[α/Fe] versus [Fe/H] (left) and [α/Fe] versus [Ref/Fe] (right) for the total sample. The red squares refer to the Jovian hosts and the blue triangles refer to the stars hosting exclusively Neptunians and super-Earths. The black dots represent the stars without planets. The black solid and green dashed lines represent the mean distributions of the planet host and non-host samples, respectively. The magenta dotted lines are the cubic fits for the low-α (thin disc) and the high-α (thick disk + high-α metal-rich) stars. The two metal-poor Jovian hosts in the red circles are the stars with low log  g values discussed in Sect. 3.2.