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

The Ca I 4227 line as a chromospheric diagnostic for M dwarfs

C.I. Short , J.G. Doyle and P.B. Byrne

Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland (cis@star.arm.ac.uk; jgd@star.arm.ac.uk)

Received 15 October 1996 / Accepted 29 January 1997

Abstract

In M dwarfs the Ca I 4227 line is saturated, therefore, the line core forms at a relatively small column mass density. As a result, the line is a potential diagnostic of chromospheric structure in these stars. We have calculated line profiles out of LTE for a grid of early M dwarf atmospheric models that spans the range from low to high chromospheric pressure and covers a range of observed activity levels. We have found that this line is sensitive to the detailed structure of the chromosphere in the same way as well studied strong lines such as H and Ca II HK: as the chromospheric pressure increases, the line changes from absorption to strong emission with a central double reversal. Therefore, observations of the Ca I 4227 line in M dwarfs would provide a useful additional constraint on the structure of the outer atmosphere. We also calculate consistently the line blanketing in the upper atmosphere of models with a chromosphere and transition region and investigate the effect of its inclusion on Ca I 4227. Finally, we demonstrate that the effects of Partial Frequency Redistribution must be taken into account if the Ca I 4227 is to be used as an accurate diagnostic of the region in dMe stars.

Key words: stars: late-type stars: activity stars: chromospheres line: formation

Send offprint requests to: C.I. Short

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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