  We present an unbiased determination of the charm content of the proton, in
which the charm parton distribution function (PDF) is parametrized on the same
footing as the light quarks and the gluon in a global PDF analysis. This
determination relies on the calculation of deep-inelastic structure functions
in the FONLL scheme, generalized to account for massive charm-initiated
contributions. In contrast to the usual situation in which the charm PDF is
assumed to be generated perturbatively by pair radiation off gluons and light
quarks, vanishing at a scale set by the value of the charm mass m_c, we find
that the fitted charm PDF vanishes within uncertainties at a scale Q~1.5 GeV
for all x<~0.1, independent of the value of m_c used in the coefficient
functions. We also find some evidence that the charm PDF at large x>~0.1 and
low scales does not vanish, but rather has an "intrinsic" component, very
weakly scale dependent and almost independent of the value of m_c, carrying
about 1% of the total momentum of the proton. The uncertainties in all other
PDFs are only slightly increased by the inclusion of fitted charm, while the
dependence of these PDFs on m_c is significantly reduced. When the EMC charm
structure function dataset is included, it is well described by the fit, and
PDF uncertainties in the fitted charm PDF are significantly reduced, though we
verify that excluding the EMC data does not qualitatively modify any of our
findings. The increased stability with respect to m_c persists at high scales
and is the main implication of our results for LHC phenomenology. Fitting the
charm PDF modifies the predictions for processes such as high p_T and large
rapidity charm pair production and Z+c production, and thus we expect that
future LHC data will further constrain the charm content of the proton.
