HotTO stars of the Medium Quality sample located within 1 kpc of the Galactic plane. Their XY spatial distribution is shown within ±4 kpc of the Sun, colour coded according to stellar count and the median of the mean metallicity (left and central panels, respectively). The solar position is indicated by a filled star coloured according to solar metallicity ([M/H] = 0.0 dex). Right panel: evolution of calcium (top) and sulfur (bottom) abundances with respect to iron abundances versus the mean metallicity.