A volume-limited sample of hot subdwarf stars

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Hot subdwarf stars (sdO/Bs) constitute a prominent population of faint blue stars. Their formation is still unclear, because they can only be formed if the progenitor star loses its envelope almost entirely after passing the red-giant branch. Binary interactions likely play an important role to explain this mass loss and several formation scenarios have been proposed. Evolutionary tracks calculated for the diverse formation scenarios are usually compared to observed samples of hot subdwarf stars. However, this comparison between models and observational data is always biased by quite severe selection effects due to the incompleteness and inhomogeneity of all the observed samples so far. Here we propose to compile the first complete, volume-limited sample of hot subdwarfs to eliminate those selection effects and put much stronger constraints on evolutionary scenarios. The complete sample will be observed spectroscopically and a data analysis pipeline will be developed and tested to prepare for the large spectroscopic surveys in the future.