Ca II H and K chromospheric emission lines in late-K and M dwarfs

We have measured the profiles of the Ca II H and K chromospheric emission lines in 147 main-sequence stars of spectral type M5 - K7 (masses 0.30 - 0.55 M-.) using multiple high-resolution spectra obtained during 6 years with the HIRES spectrometer on the Keck I telescope. Remarkably, the average FWHM, equivalent widths, and line luminosities of Ca II H and K increase by a factor of 3 with increasing stellar mass over this small range of stellar masses. We fit the Ca II H and K lines with a double-Gaussian model to represent both the chromospheric emission and the non-LTE central absorption. Most of the sample stars display a central absorption that is typically redshifted by similar to 0.1 km s(-1) relative to the emission. This implies that the higher level, lower density chromospheric material has a smaller outward velocity ( or higher inward velocity) by 0.1 km s(-1) than the lower level material in the chromosphere, but the nature of this velocity gradient remains unknown. The FWHM of the Ca II H and K emission lines increase with stellar luminosity, reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent widths and FWHM exhibit modest temporal variability in individual stars. At a given value of M-V, stars exhibit a spread in both the equivalent width and FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters, including rotation rate, age, and possibly metallicity, and to the spread in stellar 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 the H and K lines form at slightly different heights in the chromosphere where the velocities are slightly different. The equivalent width of H alpha correlates with Ca II H and K only for stars having Ca II equivalent widths above similar to 2 angstrom, suggesting the existence of a magnetic threshold above which the lower and upper chromospheres become thermally coupled.