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In this study, photometric metallicity and absolute magnitude calibrations were derived using F-G spectral type main-sequence stars in the Solar neighbourhood with precise spectroscopic, photometric and Gaia astrometric data for UBV photometry. The sample consists of 504 main-sequence stars covering the temperature, surface gravity and colour index intervals 5300 < T ef f < 7300 K, log g > 4 (cgs) and 0.3 < (B − V ) 0 < 0.8 mag, respectively. Stars with relative trigonometric parallax errors σ π /π ≤ 0.01 were preferred from Gaia DR2 data for the estimation of their M V absolute magnitudes. In order to obtain calibrations, (U − B) 0 and (B − V ) 0 colour indices of stars were preferred and a multi-variable second order equation was used. Calibrations are valid for main-sequence stars in the metallicity and absolute magnitude ranges −2 < [Fe/H] < 0.5 dex and 2.5 < M V < 6 mag, respectively. The mean value and standard deviation of the differences between original and estimated values for the metal abundance and absolute magnitude are ∆[Fe/H] = 0.00 ± 0.11 dex and ∆M V = 0.00 ± 0.22 mag, respectively. In this work, it has been shown that more precise iron abundance and absolute magnitude values were obtained with the new calibrations, compared to previous calibrations in the literature.

We derive transformation equations between GALEX and UBV colours by using the reliable data of 556 stars. We present two sets of equations: as a function of (only) luminosity class and as a function of both luminosity class and metallicity. The metallicities are provided from the literature, while the luminosity classes are determined by using the PARSEC mass tracks in this study. Small colour residuals and high squared correlation coefficients promise accurate derived colours. The application of the transformation equations to 70 stars with reliable data shows that the metallicity plays an important role in estimation of more accurate colours.