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Astron. Astrophys. 325, 709-713 (1997)

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4. Conclusions

I have presented new calculations for the generation of sub- and supersonic inflows and outflows in the chromosphere of [FORMULA]  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 [FORMULA] 10 and [FORMULA] 15 km s-1, depending on the height.

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 [FORMULA]  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 [FORMULA] 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 [FORMULA]  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-independent dissipation law for the wave energy flux in combination with the WKB approximation. Nevertheless, it should be noted that it is indeed possible that stochastic [FORMULA]  Ori velocity fields may be associated with magnetic heating - a conjecture which still needs to be explored while considering time-dependent stochastic effects. Recent studies of non-WKB waves by Charbonneau & MacGregor (1995) which also consider Alfvén wave dissipation are not applicable to [FORMULA]  Ori, as these models assume an isothermal atmosphere with [FORMULA]  K. In order to ultimately verify the significance of acoustic waves for the outer atmosphere and wind of this star, detailed comparison of synthetic line spectra with observations have to be performed. Studies focussing on that issue are planned in the future.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998