Tracers of chromospheric structure. I. CaII H$\&$K emission distribution of 13000 F, G and K stars in SDSS DR7 spectroscopic sample

TL;DR

This study measures chromospheric activity in over 13,000 F, G, and K stars from SDSS DR7, revealing how activity varies with stellar type, evolutionary stage, and Galactic location, and estimating stellar ages.

Contribution

It provides a large-scale analysis of chromospheric activity indices for a broad stellar sample, distinguishing dwarfs and giants, and examining effects of metallicity and Galactic structure.

Findings

Cooler stars are more active and show larger scatter.

Thick disk stars are generally less active than thin disk stars.

Most field stars have ages between 3 and 8 Gyr.

Abstract

We present chromospheric activity index $S\rm_{HK}$ 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 $\delta$S is defined as the difference between $S\rm_{HK}$ and a `zero' emission line fitted by several of the most inactive stars. The $S\rm_{HK}$ 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 $S\rm_{HK}$ measurements differently among F, G and K dwarfs in this sample. Using the open clusters NGC 2420, M67 and NGC6791 as calibrations, ages of most field stars in this SDSS sample range from 3-8 Gyr.