A method for identifying blue horizontal branch stars on the basis of spectra and photoelectric colours

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Abstract
We use Strömgren and broad-band B and V photoelectric photometry to classify candidate blue horizontal branch field (BHBF) stars from a sample of faint blue objects at high Galactic latitudes. The candidates had been found previously by measuring hydrogen linewidths from medium-dispersion spectra and photographic colours. Most of the objects have been confirmed to be BHBF from the Strömgren data. We develop a method using hydrogen linewidths in conjunction with photoelectric colours for identifying BHBF stars. This is as effective as Strömgren photometry and has the advantage that it can be applied to much fainter magnitudes. The accuracy of our method has been confirmed from a sample of blue horizontal branch and blue straggler stars in globular clusters. The strength of the Ca K line has been measured in our sample stars and in the globular cluster stars. Although there are some difficulties associated with the Ca K line, the data confirm that our stars are mostly metal-weak. Hence our method provides an effective way to identify BHBF stars as a tracer of the Galaxy's metal-weak halo.

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