CO isotopologues in the Perseus Molecular Cloud Complex: the X-factor and regional variations

The COMPLETE Survey of Star-Forming regions offers an unusually comprehensive and diverse set of measurements of the distribution and temperature of dust and gas in molecular clouds, and in this paper we use those data to find new calibrations of the “X-factor” and the 13CO abundance within Perseus. To carry out our analysis, we: 1) apply the NICER (Near-Infrared Color Excess Method Revisited) algorithm to 2MASS data to measure dust extinction; 2) use dust temperatures derived from re-processed IRAS data; and 3) make use of the 12CO and 13CO (1-0) transition maps gathered by COMPLETE to measure gas distribution and temperature. Here, we divide Perseus into six sub-regions, using groupings in a plot of dust temperature as a function of LSR velocity. 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. However, 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 linear fit yields a better estimation of the AV than the X factor. We derive linear