NEW INSIGHTS INTO NONRADIATIVE HEATING IN LATE A STAR CHROMOSPHERES

Using new and archival spectra from the Goddard High Resolution Spectr ograph, we have searched for evidence of chromospheric and transition region emission in six stars of mid to late A spectral type. Two of th e stars, alpha Aql (A7 IV-V) and alpha Cep (A7 IV-V), show emission in the C II 1335 Angstrom doublet, confirming the presence of hot plasma with temperatures comparable to that of the solar transition region. Using radiative equilibrium photospheric models, we estimate the net s urface fluxes in the C II emission line to be 9.4 x 10(4) ergs cm(-2) s(-1) for alpha Aql and 6.5 x 10(4) ergs cm(-2) s(-1) for alpha Cep. T hese are comparable to fluxes observed in early to mid F-type dwarfs, indicating that significant upper atmospheric heating is present in at least some stars as hot as similar to 8000 K (B-V = 0.22). We find no evidence for the blueshifted emission reported by Simon et al. (1994) . We estimate the basal flux level to be about 30% of that seen in ear ly F stars, and that the bulk of the emission is not basal in origin. We conclude that the basal flux level drops rapidly for B-V less than or similar to 0.3, but that magnetic activity may persist to B-V as sm all as 0.22.