The IACOB project IX. Building a modern empirical database of Galactic O9-B9 supergiants: Sample selection, description, and completeness

Context. Blue supergiants (BSGs) are key objects for studying the intermediate phases of massive star evolution because they are very useful to constrain evolutionary models. However, the lack of a holistic study of a statistically significant and unbiased sample of these objects has lead to several long-standing questions about their physical properties and evolutionary nature to remain unsolved. Aims. This paper and other upcoming papers of the IACOB series are focused on studying from a pure empirical point of view a sample of about 500 Galactic O9-B9 stars with luminosity classes I and II (plus 250 late O- and early B-type stars with luminosity classes III, IV, and V) that cover distances up to approximate to 4 kpc from the Sun. Methods. We compiled an initial set of approximate to 11 000 high-resolution spectra from approximate to 1600 Galactic late O- and B-type stars. We used a novel spectroscopic strategy based on a simple fitting of the H beta line to select stars in a specific region of the spectroscopic Hertzsprung-Russel diagram. We evaluated the completeness of our sample using the Alma Luminous Star catalog (ALS III) and Gaia-DR3 data. Results. We show the benefits of the proposed strategy for identifying BSGs that are descended in the context of single star evolution from stellar objects that are born as O-type stars. The resulting sample reaches a high level of completeness with respect to the ALS III catalog, gathering approximate to 80% of all-sky targets brighter than B-mag < 9 located within 2 kpc. However, we identify the need for new observations in specific regions of the southern hemisphere. Conclusions. We have explored a very fast and robust method for selecting BSGs. This provides a valuable tool for large spectroscopic surveys such as WEAVE-SCIP or 4MIDABLE-LR, and it highlights the risk of using spectral classifications from the literature. Upcoming studies will make use of this large and homogeneous spectroscopic sample to study the specific properties of these stars in detail. We initially provide first results for their rotational properties (in terms of projected rotational velocities, v sin i).