## 1. IntroductionThe M-type supergiant Ori (M2 Iab) is a
well studied object and a broad range of dynamical atmospheric
features has been identified. Alpha Ori is a noncoronal star with an
extended atmosphere and a cool, massive wind. Alpha Ori has an
extended chromosphere with temperatures in excess of 6000 K
(Newell & Hjellming 1982). The size of the chromosphere is
uncertain as it is probably strongly dependent on the spectral
diagnostics used. Basri et al. (1981) have calculated a semiempirical
chromosphere model for Ori by fitting the
Mg II and Ca II line fluxes given by IUE assuming smooth
functions for the atmospheric density and pressure. They found a
steady increase of the atmospheric temperature starting from
2820 K in the temperature minimum layer up to 7000 K at a
mass column density of g cm In the meantime, high resolution spectra obtained by HST-GHRS have
become available. Results are given by Carpenter et al. (1994a,
b),
Robinson & Carpenter (1995), Brandt et al. (1995), and Carpenter
& Robinson (1997). These data contain a large amount of novel
spectral information, largely because of the increase in spectral
resolution. They also indicate the presence of complex chromospheric
velocity fields (both sub- and supersonic, both inwardly and outwardly
directed flows), which The new observational results for Ori are a strong motivation to further explore acoustic shock wave models. It is therefore the goal of this paper to evaluate the formation of sub- and supersonic velocity patterns at different atmospheric heights. In particular, I will discuss monochromatic wave calculations having a fixed wave period and stochastic wave calculations with periods determined by a random number generator assuming modified Gaussian distributions. Understandably, the monochromatic wave models are not realistic and are only given as comparison. The paper is structured as follows: In Sect. 2, I discuss the method and model assumptions considered. Sect. 3 gives the results. Conclusions are presented in Sect. 4. © European Southern Observatory (ESO) 1997 Online publication: April 28, 1998 |