2010A&A...509A..65H

We aim to constrain the H_α, CaIIH and CaIIK line profiles of quiescent and active regions of nine dM1 stars of near solar metallicity: Gl2, GJ 1010A, Gl49, Gl150.1B, Gl205, Gl229, Gl526, G192-11A, and Gl880. We propose a new method for building two-component model chromospheres for dM1 stars-based on simple constraints and a grid of model atmospheres developed by Houdebine & Stempels (1997A&A...326.1143H). This method is based on the measurements of the equivalent width of H_α and CaII H & K. Based on the peculiar relationship between these two equivalent widths in the model atmospheres, our solutions provide an exact match of these equivalent widths. We obtain two component (quiescent and active region) model chromospheres for our nine target stars. We fit the H_α, CaIIH, and CaIIK profiles for these stars. These models show that seven of these stars lie in the intermediate activity range between H_α maximum absorption and emission. Two stars (Gl49 and G192-11A) are quite active with H_α emission profiles in plages. As far as the CaIIemission is concerned, these two stars are almost as active as dM1e stars. Two stars (GJ 1010A and Gl526) have lower activity levels with narrower and weaker H_α profiles. The range of activity covered by these stars is a factor of 13 in the CaIIlines, from low activity to activity levels almost as high as those of dM1e stars. Our method sometimes provides two solutions of the observed H_α equivalent width as a function of the quiescent region H_α equivalent width. For Gl205, one of the solutions is shown to be impossible for the assumptions that we use. For Gl49 and G192-11A, two solutions are possible; a low solution (low CaIIEW) and a high solution (high CaIIEW). The difference between these two solutions is mainly in the plage-filling factor. The two solutions give almost identical H_α and CaIIprofiles. We prefer the low solutions because the filling factors are in closer agreement with those of other stars. We find plage-filling factors typically in the range 20%-40%. We also find that it is the chromospheric pressure rather than the filling factor that increases with increasing activity. We define a minimum theoretical H_α equivalent width as a function of the mean CaII H & K equivalent width. We show that our observations agree well with this lower limit. We also show that the properties of the chromosphere in quiescent and active regions correlate with the mean CaII H & K equivalent width. This could be useful in future studies to derive an estimate of the chromospheric properties from the observed mean CaII H & K equivalent width.