Observations of the Halo Star HD 177566

We have analyzed archival FUV and optical spectra of the hot halo star HD 177566. The star has an effective temperature $T_{\mathrm{e}\mathrm{f}\mathrm{f}}=\mathrm{33,000}\pm 1000$ K, surface gravity $\log g=3.79\pm 0.11$, and helium abundance $\log N(\mathrm{H}\mathrm{e})\mathrm{/}N(\mathrm{H})=-0.86\pm 0.05$. Abundances of 13 additional elements are consistent with those of other halo stars, save for carbon, which is underabundant by about 1 dex. The low-order hydrogen Balmer lines are not well reproduced by our models. The diffuse lines of He I are often broader than predicted, but the use of more recent line-broadening parameters significantly improves the fit. Scaling our best-fit model to the star’s optical and near-IR magnitudes yields an extinction $E(B-V)=0.095\pm 0.005$, consistent with literature values, but the resulting model underpredicts the star’s FUV flux by a factor of two. The star’s effective temperature and luminosity ($\log L\mathrm{/}{L}_{\odot }=3.50\pm 0.08$) place it on the post-AGB evolutionary tracks of a star that evolved from the red horizontal branch. Its low carbon abundance, $\log N(\mathrm{C})\mathrm{/}N(\mathrm{O})=-2.18\pm 0.21$, indicates that it did not experience significant third dredge-up while ascending the AGB.