Photometric Distances to Small Dark Clouds: CB 24

A new method for determining distances to dark clouds and small Bok globules based on broadband optical photometry has been developed. The method exploits the fact that M dwarf stars do not redden into the colors of other main-sequence stars. Consequently, from analysis of color-color diagrams of V-I versus B-V, spectral types and extinctions of faint M dwarf stars located in front of and behind the molecular clouds can be established. For sufficiently deep observations (B ≈ 20–24 mag), distances to clouds closer than about 1 kpc can be bracketed to a higher degree of precision (10–40 pc) than previously possible. Photometric data for the Bok globule CB 24 are used in this paper to demonstrate this method, yielding a maximum distance of 360 pc.