A correlation between GALEX FUV magnitude and chromospheric activity among red giant stars

Abstract

It is shown that upon combining GALEX far-ultraviolet and Johnson B magnitudes a resultant FUV-B colour can be obtained that for red giant stars of luminosity classes III and II correlates well with chromospheric emission in the cores of the Mg II h and k lines. Giant stars throughout the colour range 0.8 ≤ B - V ≤ 1.6 exhibit such a phenomenon. The main result of this paper is to show that GALEX far-ultraviolet photometry can provide information about the degree of chromospheric activity among red giant stars, and as such may offer a tool for surveying the evolution of chromospheric activity from the main sequence into the red giant phases of stellar evolution.