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Description
Surface brightness-colour relations (SBCRs) are used to derive the stellar angular diameters from photometric observations. They have various astrophysical applications, such as the distance determination of eclipsing binaries or the determination of exoplanet parameters. However, strong discrepancies between the SBCRs still exist in the literature, in particular for early and late-type stars. We aim at calibrating new SBCRs as a function of the spectral type and the luminosity class of the stars. Our purpose is also to apply homogeneous criteria for the selection of the reference stars and on the basis of an exhaustive and up-to-date list of interferometric late-type targets. We implement criteria to select measurements in the JMMC Measured Diameters Catalog (JMDC). We then apply additional criteria on the photometric measurements used to build the SBCRs, together with stellar characteristics diagnostics. We build SBCRs for F5/K7-II/III, F5/K7-IV/V, M-II/III and M-V stars, with respective RMS of {sigma}_FV_=0.0022mag, {sigma}_FV_ =0.0044mag, {sigma}_FV_=0.0046mag, and {sigma}_FV_=0.0038mag. This results in a precision on the angular diameter of 1.0%, 2.0%, 2.1% and 1.7%, respectively. These relations cover a large V-K colour range of magnitude, from 1 to 7.5. Our work demonstrates that SBCRs are significantly dependent on the spectral type and the luminosity class of the star. Through a new set of interferometric measurements, we demonstrate the critical importance of the selection criteria proposed for the calibration of SBCR. Finally, using the Gaia photometry for our samples, we obtain (G-K) SBCRs with a precision on the angular diameter between 1.1% and 2.4%. By adopting a refined and homogeneous methodology we show that the spectral type and the class of the star should be considered when applying a SBCR. This is particularly important in the context of PLATO.
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