The outer atmospheres of the ‘hybrid’ bright giants: the chromospheres of α TrA (K4 II), ι AUR (K3 II), γ Aql (K3 II) and θ Her (K1 II)

Abstract

Detailed models of the chromospheres of the «hybrid» bright giants α TrA (K4 II), ι Aur (K3 II), γ Aql (K3 II) and θ Her (K1 II) are presented. The models run from the base of the chromosphere and into the transition region. The chromospheric models are based on radiative transfer calculations for a range of atoms and emission lines, including : Mg II h and k; Al II] λ2669 A; C II λ1335 A; C II] λ2325 A; Si II λλ1808.0, 1816.9 and 1817.5 A. The transition region models are made using standard emission measure techniques. The presence of steep turbulent velocity gradients leads to profiles of optically thin chromospheric emission lines that are strongly dependent on the distribution of emitting material with temperature