Modeling Anomalous Absorption Features in the FUV Spectra of Late-B Giants

Details Modeling Anomalous Absorption Features in the FUV Spectra of Late-B Giants Modeling Anomalous Absorption Features in the FUV Spectra of Late-B Giants

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21924005d&link_type=abstract

American Astronomical Society, AAS Meeting #219, #240.05

Astronomy and Astrophysics

Astronomy

Scientific paper

At optical wavelengths, the brightest member of the globular cluster 47 Tuc is the so-called Bright Star (BS). This early-type giant (B8 III) is a post-AGB star with an effective temperature T_eff = 11,000 K, a surface gravity log g = 2.2, and a metallicity [Fe/H] = -0.76, a value consistent with the cluster mean.
The star's far-ultraviolet (FUV) spectrum is well fit by Kurucz models at wavelengths longer than Lyman β, but at shorter wavelengths it is fainter than the models by nearly an order of magnitude. In particular, a spectrum of the star obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) shows broad absorption troughs with apparent ionization edges at 995 and 1010 Å and a deep absorption feature at 1072 Å. Using photo-ionization cross sections for the first and second excited states of N I (2Do and 2Po) from the Opacity Project, we can reproduce these features in Kurucz models of the stellar spectrum. Doing so requires a signficant increase in the atmospheric nitrogen abundance relative to that indicated by the ground-state N I features, suggesting that non-LTE effects boost the population of excited-state N I atoms.
The star's FUSE spectrum is similar to those of the Pop. I stars HD 1279 (B7 III) and HD 196519 (B9 III), which bracket it in the spectral sequence. We have successfully reproduced the broad absorption features in both spectra by including N I opacity in the models. We conclude that bound-free transitions of neutral nitrogen represent a signficant opacity source in the FUV spectra of late-B giants.
Presentation of this poster is supported by the STScI Director's Discretionary Research Fund.

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