Plato benchmark stars I. The initial benchmark sample

The ESA PLATO mission aims to detect and characterise Earth-size exoplanets orbiting bright, late-type stars using high-precision photometry and asteroseismology. Precise stellar parameters of host stars are essential for exoplanet characterisation and model benchmarking. This work defines the PLATO benchmark stars (PBS) catalogue, a comprehensive sample of well-characterised stars designed to support all mission phases. We compiled data from eclipsing and spectroscopic binaries, interferometry, asteroseismology, and large spectroscopic and photometric surveys. Fundamental and atmospheric stellar properties (mass, radius, luminosity, age, effective temperature, surface gravity, density, and limb darkening) were assigned quality levels (B1, B2, B3) reflecting their degree of model dependence. Homogenized ancillary parameters (including Gaia DR3 bias-corrected parallaxes, metallicities, bolometric fluxes, extinction, rotation periods, and X-ray fluxes for M dwarfs) are also provided. The PBS catalogue contains 1090 FGKM main-sequence and subgiant stars, divided into three quality tiers based on model-independent (B1) parameters: gold (15 stars with B1 mass, radius, and luminosity), silver (195 with B1 mass and radius), and bronze (416 with at least one B1 property). The remaining stars feature well-characterised model-dependent (B2) or empirical (B3) properties. About 40% of the sample lies within the PLATO fields of view (285 in LOPN1 and 143 in LOPS2). The PBS sample represents the largest catalogue of benchmark stars assembled to date, providing a key resource for validating automated pipelines, testing evolutionary models, and cross-survey calibration. The catalogue is available online at https://astro.ulb.ac.be/pbs and will be updated regularly.