  Using Hubble Space Telescope ACS/WFC data we present the photometry and
spatial distribution of resolved stellar populations of four fields within the
extended ultraviolet disk (XUV disk) of M83. These observations show a clumpy
distribution of main-sequence stars and a mostly smooth distribution of red
giant branch stars. We constrain the upper-end of the initial mass function
(IMF) in the outer disk using the detected population of main-sequence stars
and an assumed constant star formation rate (SFR) over the last 300 Myr. By
comparing the observed main-sequence luminosity function to simulations, we
determine the best-fitting IMF to have a power law slope $\alpha=-2.35 \pm 0.3$
and an upper-mass limit $\rm M_{u}=25_{-3}^{+17} \, M_\odot$. This IMF is
consistent with the observed H$\alpha$ emission, which we use to provide
additional constraints on the IMF. We explore the influence of deviations from
the constant SFR assumption, finding that our IMF conclusions are robust
against all but strong recent variations in SFR, but these are excluded by
causality arguments. These results, along with our similar studies of other
nearby galaxies, indicate that some XUV disks are deficient in high-mass stars
compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc,
many already observed with HST, thus allowing a more comprehensive
investigation of the IMF, and how it varies, using the techniques developed
here.
