CO Isotopologues in the Perseus Molecular Cloud Complex: the X ‐factor and Regional Variations

We use data gathered by the COMPLETE survey of star-forming regions to find new calibrations of the " X -factor" and 13 CO abundance within the Perseus molecular cloud. We divide Perseus into six subregions, using groupings in a dust temperature vs. LSR velocity plot. The standard X -factor, X ≡ N (H 2 )/ W ( 12 CO) , is derived both for the whole Perseus complex and for each of the six subregions with values consistent with previous estimates. However, the X -factor is heavily affected by the saturation of the emission above A V ∼ 4 mag, and variations are also found between regions. Linear fits to relate W ( 12 CO) and A V using only points below 4 mag of extinction yield a better estimate of the A V than the X -factor. Linear relations of W ( 13 CO) , N ( 13 CO) , and W (C 18 O) with A V are derived. The extinction thresholds above which 13 CO(1-0) and C 18 O(1-0) are detected are about 1 mag larger than previous estimates, so that a more efficient shielding is needed for the formation of CO than previously thought. The 12 CO and 13 CO lines saturate above 4 and 5 mag, respectively, whereas C 18 O(1-0) never saturates in the whole A V range probed by our study (up to 10 mag). Approximately 60% of the positions with 12 CO(1-0) emission have subthermally excited lines, and almost all positions have excitation temperatures below the dust temperature. PDR models, using the Meudon code, can explain the 12 CO(1-0) and 13 CO(1-0) emission with densities ranging between 10 3 and 10 4 cm −3 . In general, local variations in the volume density and nonthermal motions (linked to different star formation activity) can explain the observations. Higher densities are needed to reproduce CO data toward active star-forming sites, such as NGC 1333, 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, B5, the 12 CO and 13 CO emission appears to arise from an almost uniform thin layer of molecular material at densities around 10 4 cm −3 , and in this region the integrated intensities of the two CO isotopologues are the lowest in the whole complex.