Journal Article

Observation and modelling of main-sequence star chromospheres – XII. Two-component model chromospheres for five active dM1e stars*

E. R. Houdebine

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 397, issue 4, pages 2133-2147
Published in print August 2009 | ISSN: 0035-8711
Published online August 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.15112.x
Observation and modelling of main-sequence star chromospheres – XII. Two-component model chromospheres for five active dM1e stars*

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We aim to constrain the Hα, Ca ii H and Ca ii K profiles from quiescent and active regions on active dM1e stars.

A preliminary analysis of all the data available for dM1e stars shows that the Hα/Ca ii equivalent width (EW) ratio varies by up to a factor of 7 for different stars in our sample. We find that spectroscopic binaries have a significantly smaller ratio than single dM1e stars. We also find that the pre-main-sequence stars Gl 616.2, GJ 1264 and Gl 803 have a ratio lower than main-sequence single dM1e stars. These differences imply that different chromospheric structures are present on different stars, notably the temperature minimum must decrease with an increasing Hα/Ca ii EW ratio. For these reasons, it is impossible to reproduce all observations with only one grid of model chromospheres.

We show that the grid of model chromospheres of Paper VI is adequate to describe the physical conditions that prevail in the chromospheres of spectroscopic binaries and pre-main-sequence M1e stars, but not for the conditions in single dM1e stars. One or more additional grids of model chromospheres will be necessary to reproduce all observations.

We use the method developed in Paper XI in this series, in order to build two-component model chromospheres for five M1e field stars: FF And A, FF And B, GJ 1264, AU Mic and Gl 815A. Our solutions provide an exact match of the Hα and the mean Ca ii H & K EWs within measurement uncertainties. We compare the theoretical profiles and the observed profiles of Hα and the Ca ii H & K resonance lines. On the one hand, our fits to the Ca ii lines are reasonably good. On the other hand, our models tend to produce Hα profiles with a central absorption that is too deep. This suggests that the column mass at the transition region for plages is underestimated, but this would imply that the contrast factor between quiescent and active regions in the Ca ii lines is larger than 5.

We find that, except in the cases of FF And A and AU Mic, the total Hα profile is dominated by the contributions from plage regions. The Hα profiles in quiescent regions are typically filled in or slightly in emission. We also find that, as in the case of the less active dM1 stars, Ca ii emission in dM1e stars is generally dominated by the contribution from plage regions. We find plage filling factors typically in the range 10 to 60 per cent. These are on average (40 per cent) slightly larger than those derived for the less active dM1 stars (30 per cent).

Our modelling shows that the grid of model chromospheres of Paper VI is adequate for only a subset of dM1e stars. Physical conditions that prevail in the chromospheres of dM1e stars are very different from one star to another. For this reason, this study demonstrates the critical need for several improved grids of model chromospheres as well as higher signal-to-noise ratio data.

Keywords: line: formation; radiative transfer; stars: chromospheres; stars: emission-line, Be; stars: late-type

Journal Article.  11252 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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