2006/02/14 by Emily Rauscher, Geoffrey W. Marcy · 4 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/503021
40 pages including 12 figures and 17 pages of tables, accepted for publication in PASP
arxiv created 2006/02/14 · openalex publication_date 2006/04/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/30
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 ∼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 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.