1997A&A...317..521M

With the aim of locating the boundary for the onset of chromospheric emission in the H-R diagram we have undertaken a spectroscopic study of the hydrogen Ly-α, using the IUE short wavelength spectrograph in the high resolution mode. We report here on the analysis of IUE spectra of 11 A-F type stars. We have detected Ly-α emission in stars as early as A7 (B-V=0.19), establishing a new limit for the presence of chromospheres. The constant or the slowly decreasing trend of the Ly-α flux for B-V<0.5 is related to the decreasing trend of X-ray emission in the same spectral type region. From the interstellar absorption feature affecting the stellar Ly-α emission, the HI column density is estimated.