1 Struve & Elvey (1938) appear to be first to show that the interstellar gas is mostly hydrogen. Back.
2 Let interstellar grains be approximated by a single size of a (spherical radius) with a column density of Nd. The gas-to-dust mass ratio is
where µH and µHe are respectively the atomic weight of H and He; [He / H]ISM is the interstellar He abundance (relative to H), which is taken to be that of the solar value, ~ 0.1; d is the mass density of interstellar dust. The hydrogen (of all forms)-to-dust column density NH / Nd can be derived from
where RV is the total-to-selective extinction ratio, Qext(V) is the dust extinction efficiency at V-band ( = 5500 Å). Therefore, the gas-to-dust ratio can be readily estimated from
If we take canonical numbers of RV 3.1, Qext(V) 1.5, a = 0.1 µm, d 2.5 g cm-3, mgas / mdust would be around 210. Back.
3 In our Milky Way Galaxy, interstellar matter (gas and dust; 7 ± 3 × 109 M), contributes roughly ~ 20% of the total stellar mass (4 ± 2 × 1010 M). Therefore, the mass fraction of interstellar dust is just ~ 0.1% in our Galaxy (~ 1 ± 0.2 × 1011 M within 10 kpc, Kennicutt 2001)! Back.
4 In this method, the wavelength dependence of interstellar extinction is obtained by comparing the spectra of two stars of the same spectral type, one of which is reddened and the other unreddened. Back.
5 Falco et al. (1999) found RV 1.5 for an elliptical lensing galaxy at zl 0.96, and RV 7.2 for a spiral lensing galaxy at zl 0.68. Wang et al. (2004) found the extinction curves for two intervening quasar absorption systems at z 1.5 to have RV 0.7, 1.9. Back.
6 For example, there is at least one line of sight (Sk 143 = AvZ 456) with an extinction curve with a strong 2175Å hump detected (Lequeux et al. 1982; Prévot et al. 1984; Bouchet et al. 1985; Thompson et al. 1988; Gordon & Clayton 1998). This sightline passes through the SMC wing, a region with much weaker star formation (Gordon & Clayton 1998). The sight lines which show no 2175Å hump all pass through the SMC Bar regions of active star formation (Prévot et al. 1984; Gordon & Clayton 1998). Back.
7 Strong regional variations in extinction properties have also been found in the LMC (Clayton & Martin 1985; Fitzpatrick 1985,1986; Misselt, Clayton, & Gordon 1999): the sightlines toward the stars inside or near the supergiant shell, LMC 2, which lies on the southeast side of the 30 Dor star-forming region, have very weak 2175 Å hump (Misselt et al. 1999). Back.
8 Kemper, Vriend & Tielens (2004) found that crystalline fraction of the interstellar silicates along the sightline towards the Galactic Center is ~ 0.2%. Back.
9 The amount of Si (relative to H) required to deplete in dust to account for the observed 9.7 µm feature strength is
where silabs(9.7 µm) is the silicate mass absorption coefficient at = 9.7 µm; µsil is the silicate molecular weight. With silabs (9.7 µm) 2850 cm2 g-1 and µsil 172 µH for amorphous olivine MgFeSiO4, the local diffuse ISM ( 9.7 µm / AV 1/18.5, AV / NH 5.3 × 10-22 mag cm2) requires [Si / H] 35 ppm. Back.
10 Hough et al. (1996) reported the detection of a weak 3.47 µm polarization feature in the Becklin-Neugebauer object in the OMC-1 Orion dense molecular cloud, attributed to carbonaceous materials with diamond-like structure. See Li (2004a) and Jones & d'Hendecourt (2004) for a detailed discussion on interstellar diamond. Back.
11 Other C-H out-of-plane bending modes at 11.9, 12.7 and 13.6 µm have also been detected. The wavelengths of the C-H out-of-plane bending modes depend on the number of neighboring H atoms: 11.3 µm for solo-CH (no adjacent H atom), 11.9 µm for duet-CH (2 adjacent H atoms), 12.7 µm for trio-CH (3 adjacent H atoms), and 13.6 µm for quartet-CH (4 adjacent H atoms). Back.
12 Li & Draine (2002b) have modeled the excitation of PAH molecules in UV-poor regions. It was shown that the astronomical PAH model provides a satisfactory fit to the UIR spectrum of vdB133, a reflection nebulae with the lowest ratio of UV to total radiation among reflection nebulae with detected UIR band emission (Uchida, Sellgren, & Werner 1998). Back.
13 Let [X / H] be the interstellar abundance of X relative to H (we assume interstellar abundances to be those of the solar values: [C / H] 391 parts per million [ppm], [O / H] 501 ppm, [Mg / H] 34.5 ppm, [Fe / H] 34.4 ppm, and [Si / H] 28.1 ppm [Sofia 2004]); [X / H]gas be the amount of X in gas phase ([C / H]gas 130 ppm, [O / H]gas 375 ppm; Fe, Mg and Si are highly depleted in dust: [Fe / H]gas 1 ppm, [Mg / H]gas 2 ppm, and [Si / H]gas 2 ppm [Sofia 2004]); [X / H]dust be the amount of X contained in dust ([C / H]dust = [C / H] - [C / H]gas 261 ppm, [O / H]dust 126 ppm, [Mg / H]dust 32.5 ppm, [Fe / H]dust 27.1 ppm, [Si / H]dust 32.4 ppm). Assuming H/C = 0.5 for interstellar carbon dust, the gas-to-dust mass ratio is
where the summation is over Si, Mg, Fe, C, O and H, and µX is the atomic weight of X in unit of µH 1.66 × 10-24 g. Back.
14 Assuming all Si, Mg, and Fe elements of solar abundances are condensed in silicate dust of a stoichiometric composition MgFeSiO4 with a characteristic size a 0.1 µm, the contribution of the silicate dust to the optical extinction is
where Nsil is the column density of silicate dust, sil 3.5 g cm3 is the mass density of silicate material, and Qext(V) is the visual extinction efficiency of submicron-sized silicate dust which is taken to be Qext(V) 1.5. Back.
15 The idea of transient heating of very small grains was first introduced by Greenberg (1968). This process was not observed until many years later when the detection of the near-IR emission of reflection nebulae (Sellgren, Werner, & Dinerstein 1983) and detection by IRAS of 12 and 25 µm Galactic emission (Boulanger & Pérault 1988) were reported. Back.
16 This is because for large grains individual photon absorption events occur relatively frequently and the grain energy content is large enough that the temperature increases induced by individual photon absorptions are relatively small. Back.
17 Li & Draine (2002c) placed an upper limit of ~ 0.4% of the SMC C abundance on the amount of PAHs in the SMC Bar. But we note that the PAH emission features have been seen in SMC B1#1, a quiescent molecular cloud (Reach et al. 2000). For this region, Li & Draine (2002c) estimated that ~ 3% of the SMC C abundance to be incorporated into PAHs. Back.
18 Siebenmorgen et al. (1999) argued that the dust embedded in UV-attenuated clouds within the optical disk of typical inactive spiral galaxies cannot become colder than ~ 6 K . Back.