Local stellar kinematics from RAVE data: III. Radial and Vertical Metallicity Gradients based on Red Clump Stars

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We investigate radial and vertical metallicity gradients for a sample of red clump stars from the RAdial Velocity Experiment (RAVE) Data Release 3. We select a total of 6781 stars, using a selection of colour, surface gravity and uncertainty in the derived space motion, and calculate for each star a probabilistic (kinematic) population assignment to a thin or thick disc using space motion and additionally another (dynamical) assignment using stellar vertical orbital eccentricity. We derive almost equal metallicity gradients as a function of Galactocentric distance for the high probability thin disc stars and for stars with vertical orbital eccentricities consistent with being dynamically young, eᵥ<=0.07, i.e. d[M/H]/dRₘ = -0.041(0.003) and d[M/H]/dRₘ = -0.041(0.007) dex/kpc. Metallicity gradients as a function of distance from the Galactic plane for the same populations are steeper, i.e. d[M/H]/dzmax = -0.109(0.008) and d[M/H]/dzmax = -0.260(0.031) dex/kpc, respectively. Rₘ and zmax are the arithmetic mean of the perigalactic and apogalactic distances, and the maximum distance to the Galactic plane, respectively. Samples including more thick disc red clump giant stars show systematically shallower abundance gradients. These findings can be used to distinguish between different formation scenarios of the thick and thin discs.