The stellar content of the Hamburg/ESO survey I. Automated selection of DAwhite dwarfs

We describe automatic procedures for the selection of DA white dwarfs in th e Hamburg/ESO objective-prism survey (HES). For this purpose, and the selec tion of other stellar objects (e.g., metal-poor stars and carbon stars), a flexible, robust algorithm for detection of stellar absorption and emission lines in the digital spectra of the HES was developed. Broad band (U-B, B- V) and intermediate band (Stromogren c(1)) colours can be derived directly from HES spectra, with precisions of sigma (U-B) = 0.092 mag; sigma (B-V) = 0.095 mag; sigma (c1) = 0.15 mag. We describe simulation techniques that a llow one to convert model or slit spectra to HES spectra. These simulated o bjective-prism spectra are used to determine quantitative selection criteri a, and for the study of selection functions. We present an atlas of simulat ed HES spectra of DA and DB white dwarfs. Our current selection algorithm i s tuned to yield maximum efficiency of the candidate sample (minimum contam ination with non-DAs). DA candidates are selected in the B-V versus U-B and c(1) versus W-lambda(H beta + H gamma + H delta) parameter spaces. The con tamination of the resulting sample with hot subdwarfs is expected to be as, low as similar to8%, while there is essentially no contamination with main sequence or horizontal branch stars. We estimate that with the present set of criteria, similar to 80% of DAs present in the HES database are recover ed. A yet higher degree of internal completeness could be reached at the ex pense of higher contamination. However, the external completeness is limite d by additional losses caused by proper motion effects and the epoch differ ences between direct and spectral plates used in the HES.