Abstract
ABSTRACTThe 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 (mFUV - B) were obtained by combining GALEX FUV and Johnson B 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 (B - V) < 0.8 the photospheric flux produces a two-color relation between mFUV - B and B - V. A color excess Δ(mFUV - B), 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 (B - V) > 0.8 the photospheric contribution to the flux in the GALEX FUV bandpass is sufficiently low that the uncorrected mFUV - B 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.
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