H-alpha absolute chromospheric fluxes in G and K dwarfs and subgiants.

High resolution, high S/N H-alpha observations for a sample of 85 dwarfs and subgiants of spectral type F8 to K5 are presented. A calibration procedure to convert the observed equivalent widths to absolute fluxes at the stellar surface is developed and these fluxes are compared with those obtained in the Ca II K line are compared. Within the observed range of spectral types there is some evidence that the ratio of H-alpha to Ca II K line fluxes increases toward cooler stars, suggesting that this effect, which is prominent in M dwarfs, is already significant for K stars. For G and early K stars the H-alpha flux is found to increase with increasing activity more slowly than the Ca II K flux, giving flux-flux relationships that are similar to those observed for solar plages. It is also found that all subgiants in the sample have very low H-alpha fluxes, typically lower than for dwarfs of the same spectral type. The survey clearly shows that the H-alpha line, although being a useful diagnostic of stellar chromospheres, and easily accessible to modern solid-state detectors, is not particularly suitable for an accurate determination of absolute chromospheric fluxes.