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Astron. Astrophys. 335, 691-702 (1998)

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7. Conclusions

On the basis of the fractionation mechanism proposed by Marsch et al. (1995) a more sophisticated model for the ion-neutral separation was presented, which explains the observed abundance anomalies for a large variety of phenomena. The mechanism is based on properties dominated by atomic parameters, mainly on photoionization time and diffusion: the newly ionized particles are trapped within the wind, which causes the low-FIPs to be enriched compared to the high-FIPs. The diffusion in the neutral phase additionally regulates the fluxes of the different species. But also the velocity has an influence: in the case of a higher speed the fractionation has less time to act and thus the fractionation is weaker. Because the mechanism is mainly based on atomic properties it is not restricted to a special geometry, e.g. of the magnetic field.

This gives the possibility to apply the results presented here for the solar corona and wind also to (cool) stars. As long as the respective star is not too hot and thus somewhere in the stellar atmosphere a layer is existing, where the material is neutral and gets ionized when leaving the stellar surface into the wind, the presented model should apply. Thus e.g. the absence of the fractionation in the corona of Procyon (Drake et al. 1995) may be simply explained by a wind with high velocities in its source region. But for a proper application of the presented model to stars first of all the ionization times of the various elements have to be calculated from the respective stellar spectra.

It should be noted that the fractionation of helium cannot be understood within a purely chromospheric ionization-diffusion model (see Peter & Marsch 1998). Helium has to be described in a chromosphere-corona model as done by Hansteen et al. (1997).

As in other fractionation models before, this model gives a two-plateau structure of the fractionation as a function of the first ionization potential. But with the help of the presented analysis for the first time it is possible to understand the fractionation in a great variety of phenomena in a quantitative way:

  • The fractionation in the slow and the fast wind can be understood with the theoretically predicted velocity-dependence of the fractionation matching the observations.

  • A comparison of the theoretical fractionation in the chromosphere with the measured one in the solar wind leads to chromospheric velocities of 100 km/s and 400 km/s in the slow and fast wind respectively.

  • With the help of the velocity-dependence it can be explained why sometimes in high speed streams in the wind no fractionation is found.

  • The fractionation of the heavy noble gases as measured in lunar regolith can be understood (in a qualitative way).

  • The numerical studies enable a determination of the absolute fractionation, i.e. the fractionation in relation to hydrogen: the position of H in the fractionation pattern fits well with the in-situ measurements in the slow and fast wind.

  • The very strong fractionation as found in polar plumes can be modeled by assuming the plumes to be quasi-static as suggested by some measurements.

To explain all these different fractionation related phenomena within only one model, based on the well known processes of photoionization and diffusion, is the major achievement of the presented paper.

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998
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