Annu. Rev. Astron. Astrophys. 1990. 28: 37-70 Copyright © 1990 by Annual Reviews. All rights reserved

2.1.3 NIR CONTINUOUS EXTINCTION Figure 2 shows great differences in extinction laws among various lines of sight throughout the optical and UV portions of the spectrum. We might expect a corresponding variation at somewhat longer wavelengths, but apparently there is little if any.

Most NIR photometry is at the Johnson filters J (1.25 µm), H (1.65 µm), and K (2.2 µm). Whittet (166) tabulates E (J - H) / E (H - K) as determined by several studies, in diffuse dust and outer-cloud dust alike, and finds them to be consistent with the value E (J - H) / E (H - K) = 1.61 ± 0.04. Koornneef (87) considered a large body of data and suggests an extinction law which has a value of 1.70 for this ratio. Jones and Hyland (79) also concluded that NIR extinction is the same for both diffuse dust and outer-cloud dust, although they found E (J - H) / E(H - K) = 2.09 ± 0.10. The constancy of their ratio between lines of sight through diffuse dust and outer-cloud dust is more significant than the difference in the numerical value itself, which depends upon reduction to a standard photometric system.

The NIR extinction law is well fitted by the form A() / A(J) = (/1.25 µm)-. Recent values of are 1.70 ± 0.08 (166); 1.61 (136); 1.75 (37), and ~ 1.8 (108). The value 1.70 seems a reasonable compromise for both diffuse dust and outer-cloud dust and implies that E (J - H) / E (H - K) ~ 1.6.

The constancy of the NIR extinction law implies that the size distributions of the largest particles are almost the same in all directions. This conclusion was also reached (111) on the basis of the interstellar polarization law, which involves only the largest particles.

2.1.4. FAR-UV EXTINCTION Martin and Rouleau (107) have extended the Draine and Lee (42, hereinafter DL84) opacities through the ionizing-UV range to X-ray energies (3.5 keV), assuming that grains are composed of silicates and graphite. The opacity rises to a maximum of 2.8 x 10²¹ cm² (H atom)^-1 at 730 Å (= 17 eV) and declines to 7.4 x 10^-22 cm² (H atom)^-1 at 124 Å (= 100 eV). At energies > 300 eV, the absorption law of the dust is approximately the same as if all of its atoms were neutral in the gas phase. At lower energies, especially just above the thresholds of the abundant elements like carbon, the large grains are opaque and the effective cross section per H atom is reduced. At 24 eV, the reduction amounts to a factor of four. The major effects of the dust as regards high-energy radiation are (a) to keep its constituents absorbing as neutral atoms, rather than possibly being ionized; and (b) to scatter the radiation, with a cross-section about equal to the absorption cross-section. This scattering can be observed as an X-ray halo around point sources (117). Dust does not affect the ionization equilibrium of H II regions very significantly (112) because its absorption, peaking at 17 eV, resembles hydrogen absorption too closely.

2.1.5. EXTRAGALACTIC EXTINCTION Reasonably reliable measurements for the extinction laws and dust/gas ratios exist only for the Magellanic Clouds. In the Large Magellanic Cloud (LMC), it is found that R_V 3.2 ± 0.2, virtually Galactic (24, 86, 119). For the UV, the stars near the giant H II region 30 Doradus have weak bumps and extinctions rising steeply at the shortest IUE wavelengths, a behavior unfortunately known as ``the LMC extinction law''. However, the stars well away from 30 Doradus (> 500 pc projected distance), spread throughout the galaxy, have approximately galactic extinction laws (24, 53). The N(H) / E (B - V) is 2 x 10²² atoms mag^-1 (86, 53), about four times the Galactic value (9) and about proportional to the gaseous carbon abundance in the LMC.

In the Small Magellanic Cloud (SMC), there are almost no suitably reddened stars. In general there seems to be a low value of R_V, almost no bump, and a very steep far-UV rise (11, 121), as might be expected from a small R_V. One star, though, shows an extinction law similar to the Galaxy (96). The N(H) / E (B - V) is 4.5 x 10²² atoms mag^-1, about 10 times galactic and consistent with the gaseous C abundance in the SMC (104).