Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981apj...244..483m&link_type=abstract
Astrophysical Journal, Part 1, vol. 244, Mar. 1, 1981, p. 483-492.
Astronomy and Astrophysics
Astronomy
90
Cosmic Dust, Infrared Astronomy, Interstellar Extinction, Particle Size Distribution, Colorimetry, Graphite, Hydrogen, Silicates
Scientific paper
The effects of changing the upper and lower size limits of a distribution of bare graphite and silicate particles with n(a)αa-q is investigated. Mathis, Rumpl, and Nordsieck showed that the normal extinction is matched very well by having the small-size cutoff, a-, ≍ 0.005 or 0.01 μm, and the large size, a+, about 0.25 μm, and q = 3.5 for both substances. We consider the progressively peculiar extinctions exhibited by the well-observed stars, σ Sco, ρ Oph, and θ1 Ori C, with values of RV[≡ AV/E(B-V)] of 3.4, 4.4, and 5.5 compared to the normal 3.1. Two (σ Sco, ρ Oph) are in a neutral dense cloud; θ1 Ori C is in the Orion Nebula. We find that σ Sco has a normal graphite distribution but has had its small silicate particles removed, so that a- (sil) ≍ 0.04 μm if q = 3.5, or q(sil) = 2.6 if the size limits are fixed. However, the upper size limit on silicates remains normal. In ρ Oph, the graphite is still normal, but both a- (sil) and a+ (sil) are increased, to about 0.04 μm and 0.4 or 0.5 μm, respectively, if q = 3.5, or q(sil) ≍ 1.3 if the size limits are fixed. In θ1 Ori, the small limit on graphite has increased to about 0.04 μm, or q(gra) ≍ 3, while the silicates are about like those in ρ Oph. The calculated λ2175 bump is broader than the observed, but normal foreground extinction probably contributes appreciably to the observed bump. The absolute amount of extinction per H atom for ρ Oph is not explained. The column density of H is so large that systematic effects might be present. Very large graphite particles (a > 3 μm) are required to "hide" the graphite without overly affecting the visual extinction, but a normal (small) graphite size distribution is required by the λ2175 bump. We feel that it is unlikely that such a bimodal distribution exists.
The infrared color ratio (I - L)/(B - V) is also calculated and found to be in satisfactory agreement with observations for both normal extinction and for that in the ρ Oph cloud. There are no free parameters in the size distribution to fit the infrared observations, but there is considerable uncertainty and rapid variation in the indices of refraction. However, the same indices which give good agreement with the normal interstellar medium when used with a normal size distribution give agreement for the ρ Oph cloud when used with the size distribution needed to fit the ultraviolet there. This comparison is more free from uncertainties in the refractive indices than is either calculation alone. The ratio AV/τ (9.7 μm) is calculated to be 14, in agreement with observation.
The paper concludes with a summary of properties of interstellar dust which the graphite-silicate mixture fits.
Mathis John S.
Wallenhorst S. G.
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