed to date. A new method to derive mass-l oss rates from IR data is presented, and its results agree with other determinations. The theoretical spectra and colors are in good agreeme nt with observations. IRAS Low Resolution Spectrometer classes are ass ociated with tau(F), for various grain materials and the regions of co lor-color diagrams expected to be populated by late-type stars are ide ntified. For a given grain composition, location in the color-color di agram follows a track with position along the track determined by tau( F). We show that cirrus emission can severely affect point source meas urements to the extent that their listed IRAS long-wavelength fluxes a re 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 ar e no longer indicative of the underlying point source. After accountin g of cirrus contamination, essentially all IRAS point sources (95%) lo cated in the relevant regions of the color-color diagrams can be expla ined as late-type stars. There is no need to invoke time dependent eff ects, such as detached shells, for example, to explain either the colo rs or mass-loss rates of these sources. Although various indications o f 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.