Abstract
Evidence relating to the value ofR=A V/EB-V appropriate to discrete dark clouds in the interstellar medium is discussed. The polarimetric method of evaluatingR recently proposed by Serkowski, Mathewson and Ford gives results consistent with extinction curve determinations. A value ofR∼4.4 appears to apply to the most reddened stars in the ϱ Oph complex, suggesting the existence of an upper size limit for the grains. If the grains grow by accretion of the heavier elements from the surrounding gas, as suggested by Carrasco, Strom and Strom, then the availability of material sets a limitation on mantle growth consistent with the observed increase inR.
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Whittet, D.C.B., Van Breda, I.G. The ratio of total to selective absorption in dark clouds. Astrophys Space Sci 38, L3–L8 (1975). https://doi.org/10.1007/BF00646110
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DOI: https://doi.org/10.1007/BF00646110