Local Stellar Kinematics from RAVE data - VII. Metallicity Gradients
  from Red Clump Stars
  We investigate the Milky Way Galaxy's radial and vertical metallicity
gradients using a sample of 47,406 red clump stars from the RAVE DR4. This
sample is more than twice the size of the largest sample in the literature
investigating radial and vertical metallicity gradients. The absolute magnitude
of Groenewegen (2008) is used to determine distances to our sample stars. The
resulting distances agree with the RAVE DR4 distances Binney et al. (2014) of
the same stars. Our photometric method also provides distances to 6185 stars
that are not assigned a distance in RAVE DR4. The metallicity gradients are
calculated with their current orbital positions ($R_{gc}$ and $Z$) and with
their orbital properties (mean Galactocentric distance, $R_{m}$ and $z_{max}$),
as a function of the distance to the Galactic plane:
d[Fe/H]/d$R_{gc}=$-$0.047\pm0.003$ dex/kpc for $0\leq |Z|\leq0.5$ kpc and
d[Fe/H]/d$R_m=$-$0.025\pm0.002$ dex/kpc for $0\leq z_{max}\leq0.5$ kpc. This
reaffirms the radial metallicity gradient in the thin disc but highlights that
gradients are sensitive to the selection effects caused by the difference
between $R_{gc}$ and $R_{m}$. The radial gradient is flat in the distance
interval 0.5-1 kpc from the plane and then becomes positive greater than 1 kpc
from the plane. The radial metallicity gradients are also eccentricity
dependent. We showed that d[Fe/H]/d$R_m=$-$0.089\pm0.010$, -$0.073\pm0.007$,
-$0.053\pm0.004$ and -$0.044\pm0.002$ dex/kpc for $e_p\leq0.05$, $e_p\leq0.07$,
$e_p\leq0.10$ and $e_p\leq0.20$ sub-samples, respectively, in the distance
interval $0\leq z_{max}\leq0.5$ kpc. Similar trend is found for vertical
metallicity gradients. Both the radial and vertical metallicity gradients are
found to become shallower as the eccentricity of the sample increases. These
findings can be used to constrain different formation scenarios of the thick
and thin discs.
