Dust Extinction and Molecular Cloud Structure: L977

Abstract

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 and coworkers. 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 M_L977 = (660 ± 30)(D/500 pc)² 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. To further investigate the structure of the cloud we construct the frequency distribution of the 1628 individual extinction measurements in the L977 cloud. The shape of the distribution is similar to that of the IC 5146 cloud. Monte Carlo modeling of this distribution suggests that in the range 2 mag < A_V < 40 mag (or roughly 1 pc < r < 0.1 pc) the material inside L977 is characterized by a density profile ρ(r) ∝ r^-2. Direct measurement of the radial profile of a portion of the cloud confirms this result. At the lower Galactic latitude of L977, we find both the mean and dispersion of the infrared colors of field stars to be larger than those observed toward IC 5146. This produces an increase of about a factor of 2 in the minimum or threshold value of extinction that can be reliably measured toward L977 with this technique. Nevertheless, the accuracy in an extinction map pixel is not significantly different toward L977 because of the increased number of field stars at this latitude. We also find an increase in the number of detected giant stars at the lower Galactic latitude of the survey by almost a factor of 2. Most of these excess stars suffer extraneous extinction and are probably red giants seen along the disk of the Milky Way up to distances of ~15 kpc and reddened by unrelated background molecular clouds along this direction of the Galaxy. We discuss a possible application of this observable to Galactic structure studies on the plane of the Galaxy.