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

¹ Observatoire Astronomique, 11 rue de l'Université, 67000 Strasbourg, France
² Institut d'Astrophysique Spatiale, Université Paris XI, Bât. 121, 91405 Orsay Cedex, France

Abstract

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