Abstract
Abstract The onset of chromospheric activity in late-A and early-F stars is here discussed. The detection of Ly-α emission core in several A and F atars with the IUE satellite, gives evidence for the presence of chromospheric layers in these stars up to B—V = 0m.19 (Marilli et al., 1996). Semiempirical chromospheric models for Altair allowed us (Freire Ferrero et al., 1995) to explain the observed emission profiles taking into account normal H I interstellar (IS) absorption. However, due to the very high rotational velocity, we analysed alternative hypotheses to explain the observed emissions: (1) circumstellar or shell matter; (2) co-rotating expanding optically thin wind. We ruled out these hypotheses because their effects are negligible and as a consequence, this result reinforces the chromospheric origin of the observed Ly-α core in Altair. The stars of our sample, having observed Ly-α profilies similar to Altair's and similar stellar and IS properties, should reproduce similar chromospheric behaviour...
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