Observation and modelling of main-sequence star chromospheres

Abstract

Aims. We aim to constrain the H α , 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. Methods. 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 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. Results. We obtain two component (quiescent and active region) model chromospheres for our nine target stars. We fit the H α , 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 α maximum absorption and emission. Two stars (Gl 49 and G192-11A) are quite active with H α 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 α 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 α equivalent width as a function of the quiescent region H α 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 EW) and a high solution (high CaII EW). The difference between these two solutions is mainly in the plage-filling factor. The two solutions give almost identical H α 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 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.

A young hierarchical triple system harbouring a candidate debris disc★

“…This time the Hα profile appears as two emission features either side of the expected wavelength with absorption in between. Such self-absorption of Hα emission is commonly seen in active M dwarfs (Worden et al 1981;Houdebine et al 1995;Mauas 2000). The radial velocity measurement for this second spectrum was -6.7±0.1 km s −1 .…”

Semi-Empirical Modeling of the Photosphere, Chromopshere, Transition Region, and Corona of the M-Dwarf Host Star Gj 832*

“…Previous chromospheric models for M dwarf stars were usually computed to fit only the Ca II, Hα, and Na I D lines (e.g., Mauas et al 1997;Houdebine & Stempels 1997;Short & Doyle 1998;Fuhrmeister, Schmitt, & Hauschildt 2005;Houdebine 2009Houdebine , 2010a).…”

Observation and modelling of main-sequence star chromospheres - XVIII. Observations of the Ca ii resonance lines and Hα line for dM4 stars and dK5 stars★

“…We aim here at collecting large samples of high‐resolution spectra for two different spectral types, dK5 and dM4, in order to develop two‐component model chromospheres for these stars such as those developed by Houdebine (2009a, hereafter Paper XII) and Houdebine (2010a, hereafter Paper IX) for dM1 stars. Two‐component model chromospheres are composed of a plage component and a quiescent chromosphere component with a fixed ratio in the Ca ii lines between these two components (see Paper IX).…”