Chemical gradients in the Milky Way from the RAVE data. II : giant stars

Aims. We provide new constraints on the chemo-dynamical models of the Milky Way by measuring the radial and vertical chemical gradients for the elements Mg, Al, Si, Ti, and Fe in the Galactic disc and the gradient variations as a function of the distance from the Galactic plane (Z). Methods. We selected a sample of giant stars from the RAVE database using the gravity criterium 1.7 < log g < 2.8. We created a RAVE mock sample with the Galaxia code based on the Besançon model and selected a corresponding mock sample to compare the model with the observed data. We measured the radial gradients and the vertical gradients as a function of the distance from the Galactic plane Z to study their variation across the Galactic disc. Results. The RAVE sample exhibits a negative radial gradient of d[Fe/H]/dR = -0.054 dex kpc-1 close to the Galactic plane (|Z| < 0.4 kpc) that becomes flatter for larger |Z|. Other elements follow the same trend although with some variations from element to element. The mock sample has radial gradients in fair agreement with the observed data. The variation of the gradients with Z shows that the Fe radial gradient of the RAVE sample has little change in the range |Z| ≲ 0.6 kpc and then flattens. The iron vertical gradient of the RAVE sample is slightly negative close to the Galactic plane and steepens with |Z|. The mock sample exhibits an iron vertical gradient that is always steeper than the RAVE sample. The mock sample also shows an excess of metal-poor stars in the [Fe/H] distributions with respect to the observed data. These discrepancies can be reduced by decreasing the number of thick disc stars and increasing their average metallicity in the Besançon model