Tracers of Chromospheric Structure. I. Ca Ii H&K Emission Distribution of 13,000 F, G, and K Stars in SDSS DR7 Spectroscopic Sample

We present chromospheric activity index S _HK measurements for over 13,000 F, G, and K disk stars with high signal-to-noise ratio ( > 60) spectra in the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) spectroscopic sample. A parameter δS is defined as the difference between S _HK and a “zero” emission line fitted by several of the most inactive stars. The S _HK indices of subgiant stars tend to be much lower than dwarfs, which provide a way to distinguish dwarfs and giants with relatively low-resolution spectra. Cooler stars are generally more active and display a larger scatter than hotter stars. Stars associated with the thick disk are in general less active than those of the thin disk. The fraction of K dwarfs that are active drops with vertical distance from the Galactic plane. Metallicity affects S _HK measurements differently among F, G, and K dwarfs in this sample. Using the open clusters NGC 2420, M67, and NGC 6791 as calibrations, ages of most field stars in this SDSS sample range from 3 to 8 Gyr.