2021ApJ...908...76L

We study the relationship between molecular gas and dust in the California Molecular Cloud over an unprecedented dynamic range of cloud depth (A_V = 3-60 mag). We compare deep Herschel-based measurements of dust extinction with observations of the ¹²CO, ¹³CO, and C¹⁸O J = 2 - 1 lines on sub-parsec scales across the cloud. We directly measure the ratio of CO integrated intensity to dust extinction to derive the CO X-factor at over 10⁵ independent locations in the cloud. Confirming an earlier study, we find that no single ¹²CO X-factor can characterize the molecular gas in the cold (T_dust <= 20) regions of the cloud that account for most of its mass. We are able to derive a single-valued X-factor for all three CO isotopologues in the warm (T_dust > 25 K) material that is spatially coincident with an H II region surrounding the star LkHα 101. We derive the LTE CO column densities for ¹³CO and C¹⁸O since we find both lines are relatively optically thin. In the warm cloud material, CO is completely in the gas phase and we are able to recover the total ¹³CO and C¹⁸O abundances. Using CO abundances and deep Herschel observations, we measure lower bounds to the freeze-out of CO onto dust across the whole cloud, finding some regions having CO depleted by a factor of >20. We construct the first maps of depletion that span the extent of a giant molecular cloud. Using these maps we identify 75 depletion-defined cores and discuss their physical nature.