The T-GEX project. I. Stealth UV-bright Sun-like stars in the Galaxy as candidate contributors to the UV upturn

We constructed the Tiny Gaia-ESO + GALEX (T-GEX) catalogue by combining Gaia-ESO spectroscopy, Gaia DR3 astrometry, GALEX UV photometry, and 2MASS and AllWISE infrared measurements for 37 stars. Astrometric and spectroscopic quality cuts minimise obvious multiplicity and spectral peculiarity, although unresolved companions cannot be excluded. We classified the stars relative to the empirical FGK UV-normal locus, applied a 3-component Gaussian mixture model in the FUV-NUV versus NUV- G plane, measured H alpha lambda 6563 diagnostics for 24 stars, and performed an IMF-based empirical scaling calculation for three UV-upturn galaxies. Approximately 2/3 of the sample are UV-abnormal. The GMM identifies a cool, strongly UV-excess group (G1), a hotter UV-normal group (G2), and an intermediate UV-excess group (G3). G1 occupies a narrow, predominantly sub-solar [Fe/H] range and is alpha-enhanced, with the highest median [Mg/Fe]. G3 has the highest median [Fe/H], while G2 spans the broadest metallicity range and has the youngest median age. G1 and G3 extend to old ages, although their youngest estimates are affected by isochrone degeneracy. Enhanced H alpha core emission occurs only among hotter G2 stars in the available subsample, but only one G1 star has H alpha coverage. The extragalactic contribution is strongly template-dependent: the median G1 template accounts for at most about20% of the observed UV output, whereas the bluest template could match most or all of the NUV and FUV emission if shared by about10 -- 15% of surviving FGK stars.