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The far-ultraviolet (FUV) spectrum of the Bright Star (B8 III) in 47 Tuc (NGC104) shows a remarkable pattern: it is well fit by LTE models at wavelengthslonger than Lyman beta, but at shorter wavelengths it is fainter than themodels by a factor of two. A spectrum of this star obtained with the FarUltraviolet Spectroscopic Explorer (FUSE) shows broad absorption troughs withsharp edges at 995 and 1010 A and a deep absorption feature at 1072 A, none ofwhich are predicted by the models. We find that these features are caused byresonances in the photoionization cross sections of the first and secondexcited states of atomic nitrogen (2s$^2$ 2p$^3$ $^2$D$^0$ and $^2$P$^0$).Using cross sections from the Opacity Project, we can reproduce these features,but only if we use the cross sections at their full resolution, rather than theresonance-averaged cross sections usually employed to model stellaratmospheres. These resonances are strongest in stellar atmospheres withenhanced nitrogen and depleted carbon abundances, a pattern typical of post-AGBstars.

Resonances in the Photoionization Cross Sections of Atomic Nitrogen Shape the Far-Ultraviolet Spectrum of the Bright Star in 47 Tucanae