2010PASP..122.1303S

The magnitudes of F, G, and K dwarf stars measured in the FUV bandpass of the GALEX satellite are found to show correlations with chromospheric activity. Values of a color were obtained by combining GALEX FUV and Johnson magnitudes for a large sample of dwarfs for which published measurements of flux in the chromospheric Ca II H and K emission lines are available. Among dwarfs with the photospheric flux produces a two-color relation between and. A color excess, obtained by making a first-order correction for this photospheric effect, shows a correlation with chromospheric activity for stars with, where is the ratio between the luminosity in the H and K emission lines and the bolometric luminosity. Among dwarfs with the photospheric contribution to the flux in the GALEX FUV bandpass is sufficiently low that the uncorrected color shows a clear variation with. The approximately linear nature of this relation indicates that the combined flux in the FUV band, which for such late-type dwarfs arises from transition-region and chromospheric emission lines, is related in a power-law manner to the flux in the Ca II H and K emission lines.