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We measure the parsec-scale relationship between integrated CO intensity(I_CO) and visual extinction (A_V) in 24 local molecular clouds using maps ofCO emission and dust optical depth from Planck. This relationship informs ourunderstanding of CO emission across environments, but clean Milky Waymeasurements remain scarce. We find uniform I_CO for a given A_V, with theresults bracketed by previous studies of the Pipe and Perseus clouds. Ourmeasured I_CO-A_V relation broadly agrees with the standard Galactic CO-to-H2conversion factor, the relation found for the Magellanic clouds at coarserresolution, and numerical simulations by Glover & Clark (2016). This supportsthe idea that CO emission primarily depends on shielding, which protectsmolecules from dissociating radiation. Evidence for CO saturation at high A_Vand a threshold for CO emission at low A_V varies remains uncertain due toinsufficient resolution and ambiguities in background subtraction. Resolutionof order 0.1 pc may be required to measure these features. We use this I_CO-AVrelation to predict how the CO-to-H2 conversion factor (X_CO) would change ifthe Solar Neighborhood clouds had different dust-to-gas ratio (metallicity).The calculations highlight the need for improved observations of the COemission threshold and HI shielding layer depth. They are also sensitive to theshape of the column density distribution. Because local clouds collectivelyshow a self-similar distribution, we predict a shallow metallicity dependencefor X_CO down to a few tenths of solar metallicity. However, our calculationsalso imply dramatic variations in cloud-to-cloud X_CO at subsolar metallicity.

The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular Clouds