DUST EXTINCTION AND MOLECULAR GAS IN THE DARK CLOUD IC 5146

In this paper we describe a powerful method for mapping the distributi on of dust through a molecular cloud using data obtained in large-scal e, multiwavelength, infrared imaging surveys. This method combines dir ect measurements of near-infrared color excess and certain techniques of star counting to derive mean extinctions and map the dust column de nsity distribution through a cloud at higher angular resolutions and g reater optical depths than those achieved previously by optical star c ounting. We report the initial results of the application of this meth od to a dark cloud complex near the cluster IC 5146, where we have per formed coordinated, near-infrared, JHK imaging and (CO)-C-13, (CO)-O-1 8, and CS millimeter-wave, molecular-line surveys of a large portion o f the complex. More than 4000 stars were detected in our JHK survey of the cloud. Of these, all but about a dozen appear to be field stars n ot associated with the cloud. Star count maps at J band show a strikin g and detailed anticorrelation between the surface density of J-band s ources and CO and CS molecular-line emission. We used the (H-K) colors and positions of nearly 1300 sources to directly measure and map the extinction and thus trace the dust column density through the cloud at an effective angular resolution of 1.5'. We report an interesting cor relation between the measured dispersion in our extinction determinati ons and the extinction. Modeling this relation indicates that effects of small-scale cloud structure dominate the uncertainties in our measu rements. Moreover, we demonstrate that such observations can be used t o place constraints on the nature of the spatial distribution of extin ction on scales smaller than our resolution. In particular, we show th at models in which the dust is distributed uniformly or in discrete hi gh-extinction clumps on scales smaller than 1.5' are inconsistent with the observations. We have derived extinctions at the same positions a nd at the same angular resolution (1.7') as our molecular-line observa tions. This enabled a direct comparison of (CO)-C-13, (CO)-O-18, and C S integrated intensities and column densities with A(V) for more than 500 positions in the cloud, corresponding to a range in A(V) between 0 -32 mag of extinction. We found the integrated intensities of (CO)-C-1 3, (CO)-O-18, and CS to be roughly linearly correlated with extinction over different ranges of extinction. However, for all three molecules we find the scatter in the observed relations to be larger than can b e accounted for by instrumental error, suggesting that there are large intrinsic variations in the abundances or excitation of the molecules through the cloud. Mean abundances for all the molecules relative to hydrogen were directly derived from the data. The ratio of (CO)-C-13 t o (CO)-O-18 abundances was found to be significantly higher than the t errestrial ratio in regions where extinction is less than 10 mag. In t he same region, the dispersion in the abundance ratio is also found to be very large, suggesting that the abundances of one or both molecule s are very unstable even at relatively large cloud optical depths. Bey ond 10 mag of extinction the abundances of both species appear very st able with their ratio close to the terrestrial value.