The outer atmosphere of Tau -- III. Inhomogeneities deduced from cold CO fluorescence

Abstract

Ultraviolet spectra of α Tau obtained with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope show clusters of CO emission lines which are excited by the O i (uv2) multiplet near 1304 A. Identifications of these fluorescent lines and their pumping routes are presented. The absolute intensities of the CO lines and their ratios cannot be explained by the one-dimensional chromospheric model based on collisionally excited lines. The excitation temperature of the CO is found to be only ≃2000 K, lower than that of the minimum value in the chromospheric model. The CO line intensities are used to deduce the O i radiation field, linewidths and opacity where the CO is pumped. The low opacity deduced implies that the O i radiation and CO molecules must be in close proximity, implying a two-component atmosphere. Although a combination of regions of traditional ‘chromosphere’ within a cool radiative equilibrium atmosphere cannot yet be excluded, the uv spectrum could instead be accounted for, in principle, by shocks within such a cool atmosphere.