The outer atmosphere of Tau - I. A new chromospheric model

Abstract

A new model of the chromosphere of α Tau (K5 III) has been developed. The lower part of the model is based on an earlier one designed mainly to fit the Ca ii H and K lines. The new model extends through the upper chromosphere, to account for lines of C i, C ii, Si ii and Mg ii, and into the transition region up to 10⁵ K to account for emission from C iv. It is based on observations made with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST). The model produces fluxes that for collisionally excited lines agree to within a factor of 2 with those observed, and in particular gives an improved fit to the C ii (uv1) lines. Thus the collisionally excited lines can be accounted for by a plane-parallel, hydrostatic, one-component atmosphere. The new model also gives an improved fit to the O i (uv2) lines, which depend on the H Lyβ flux.