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

TL;DR

This study identifies how resonances in atomic nitrogen's photoionization cross sections influence the far-ultraviolet spectrum of a bright star in 47 Tucanae, explaining observed spectral features not predicted by LTE models.

Contribution

It reveals the importance of high-resolution atomic nitrogen cross sections in modeling stellar spectra, especially for stars with altered chemical abundances.

Findings

Resonances cause specific absorption features at 995, 1010, and 1072 Å.

Full-resolution cross sections are necessary to reproduce observed spectral features.

Enhanced nitrogen and depleted carbon abundances amplify these resonance effects.

Abstract

The far-ultraviolet (FUV) spectrum of the Bright Star (B8 III) in 47 Tuc (NGC 104) shows a remarkable pattern: it is well fit by LTE models at wavelengths longer than Lyman beta, but at shorter wavelengths it is fainter than the models by a factor of two. A spectrum of this star obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) shows broad absorption troughs with sharp edges at 995 and 1010 A and a deep absorption feature at 1072 A, none of which are predicted by the models. We find that these features are caused by resonances in the photoionization cross sections of the first and second excited 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 the resonance-averaged cross sections usually employed to model stellar atmospheres. These resonances are strongest in stellar atmospheres with enhanced nitrogen and depleted carbon abundances, a pattern typical of post-AGB stars.