CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor   and Regional Variations
  We use the COMPLETE data to derive new calibrations of the X-factor and the 13CO abundance within Perseus. We divide Perseus into six sub-regions. The standard X factor, X=N(H2)/W(12CO), is derived both for the whole Perseus Complex and for each of the six sub-regions with values consistent with previous estimates. The X factor is heavily affected by the saturation of the emission above AV~4 mag, and variations are found between regions. We derive linear fits to relate W(12CO) and AV using only points below 4 mag of extinction, this yields a better estimation of the AV than the X-factor. We derive linear relations of W(13CO), N(13CO) and W(C18O) with AV . The extinction threshold above which 13CO(1-0) and C18O(1-0) are detected is about 1 mag larger than previous estimates. 12CO(1-0) and 13CO(1-0) lines saturate above 4 and 5 mag, respectively, whereas C18O(1-0) never saturates (up to 10 mag). Approximately 60% of the positions with 12CO emission have sub-thermally excited lines, and almost all positions have 12CO excitation temperatures below the dust temperature. Using the Meudon PDR code we find that 12CO and 13CO emission can be explained by uniform slab models with densities ranging between about 10^3 and 10^4 cm-3. Local variations in the volume density and non-thermal motions (linked to different star formation activity) can explain the observations. Higher densities are needed to reproduce CO data toward active star forming sites, where the larger internal motions driven by the young protostars allow more photons from the embedded high density cores to escape the cloud. In the most quiescent region, the 12CO and 13CO emission appears to arise from an almost uniform thin layer of molecular material at densities around 10^4 cm-3. 
Jaime E. Pineda, Paola Caselli, and Alyssa A. Goodman