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Astron. Astrophys. 336, 613-625 (1998)

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Chromospheric modelling of the H[FORMULA] and Na I D lines in five M dwarfs of low to high activity level

C.I. Short 1 and J.G. Doyle 2

1 Department of Physics & Astronomy, University of Georgia, Athens, GA, 30602-2451, USA (cis@calvin.physast.uga.edu)
2 Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland (jgd@star.arm.ac.uk)

Received 23 October 1997 / Accepted 25 May 1998

Abstract

We have obtained simultaneous high resolution H[FORMULA] and Na I D spectra of five dwarf M stars that span a wide range in chromospheric activity level. The observed Na I D lines exhibit behavior that is qualitatively similar to that of more well established diagnostics such as the Ca II HK lines: as the activity level, as indicated by the H[FORMULA] line, increases, the absorption core brightens and then develops an emission reversal. We compare the observed profiles with computed non-LTE profiles from a grid of chromospheric/transition region models. We find that the H[FORMULA] and Na I D lines tend to be in general agreement as indicators of approximate chromospheric activity level. However, the H[FORMULA] line systematically indicates a value for the mass loading at the onset of the Transition Region and the location of [FORMULA] that is 0.4 dex lower in column mass density than that indicated by Na I D. Therefore, the profile of both lines cannot be simultaneously well fit for all but one of our stars. We also find, in agreement with the pioneering study of Andretta et al. (1997), that for dMe stars the shape of the Na I D emission cores is a much more sensitive indicator of chromospheric thickness (or, equivalently, chromospheric steepness) than is H[FORMULA], and, therefore, provides a powerful diagnostic complement to H[FORMULA]. Finally, we investigate the dependence of the predicted line profiles on the values of the stellar parameters and conclude that the inferred chromospheric pressure is sensitive to the choice of [FORMULA] and [FORMULA], especially among dMe stars. Specifically, among dMe stars, a model in which the value of [FORMULA] is too small or too large by approximately 200 K, or in which the value of [FORMULA] is too large or too small by 0.5 dex, will give rise to closest fit values of the column mass density at the location of the Transition Region and [FORMULA] that are too small or too large, respectively, by approximately 0.3 dex. As a result, discrepancies between the stellar parameters of our photospheric model and those of the program objects allow us to extract only upper or lower limits for the values of the column mass density at critical points in the chromospheric structure

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

Send offprint requests to: C.I. Short, Department of Physics & Astronomy, University of Georgia, Athens, GA, 30602-2451, USA

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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