Observation and modelling of main sequence star chromospheres^*

VIII. High resolution observations of M and K dwarf chromospheric lines

¹ 25 Rue du Dr. Laulaigne, 49670 Valanjou, France e-mail: eric_houdebine@yahoo.fr
² Armagh Observatory, College Hill, BT61 9DG Armagh, N. Ireland
³ Dublin Institute for Advanced Studies, Dunsink Observatory, Castleknock, Dublin 15, Ireland

Received: 14 November 2008
Accepted: 10 May 2009

Abstract

Aims. We report on high resolution observations of chromospheric lines for M and K dwarfs.

Methods. We observed up to 13 spectral lines per star: the H_α, H, H, Pa, Pa₈, CaII H, CaII IR triplet, CaI 6572Å, NaI D₁/D₂ and HeI D₃ lines. We observed two dMe stars, one dM(e) star, 5 dM stars and 3 dK stars.

Results. We observed a self-reversal in the emission core of the CaII H line for the brightest stars only, indicating a rather optically thick region of formation.
We present original spectra of the NaI doublet and the CaII IR triplet for active dMe stars and less active dM stars. Core emission is detected in the Sodium lines and the CaII IR triplet lines for the most active M dwarf AU Mic. We investigate the difference spectra between active dMe stars and dM stars and show that these provide interesting new constraints for the NLTE-radiation transfer modelling of the chromospheres.
In our sample, emission H_α profiles have a rather homogeneous FWHM of about 1.5 Å. This, according to our previous modelling, can be interpreted as the signature of a rather constant temperature break in the chromosphere. We found that one of our targets (MCC 332) is a binary with a faint but active H_α emission component.
For the first time we detect the Pa line for six dwarfs. It appears as weak absorption with possible weak wing emission in AU Mic. The region of the Pa₈ line was observed but the line was not detected.