5.1. Nano Carbon Grains: Polycyclic Aromatic Hydrocarbon Dust
As the most abundant interstellar nanoparticle species, nano carbon grains (mainly PAHs), containing ~ 15% of the interstellar C abundance (Li & Draine 2001b), reveal their presence in the ISM by emitting a prominent set of "UIR" bands at 3.3, 6.2, 7.7, 8.6, and 11.3 µm (see Section 1 and Section 2). Modern research on astrophysical PAHs started in the mid 1980s with the seminal studies of Léger & Puget (1984) and Allamandola et al. (1985) who by the first time explicitly proposed PAH molecules as the "UIR" band carrier. The PAH model is now gaining increasing acceptance because of (1) the close resemblance of the "UIR" spectra (frequencies and relative intensities) to the vibrational spectra of PAH molecules (see Allamandola & Hudgins 2003 for a recent review); (2) the ability of a PAH molecule to emit efficiently in the "UIR" wavelength range following single photon heating (Léger & Puget 1984; Allamandola et al. 1985, 1989; Draine & Li 2001; also see Section 3); and (3) the success of the PAH model in quantitatively reproducing the observed mid-IR spectra of the Milky Way diffuse ISM (Li & Draine 2001b), the quiescent molecular cloud SMC B1#1 in the Small Magellanic Cloud (Li & Draine 2002c), and the "UIR" band ratios for a wide range of environments ranging from reflection nebulae, HII regions, photodissociation regions, molecular clouds in the Milky Galaxy to normal galaxies, starburst galaxies, and a Seyfert 2 galaxy (Draine & Li 2001).
Recently, the PAH model further gained its strength from a close fit to the observed "UIR" bands of vdB 133, a UV-poor reflection nebula, which was considered as one of the major challenges to identification of the "UIR" bands with PAH molecules (see Uchida, Sellgren, & Werner 1998), since small, neutral PAH molecules have little or no absorption at visible wavelengths and therefore require UV photons for excitation. Li & Draine (2002b) have shown that the "astronomical" PAH model, incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases (see Appendix A2 of Li & Draine 2002b and references therein), can closely reproduce the observed IR emission bands of this reflection nebula, and is also able to account for the observed dependence of the 12 µm IRAS emission on the effective temperature of the illuminating star (Sellgren, Luan, & Werner 1990).
Henning & Schnaiter (1999) suggested that nano-sized hydrogenated carbon grains may be responsible for the 2175 Å extinction hump as well as the 3.4 µm C-H absorption feature. But this seems to be in conflict with the detection of the 3.4 µm feature in regions where the 2175 Å hump is not seen, e.g., along the sightline toward HD204827 (Valencic et al. 2003) and the Taurus cloud (Whittet et al. 2003).
Kroto et al. (1985) first proposed that C60 could be present in the ISM with a considerable quantity. This molecule and its related species have later been proposed as the carriers of the 2175 Å extinction hump, the diffuse interstellar bands (DIBs), the "UIR" bands, and the ERE (see Webster 1993 and references therein). Foing & Ehrenfreund (1994) attributed the two DIBs at 9577 Å and 9632 Å to C60+. However, attempts to search for these molecules in the UV and IR were unsuccessful (Snow & Seab 1989; Somerville & Bellis 1989; Moutou et al. 1999; Herbig 2000). These molecules were estimated to consume at most < 0.7 ppm carbon (Moutou et al. 1999). Therefore, C60 is at most a minor component of the interstellar dust family.