The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular
  Clouds
  We measure the parsec-scale relationship between integrated CO intensity
(I_CO) and visual extinction (A_V) in 24 local molecular clouds using maps of
CO emission and dust optical depth from Planck. This relationship informs our
understanding of CO emission across environments, but clean Milky Way
measurements remain scarce. We find uniform I_CO for a given A_V, with the
results bracketed by previous studies of the Pipe and Perseus clouds. Our
measured I_CO-A_V relation broadly agrees with the standard Galactic CO-to-H2
conversion factor, the relation found for the Magellanic clouds at coarser
resolution, and numerical simulations by Glover & Clark (2016). This supports
the idea that CO emission primarily depends on shielding, which protects
molecules from dissociating radiation. Evidence for CO saturation at high A_V
and a threshold for CO emission at low A_V varies remains uncertain due to
insufficient resolution and ambiguities in background subtraction. Resolution
of order 0.1 pc may be required to measure these features. We use this I_CO-AV
relation to predict how the CO-to-H2 conversion factor (X_CO) would change if
the Solar Neighborhood clouds had different dust-to-gas ratio (metallicity).
The calculations highlight the need for improved observations of the CO
emission threshold and HI shielding layer depth. They are also sensitive to the
shape of the column density distribution. Because local clouds collectively
show a self-similar distribution, we predict a shallow metallicity dependence
for X_CO down to a few tenths of solar metallicity. However, our calculations
also imply dramatic variations in cloud-to-cloud X_CO at subsolar metallicity.
