Kiel diagram (upper panel) and Hertzsprung-Russell diagram (lower panel) with isochrones and helium burning tracks. HD 20’s position is depicted by a blue filled circle with error bars. Upper panel: the error on the gravity is smaller than the circle size. The red line represents a He-normal 11 Gyr isochrone at [Fe/H] = −1.60 dex, and [α/Fe] = +0.4 dex with age and metallicity error margins shown by orange and blue ranges. The RGB luminosity bump for this particular model at log L/L_⊙ ∼ 2.0 is highlighted in the lower panel by an arrow and the label “LB”. The light blue curve is a model with the same parameters except for Y = 0.4. He-burning tracks for three different masses are shown by gray lines of different line styles with the stellar masses being indicated next to the respective tracks.