  In our previous work, we found that only two scenarios are capable of
reproducing the observed integrated mass-metallicity relations for the gas and
stellar components of local star-forming galaxies simultaneously. One scenario
invokes a time-dependent metal outflow loading factor with stronger outflows at
early times. The other scenario uses a time-dependent IMF slope with a steeper
IMF at early times. In this work, we extend our study to investigate the radial
profile of gas and stellar metallicity in local star-forming galaxies using
spatially resolved spectroscopic data from the SDSS-IV MaNGA survey. We find
that most galaxies show negative gradients in both gas and stellar metallicity
with steeper gradients in stellar metallicity. The stellar metallicity gradient
tend to be mass dependent with steeper gradients in more massive galaxies while
no clear mass dependence is found for the gas metallicity gradient. Then we
compare the observations with the predictions from a chemical evolution model
of the radial profiles of gas and stellar metallicities. We confirm that the
two scenarios proposed in our previous work are also required to explain the
metallicity gradients. Based on these two scenarios we successfully reproduce
the radial profiles of gas metallicity, stellar metallicity, stellar mass
surface density, and star formation rate (SFR) surface density simultaneously.
The origin of the negative gradient in stellar metallicity turns out to be
driven by either radially dependent metal outflow or IMF slope. In contrast,
the radial dependence of the gas metallicity is less constrained because of the
degeneracy in model parameters.
