## 4. ConclusionsI have presented new calculations for the generation of sub- and supersonic inflows and outflows in the chromosphere of Ori (M2 Iab) as a consequence of stochastic energy deposition by acoustic shocks. These models are motivated by observational results from HST-GHRS which give information about the chromospheric dynamics in this star. I found the following results: 1. In case of stochastic shock waves the mechanical energy and momentum input to the atmosphere occurs episodically and is overwhelmingly controlled by strong shocks generated by shock merging. These shocks are thus responsible for generating stochastic chromospheric velocity fields. 2. In case of stochastic shocks, a relatively broad range of
chromospheric velocities is encountered, which increases with
decreasing mass column density. In the middle and outer chromosphere,
the characteristic velocity range encompasses
10 and 15 km s 3. The difference between the chromospheric velocity distributions for the two inserted spectra appear to be insignificant. A much narrower velocity distribution however is found in case of the monochromatic wave model considered. 4. The range of velocities found in the stochastic wave computations appear to be consistent with the velocity intervals revealed by the Fe II emission line components observed by HST-GHRS, which are considered an important diagnostic tool for chromospheric dynamics of Ori (Carpenter & Robinson 1997). Detailed studies of Fe II line formation are however needed to verify this picture. 5. Regarding the Mach numbers of the flow, it is found that supersonic inflows and outflows are easily produced by the stochastic wave models, contrary to the monochromatic wave model calculated. This result is in agreement with earlier results given by Cuntz (1992a, b). 6. In case that noninstantaneous ionization of hydrogen is considered, some of the results are expected to change, notably the ratios. It is found that noninstantaneous hydrogen ionization tends to produce larger temperatures behind shocks (Carlsson & Stein 1991, 1992), while leaving the local velocity fields largely unaffected. This work is part of ongoing efforts to understand outer
atmospheric heating in stars of different spectral type and
evolutionary status. In case of Ori, it
has previously been argued that the outer atmospheric structure might
be attributable to the propagation of Alfvén waves (Hartmann
& Avrett 1984). The problem however is that the models calculated
so far are solely based on the adoption of a time- © European Southern Observatory (ESO) 1997 Online publication: April 28, 1998 |