Astron. Astrophys. 324, 196-202 (1997)

4. Conclusion

We have demonstrated that the Ca I 4227 line is sensitive to the chromospheric structure of dM stars. In particular, as chromospheric pressure increases from that of a quiescent chromosphere to that of a highly active chromosphere, the line changes from being strongly in absorption to being strongly in emission with a central double reversal. In the most active stars, the line source function is strongly coupled to the thermal temperature at, and just above . Therefore, the doubly reversed emission core of stars with high chromospheric pressure may be used as a "thermometer" for the region in the same way that the Ca II HK lines are used. However, the Ca I 4227 line has the advantage of being more observable in M stars than is the Ca II HK line because it is closer to the Wien peak of the emergent flux distribution. Also, as in the case of the Ca II HK lines, the effect of PRD must be taken into account in the line profile calculation if the line is to be used as an accurate diagnostic of . Unfortunately, there is a lack of observations of this line in dM stars of sufficient resolution and signal-to-noise to fit the core. Observations of this line with and in the pseudo-continuum of for dM stars of a variety of activity levels are needed to exploit the modelling results shown here.

The inclusion of affects the Ca I 4227 profile, both by its direct effect on the radiative transfer of the Ca I atom, and by its affect on the chromospheric structure that arises from the ionization balance of H I /II. Because the Lyman and Balmer transitions of H I form in the upper atmosphere, it is necessary to include not just photospheric line blanketing in radiation fields forming below , but also chromospheric and transition region line blanketing in radiation fields that form above .

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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