The Relationship Between the Dust and Gas-Phase CO Across the California Molecular Cloud

Abstract

A deep, wide-field, near-infrared imaging survey was used to construct an extinction map of the southeastern part of the California Molecular Cloud (CMC) with $\sim$ 0.5 arc min resolution. The same region was also surveyed in the ${}^{12}$CO(2-1), ${}^{13}$CO(2-1), C${}^{18}$O(2-1) emission lines at the same angular resolution. Strong spatial variations in the abundances of ${}^{13}$CO and C${}^{18}$O were found to be correlated with variations in gas temperature, consistent with temperature dependent CO depletion/desorption on dust grains. The ${}^{13}$CO to C${}^{18}$O abundance ratio was found to increase with decreasing extinction, suggesting selective photodissociation of C${}^{18}$O by the ambient UV radiation field. The cloud averaged X-factor is found to be $<$X${}_{\mathrm{CO}}$$>$ $=$ 2.53 $\times$ 10${}^{20}$ ${\mathrm{cm}}^{-2}({\mathrm{K}\mathrm{km}\mathrm{s}}^{-1}{)}^{-1}$, somewhat higher than the Milky Way average. On sub-parsec scales we find no single empirical value of the X-factor that can characterize the molecular gas in cold (T${}_{\mathrm{k}}$ $\lesssim$ 15 K) regions, with X${}_{\mathrm{CO}}$ $\propto$ A${}_{\mathrm{V}}$${}^{0.74}$ for A${}_{\mathrm{V}}$ $\gtrsim$ 3 magnitudes. However in regions containing relatively hot (T${}_{\mathrm{ex}}$ $\gtrsim$ 25 K) gas we find a clear correlation between W(${}^{12}$CO) and A${}_{\mathrm{V}}$ over a large (3 $\lesssim$ A${}_{\mathrm{V}}$ $\lesssim$ 25 mag) extinction range. This suggests a constant X${}_{\mathrm{CO}}$ $=$ 1.5 $\times$ 10${}^{20}$ ${\mathrm{cm}}^{-2}({\mathrm{K}\mathrm{km}\mathrm{s}}^{-1}{)}^{-1}$ for the hot gas, a lower value than either the average for the CMC or Milky Way. We find a correlation between X${}_{\mathrm{CO}}$ and T${}_{\mathrm{ex}}$ with X${}_{\mathrm{CO}}$ $\propto$ T${}_{\mathrm{ex}}$${}^{-0.7}$ suggesting that the global X-factor of a cloud may depend on the relative amounts of hot gas within it.

21 pages, 17 figures, accepted by ApJ