Absolute extinction and the influence of environment: Dark cloud sight lines toward VCT 10, 30, and walker 67

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We present absolute near-infrared, optical, and ultraviolet extinction data, A(λ)/A(V), for three sight lines passing through dense, dark cloud environments with large values of total-to-selective extinction, Rv[=A(V)/E(B-V)]. The absolute extinction curves for VCT 10(Rv, = 4.3), VCT 30 (Rv = 4.7), and Walker 67 (Rv = 5.4) exhibit no significant deviations from the Rv dependent extinction law of Cardelli et al. [ApJ, 345, 245 (1989)] from the near IR through the near UV (3.4 μm > λ > 0.25 μm). However, for λ≤0.25 μm, all three sight lines exhibit similar and relatively large deviations from the Rv, dependent extinction law. The contrast of large deviations from the Rv dependent extinction law for λ≤0.25 μm to strong adherence for λ > 0.25 μm suggests that (1) the grains that give rise to the optical/near-IR extinction are processed in an efficient and systematic way and (2) the bump and far-UV extinction components produce little or no contribution at optical wavelengths and are modified by processes independent of those responsible for the Rv dependence. In addition, we have fitted the UV extinction data with the three component fitting techniques of Fitzpatrick and Massa and find the fitted "0.2175 μm (2175 Å)" bump toward VCT 10 to be very broad and shifted to a central wavelength of λ0≈0.2120 μm. The UV extinction characteristics observed toward VCT 10, VCT 30, and to a smaller degree Walker 67, are similar in nature to other dark cloud sight lines which sample a large range of Rv. These characteristics are in sharp contrast to sight lines associated with bright nebulosity which generally exhibit only minor deviations from the Cardelli et al. Rv dependent average in the UV and some of the narrowest "2175 Å" bumps yet observed. The general form of the UV extinction deviations of the dark cloud sight lines from the Rv dependent extinction law along with their sharp contrast to sight lines in regions of bright nebulosity seems consistent with the affects of coatings on both the small bump and far-UV grains. The existence of these coatings appear to be a sensitive function of the UV radiation field.

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Astronomical Journal

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