2008ApJ...679..481P

We use data gathered by the COMPLETE survey of star-forming regions to find new calibrations of the ``X-factor'' and ¹³CO 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(H₂)/W(¹²CO), 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(¹²CO) 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(¹³CO), N(¹³CO), and W(C¹⁸O) with A_V are derived. The extinction thresholds above which ¹³CO(1-0) and C¹⁸O(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 ¹²CO and ¹³CO lines saturate above 4 and 5 mag, respectively, whereas C¹⁸O(1-0) never saturates in the whole A_V range probed by our study (up to 10 mag). Approximately 60% of the positions with ¹²CO(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 ¹²CO(1-0) and ¹³CO(1-0) emission with densities ranging between 10³ and 10⁴/cm3. 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 ¹²CO and ¹³CO emission appears to arise from an almost uniform thin layer of molecular material at densities around 10⁴/cm3, and in this region the integrated intensities of the two CO isotopologues are the lowest in the whole complex.