Volume-limited sample of low-mass red giant stars, the progenitors of hot subdwarf stars

Context. Binary hot subdwarf B (sdB) stars are typically produced from low-mass red giant branch (RGB) stars that have lost almost all their envelopes through binary mass transfer while still fusing helium in their cores. Particularly, when a low-mass red giant enters stable Roche lobe overflow (RLOF) mass transfer near the tip of the RGB, a long-period sdB binary may be formed. Aims. We aim to extend our previous volume-limited sample of 211 stars within 200 pc, a homogeneous sample of low-mass red giants, predicted progenitors of wide sdB binaries, to 500 pc and validate it. Additionally, our goal is to provide the distribution of stellar parameters for these stars. Methods. We refined our original 500 pc sample by incorporating Gaia DR3 parallax values and interstellar extinction measurements. Next, we collected multi-epoch high-resolution spectra for 230 stars in the volume-limited sample using the CORALIE échelle spectrograph from 2019 to 2023. To confirm or discard binarity, we combined astrometric parameters from Gaia with the resulting radial velocity variations. We derived the distribution of stellar parameters using evolutionary models and employed the equivalent evolutionary phase to verify the evolutionary stage of the stars in our sample. Finally, we compared our stellar parameters with the literature. Results. The derived stellar parameters confirmed that 82% of stars in our sample are indeed in the RGB phase, while 18% are red clump (RC) contaminants. This was expected due to the overlapping of RGB and RC stars in the colour-magnitude diagram. Additionally, 75% of the confirmed RGB stars have a high probability of being part of a binary system. Comparison with the literature shows good overall agreement with a scatter ≲15% in stellar parameters, while the masses show somewhat higher dispersion (∼20%). Conclusions. We have obtained the most complete volume-limited sample of binary RGB star candidates within 500 pc. These systems are likely progenitors of hot subdwarfs and other classes of stripped helium stars.