Probing the Cold Deep Depths of the California Molecular Cloud: The Icy
  Relationship between CO and Dust
  We study the relationship between molecular gas and dust in the California
Molecular Cloud over an unprecedented dynamic range of cloud depth (Av = 3 - 60
magnitudes). 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 1e5 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 ( Tdust<20K) 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 ( Tdust>25 K ) material
that is spatially coincident with an HII region surrounding the star LKHa101.
We derive 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.
