Comparing the Ca II H and K Emission Lines in Red Giant Stars

TL;DR

This study compares the asymmetry of Ca II H and K emission lines in 105 red giant stars, revealing temperature-dependent differences likely caused by chromospheric layer motions.

Contribution

It provides the first detailed comparison of Ca II H and K line asymmetries in red giants, highlighting how these differences vary with stellar color and suggesting chromospheric dynamics.

Findings

H line V/R is slightly larger than K line V/R on average.

The difference in asymmetry depends on B-V color, with opposite trends for cooler and hotter giants.

Chromospheric layer motions may cause the observed asymmetry differences.

Abstract

Measurements of the asymmetry of the emission peaks in the core of the Ca II H line for 105 giant stars are reported. The asymmetry is quantified with the parameter V/R, defined as the ratio between the maximum number of counts in the blueward peak and the redward peak of the emission profile. The Ca II H and K emission lines probe the differential motion of certain chromospheric layers in the stellar atmosphere. Data on V/R for the Ca II K line are drawn from previous papers and compared to the analogous H line ratio, the H and K spectra being from the same sets of observations. It is found that the H line V/R value is +0.04 larger, on average, than the equivalent K line ratio, however, the difference varies with B-V color. Red giants cooler than B-V = 1.2 are more likely to have the H line V/R larger than the K line V/R, whereas the opposite is true for giants hotter than B-V = 1.2. The differences between the Ca II H and K line asymmetries could be caused by the layers of chromospheric material from which these emission features arise moving with different velocities in an expanding outflow.