Abstract
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 -->AV ~ 4 mag, and variations are also found between regions. Linear fits to relate -->W(12CO) and -->AV using only points below 4 mag of extinction yield a better estimate of the -->AV than the X-factor. Linear relations of -->W(13CO) , N(13CO) , and -->W(C18O) with -->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 -->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.
Highlights
H2 is the most abundant molecule in the interstellar medium, it cannot be used as a tracer of the physical conditions in a molecular cloud
The linear fit performed to positions with AV < 4 mag gives the best estimate for the extinction in the unsaturated regions but only provides a lower limit extinction estimate for the saturated regimes, while the standard X factor provides a poor description of the data in both saturated and unsaturated regimes
Using the FCRAO 12CO, 13CO and C18O data, and a Near Infrared Color Excess Revisited (NICER) extinction map produced by COMPLETE we perform a calibration of the column density estimation using 12CO, 13CO and
Summary
H2 is the most abundant molecule in the interstellar medium (by about four orders of magnitude), it cannot be used as a tracer of the physical conditions in a molecular cloud. This relation enabled them to derive a ratio of 3.0 × 1020 cm−2 K−1 km−1 s for the median mass of the sample (105 M⊙) In this case, the main source of uncertainty is the assumption of virial equilibrium for the molecular clouds. Lombardi et al (2006) studied the Pipe Cloud using an extinction map derived using the Near Infrared Color Excess Revisited (NICER) technique on 2MASS and 12CO (1-0) data They derived an X factor of (2.91 ± 0.05) × 1020 cm−2 K−1 km−1 s, similar to what is found in cloud core regions and dark nebulae (see compilation by Young & Scoville 1982). Bensch (2006) used 12CO and 13CO maps with C I pointing observations of 12 positions in a North-South stripe from the central B5 region to model the emission with a PDR code From this analysis he derives average densities ∼ 3 × 103 − 3 × 104 cm−3.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
