A METHOD FOR IDENTIFYING BLUE HORIZONTAL-BRANCH STARS ON THE BASIS OFSPECTRA AND PHOTOELECTRIC COLORS

We use Stromgren and broad-band B and V photoelectric photometry to cl assify candidate blue horizontal branch field (BHBF) stars from a samp le of faint blue objects at high-Galactic latitudes. The candidates ha d been found previously by measuring hydrogen linewidths from medium-d ispersion spectra and photographic colours. Most of the objects have b een confirmed to be BHBF from the Stromgren data. We develop a method using hydrogen linewidths in conjunction with photoelectric colours fo r identifying BHBF stars. This is as effective as Stromgren photometry and has the advantage that it can be applied to much fainter magnitud es. The accuracy of our method has been confirmed from a sample of blu e horizontal branch and blue straggler stars in globular clusters. The strength of the Ca K line has been measured in our sample stars and i n the globular cluster stars. Although there are some difficulties ass ociated with the Ca K line, the data confirm that our stars are mostly metal-weak. Hence our method provides an effective way to identify BH BF stars as a tracer of the Galaxy's metal-weak halo.