CHALLENGES TO UNDERSTAND STELLAR CHROMOSPHERES AND STELLAR ACTIVITY: THE LIMIT CASE OF LATE-AAND EARLY-F STARS

Abstract

The onset of chromospheric activity appears at late-A and early-F stars where theories predict atmospheres in radiative equilibriumand shallow or non-existent convective zones.The detection of Ly-α emission coresin several A and F stars, first with the IUE satellite and then withthe HST, gives evidence for the presence of chromospheric layersin these stars up to B-V=0.^m19 (Catalano et al. 1986). Semiempirical chromospheric models for Altair allowed us(Freireet al. 1995) to explain the observed emission profilestaking into account normal HI IS absorption.However, due to the very high rotational velocity we analyzedalternative hypotheses like the formation of Ly-α emissionsinto a corotating expanding wind, but we ruled out this alternativebecause we obtained inconsistent results. In addition, X-ray emission(originated surely in a corona) strengthenthe presence of a chromosphere.Here we place the problem of chromospheric activity oflate-A and early-F stars in the general context of the formationof over-photospheric stellar layers, comparing them with late-type star and solar cases.