Temperature – gravity grid for atmospheres composed purely of hydrogen (upper panel), purely of helium (middle panel), and of heavy elements (bottom panel). The gray dots show the gravity grid with Δg = 0.05 in logarithmic scale, because for Δg = 0.01, the figure would not be legible. The different dashed curves represent models with constant l at the values described in the legend. The horizontal solid red line displays the value of the gravity for the best-fit mass and radius measured by Nättilä et al. (2017), and the two horizontal dashed red lines correspond to the measured errors (see Sect. 2.3).