Dust Extinction and Molecular Cloud Structure : L 977

We report results of a near–infrared imaging survey of L977, a dark cloud in Cygnus seen in projection against the plane of the Milky Way. We use measurements of the near–infrared color excess and positions of the 1628 brightest stars in our survey to measure directly dust extinction through the cloud following the method described by Lada et al. (1994). We spatially convolve the individual extinction measurements with a square filter 90 in size to construct a large–scale map of extinction in the cloud. We integrate over this map to derive a total mass of ML977 = (660 ± 30) (D/500pc) M and, via a comparison of source counts with predictions of a galactic model, estimate a distance to L977 of 500 ± 100 pc. We find a correlation between the measured dispersion in our extinction determinations and the extinction which is very similar to that found for the dark cloud IC 5146 in a previous study. We interpret this as evidence for the presence of structure on scales smaller than the 90 resolution of our extinction map. Visiting Astronomer, Kitt National Observatory, part of the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation. Also Physics Department, University of Lisbon, Lisbon Portugal