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 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, <IMG SRC="eq-00001.gif" ALT="X\equiv N(\mathrm{H}\,_{2})/W(^{12}\mathrm{CO}\,) "/> 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 <IMG SRC="eq-00002.gif" ALT="A_{V}\sim 4"/> AV [?] 4 mag, and variations are also found between regions. Linear fits to relate <IMG SRC="eq-00003.gif" ALT="W(^{12}\mathrm{CO}\,) "/> W(12CO) and <IMG SRC="eq-00004.gif" ALT="A_{V}"/> AV using only points below 4 mag of extinction yield a better estimate of the <IMG SRC="eq-00005.gif" ALT="A_{V}"/> AV than the X-factor. Linear relations of <IMG SRC="eq-00006.gif" ALT="W(^{13}\mathrm{CO}\,) ,\ N(^{13}\mathrm{CO}\,) "/> W(13CO) , N(13CO) , and <IMG SRC="eq-00007.gif" ALT="W(\mathrm{C}\,^{18}\mathrm{O}\,) "/> W(C18O) with <IMG SRC="eq-00008.gif" ALT="A_{V}"/> AV 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 <IMG SRC="eq-00009.gif" ALT="A_{V}"/> AV 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 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 12CO and 13CO emission appears to arise from an almost uniform thin layer of molecular material at densities around 104 cm[?]3, and in this region the integrated intensities of the two CO isotopologues are the lowest in the whole complex.