New Insights into Nonradiative Heating in Late A Star Chromospheres

Abstract

Using new and archival spectra from the Goddard High Resolution Spectrograph, we have searched for evidence of chromospheric and transition region emission in six stars of mid to late A spectral type. Two of the stars, α Aql (A7 IV-V) and alpha Cep (A7 IV-V), show emission in the C II 1335 Å 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 surface fluxes in the C II emission line to be 9.4 × 10⁴ ergs cm^-2s^-1 for α Aql and 6.5 × 10⁴ ergs cm^-2 s^-1 for α Cep. These 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 ∼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 early 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 ≲ 0.3, but that magnetic activity may persist to B - V as small as 0.22.