CaiiH and K Chromospheric Emission Lines in Late‐K and M Dwarfs

We have measured the profiles of the Ca II H and K chromospheric emissionlines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solarmasses) using multiple high resolution spectra obtained during six years withthe HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM,equivalent widths, and line luminosities of Ca II H and K increase by a factorof 3 with increasing stellar mass over this small range of stellar masses. Wefit the H and K lines with a double Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of the samplestars display a central absorption that is typically redshifted by ~0.1 km/srelative to the emission, but the nature of this velocity gradient remainsunknown. The FWHM of the H and K lines increase with stellar luminosity,reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalentwidths and FWHM exhibit modest temporal variability in individual stars. At agiven value of M_v, stars exhibit a spread in both the equivalent width andFWHM of Ca II H and K, due both to a spread in fundamental stellar parametersincluding rotation rate, age, and possibly metallicity, and to the spread instellar mass at a given M_v. The K line is consistently wider than the H line,as expected, and its central absorption is more redshifted, indicating that theH and K lines form at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of H-alpha correlateswith Ca II H and K only for stars having Ca II equivalent widths above ~2angstroms, suggesting the existence of a magnetic threshold above which thelower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for publication in PAS