A Study of General and Selective Absorption in Four Small Dark Nebulae

Abstract

Four small dark clouds of Barnard's list were studied by star counts according to photographic and photovisual magnitudes in the obscured regions and the surrounding star fields. Magnitude sequences were set up by comparisons with selected areas and extended by long-exposure objective-grating plates. Attempts were made to find the star density and general interstellar absorption in the comparison regions surrounding the dark clouds, but no unique solutions could be found. The total absorptions by each cloud were determined directly from the Wolf diagrams. All four clouds showed a selective absorption; and the mean ratio of photographic to photovisual absorption is 1.31. The distances of the clouds could not be determined accurately, only an upper limit of the distance (1000 parsecs) could be ascertained. The angular diameters then indicate that these objects may be comparable in size with the Orion nebula and that the absorption coefficients in the clouds themselves are at least of the order of 0.2 per parsec