Searching for chromospheric Lyman-α emission in A–F stars from IUE high resolution spectra

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 detectedLy-α emission in stars as early asA7 (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 H i column density is estimated.