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 eff = 33, 000 plus or minus 1000 K, surface gravity g = 3.79 plus or minus 0.11, and helium abundance N(He)/N(H) = -0.86 plus or minus 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 plus or minus 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 (L / L = 3.50 plus or minus 0.08) place it on the post-AGB evolutionary tracks of a star that evolved from the red horizontal branch. Its low carbon abundance, N(C)/N(O) = -2.18 plus or minus 0.21, indicates that it did not experience significant third dredge-up while ascending the AGB.