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Improved Baade–Wesselink surface brightness relations




Recent, and older accurate, data on (limb-darkened) angular diameters is compiled for 221 stars, as well as BVRIJK[12][25] magnitudes for those objects, when available. Nine stars (all M-giants or supergiants) showing excess in the [12–25] colour are excluded from the analysis as this may indicate the presence of dust influencing the optical and near-infrared colours as well. Based on this large sample, Baade–Wesselink surface brightness (SB) relations are presented for dwarfs, giants, supergiants and dwarfs in the optical and near-infrared. M-giants are found to follow different SB relations from non-M-giants, in particular in V versus VR. The preferred relation for non-M-giants is compared to the earlier relation by Fouqué and Gieren (based on 10 stars) and Nordgren et al. (based on 57 stars). Increasing the sample size does not lead to a lower rms value. It is shown that the residuals do not correlate with metallicity at a significant level. The finally adopted observed angular diameters are compared to those predicted by Cohen et al. for 45 stars in common, and there is reasonable overall, and good agreement when θ < 6 mas. Finally, I comment on the common practice in the literature to average, and then fix, the zero-point of the V versus VK, V versus VR and K versus JK relations, and then rederive the slopes. Such a common zero-point at zero colour is not expected from model atmospheres for the VR colour and depends on gravity. Relations derived in this way may be biased.