Ca ii H and K Chromospheric Emission Lines in Late‐K and M Dwarfs11Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. Keck time has been granted by both NASA and the University of California.

Abstract

ABSTRACTWe 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 ∼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 MV, 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 MV. 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 Hα correlates with Ca ii H and K only for stars having Ca ii equivalent widths above ∼2 Å, suggesting the existence of a magnetic threshold above which the lower and upper chromospheres become thermally coupled.