Observation and modelling of main-sequence star chromospheres

Abstract

<i>Aims. <i/> We aim to constrain the H, CaII H and CaII K line profiles of quiescent and active regions of nine dM1 stars of near solar metallicity: Gl 2, GJ 1010A, Gl 49, Gl 150.1B, Gl 205, Gl 229, Gl 526, G192-11A, and Gl 880.<i>Methods. <i/>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. This method is based on the measurements of the equivalent width of H 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.<i>Results. <i/>We obtain two component (quiescent and active region) model chromospheres for our nine target stars. We fit the H, CaII H, and CaII K profiles for these stars. These models show that seven of these stars lie in the intermediate activity range between H maximum absorption and emission. Two stars (Gl 49 and G192-11A) are quite active with H emission profiles in plages. As far as the CaII emission is concerned, these two stars are almost as active as dM1e stars. Two stars (GJ 1010A and Gl 526) have lower activity levels with narrower and weaker H profiles. The range of activity covered by these stars is a factor of 13 in the CaII lines, from low activity to activity levels almost as high as those of dM1e stars. Our method sometimes provides two solutions of the observed H equivalent width as a function of the quiescent region H equivalent width. For Gl 205, one of the solutions is shown to be impossible for the assumptions that we use. For Gl 49 and G192-11A, two solutions are possible; a low solution (low CaII <i>EW<i/>) and a high solution (high CaII <i>EW<i/>). The difference between these two solutions is mainly in the plage-filling factor. The two solutions give almost identical H and CaII profiles. 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 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.