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

We describe automatic procedures for the selection of DA white dwarfs in the Hamburg/ESO objective-prism survey (HES). For this purpose, and the selection 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 allow one to convert model or slit spectra to HES spectra. These simulated objective-prism spectra are used to determine quantitative selection criteria, and for the study of selection functions. We present an atlas of simulated HES spectra of DA and DB white dwarfs. Our current selection algorithm is tuned to yield maximum efficiency of the candidate sample (minimum contamination 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 contamination 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 recovered. A yet higher degree of internal completeness could be reached at the expense of higher contamination. However, the external completeness is limited by additional losses caused by proper motion effects and the epoch differences between direct and spectral plates used in the HES.