Spectroscopy of hot stars in the galactic halo. II. The identification and classification of horizontal-branch and other A-type stars

We discuss a spectroscopic and photometric technique that enables the identification and classification of field horizontal-branch (FHB) and other A-type stars, even from relatively low signal-to-noise ratio medium-resolution spectra. This technique makes use of broadband UBV colors predicted from model atmosphere calculations and Balmer line profiles and Ca II K equivalent widths determined from synthetic spectra to estimate the physical parameters T_eff, log g, and [Fe/H] for stars in the effective temperature range 6000-10,000 K. A comparison of our method with high signal-to-noise ratio spectra of standard stars indicates a scatter in the derived parameters of σ(T_eff) = ±250 K, σ(log g) = ±0.14 dex, and σ([Fe/H]) = ±0.12 dex. This precision allows for a separation of low surface gravity FHB and other, generally higher surface gravity, A-type (and somewhat later) stars. We also develop a synthetic-template comparison technique, which is very effective in the identification of metallic-line and peculiar A-type stars. A detailed investigation of the influence of noise in the spectra on the determination of physical parameters shows that, for spectra with signal-to-noise ratios in the range 10 < S/N < 20, the scatter in estimated surface gravity, σ(log g) = ±0.25 dex, is sufficiently small to keep the noise-induced misclassification rate for the hot stars we consider to less than 10%. Effective temperatures of precision σ(T_eff) = ±225 K and metallicities of precision σ([Fe/H]) = ±0.3 dex can be obtained.