GENERATION OF EXTENDED CHROMOSPHERES AND MASS LOSS OF LATE-TYPE GIANT STARS DUE TO ACOUSTIC SHOCK WAVES

Abstract

We investigate the generation of extended chromospheres and mass loss of late-type giant stars due to acoustic shock waves. Effects of both monochromatic waves and acoustic frequency spectra are studied. For our wave computations we use an eulerian time-dependent hydrodynamic code based on the method of characteristics. The possible boundary conditions for the cases of suband supersonic inand outflows are discussed in detail. Ionization of hydrogen is explicitly taken into account. Radiation damping is considered in the thin plasma approximation using a Cox and Tucker type law, fitted on the emission function as given by McWhirter, Thonemann, and Wilson (1975). We obtain three different types of stellar wind solutions: Short-period, acoustic-wave calculations show that a persistent wave energy flux can produce an extended chromosphere. If monochromatic shock waves are used no appreciable mass loss is generated. However, in the case of wave models with stochastically changing wave periods episodic mass loss occurs. This behavior is caused by overtaking and merging of shocks. Occasionally very strong shocks are produced. But the time-averaged mass-loss rates related to these stochastic wave models remain very small. The third type of stellar wind solutions is the generation of continuous mass loss in adiabatic wave models with large wave periods. For these models the wavelengths must be comparable to or larger than a stellar radius. In the case of Arcturus, a mass-loss rate between 10~10 and ΚΓ11 3KO y1 is found. The final flow speed of the wind is larger than 40 km s1. We have applied our method to individual stars, which are: Arcturus (a Bootis), Aldebaran (a Tauri), and Betelgeuse (α Orionis). The chromospheric extents which we find in our time-dependent wave models are 1.12 r*, 1.13 r*, and 1.22 r*, respectively, corresponding to a time-averaged electron density of 107 cm3. These extents are significantly smaller than those derived by Stencel (1981) and Carpenter, Brown, and Stencel (1985), but in the case of Arcturus and Aldebaran, they confirm the results of Judge (1986a, b). The chromospheric temperatures in our wave models increase with height, starting from 6000 Κ up to about 15,000 Κ at the temperature maximum.