Interstellar dust emission

Again, the emissivity varies as 1/A2 in the far infrared. But various observations of emission from interstellar dust suggest that the wavelength dependence of emissivity is closer to 1/A than to 1/A2 (Seki and Yamamoto, 1980). This may be a consequence of the failure of the conditions underlying the 1/A2 dependence—the particles are crystalline and spherical—to be satisfied. 

From these considerations we conclude that the failure of the emission spectrum of interstellar dust to vary as 1/A2 in the far infrared, which is predicted for small crystalline spheres, may be the result of either noncrystallinity or nonsphericity (or both). Therefore, the infrared emission spectrum may not prove to be as uniquely diagnostic of interstellar grain characteristics as it once was thought to be. 

In the past 10 years a large number of organic molecules have been found in interstellar dust clouds mostly by emission lines in the microwave region of the spectrum (for a summary see Ref. 38). The concentration of these molecules is very low (a few molecules per cm3 at the most) but the total amount in a dust cloud is large. The molecules found include formaldehyde, hydrogen cyanide, acetaldehyde, and cyanoacetylene. These are important prebiotic molecules, and this immediately raises the question of whether the interstellar molecules played a role in the origin of life on the earth. In order for this to have taken place it would have been necessary for the molecules to have been greatly concentrated in... 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

The term unidentified infrared emission is used to refer to the long-known emission features of interstellar dusts in the spectral region from just over 3,000 cm-1 to below 800 cm-1 (Gillett et al. 1973). These features comprise sharp IR bands at 2,920,1,610, and 880 cm-1, as well as a broader envelope near 1,300 cm-1. In addition, a recurrent mode at 3,050 cm-1, a weak mode near 1,450 cm-1, and a shoulder near 1,150 cm-1 are observed. These spectral features can all be attributed to vibrational modes of hydrogenated carbon species, as summarized in Table 2.1. The chemical structure of these species remains the subject of debate. Furthermore, a number of carbon-rich astronomical objects reveal an emission feature in the far-IR at 490 cm-1, of unclear attribution (Kwok et al. 1989). 

The discovery of C60 by Kroto and coworkers (1985) was motivated in part by the interstellar dust problem. C60 would seem to be an ideal candidate, as it is spherical and graphite-like, it forms spontaneously in harsh environments with carbon dust, and is stable in intense radiation fields, a condition analogous to that found in the diffuse ISM (Kroto and Jura 1992). In fact, the observation of two DIBs at 957.7 and 963.2 nm are tentatively considered the first evidence of C60+ in interstellar dust (Foing and Ehrenfreund 1997). Moreover, a mixture of hydrides of C60 is shown to exhibit spectral features remarkably similar to those seen in the unidentified infrared emission (Stoldt et al. 2001). The UV absorption spectrum of synthetic C60H36 was also observed to possess abroad bump at 217.5 nm (Cataldo 2003). 

The polycyclic aromatic hydrocarbons (PAHs) have been inferred to exist in the interstellar dust by the correlation of their general infrared spectral characteristics with observed celestial infrared emission bands [13-15]. 

The mid-IR dust emission, particularly the PAH feature at A = 7.7 pm, is a clear tracer of the presence of interstellar matter. The emission shows high contrast against stellar emission at the same wavelength. 

We give an estimation of the carbon fraction locked in these molecules. We discuss the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes in various phases of the interstellar medium. These molecules could be the carriers of the anomalous microwave emission detected by Watson et al. (Astrophys. J. 624 L89,2005) in the Perseus molecular complex and Cassasus et al. (2006) in the dark cloud LDN 1622. Hydrogenated forms of fullerenes may account for the dust-correlated microwave emission detected in our Galaxy by Cosmic Microwave Background experiments. 

Cataldo F. (2003) the first have shown that the absorption spectrum of C60H36 synthesized by chemical method is able to match several IR emission lines detected from interstellar carbon dust. Background in the spectral range 700 4- 1,700 cm-1 was not detected in this paper. 

Figure 7.2 The relation between the particle growth in the disk mid-plane traced by the millimeter opacity index and that of the inner disk surface traced by the 9.7 pm silicate emission feature. The star symbols represent individual disks. Data points are from van Boekel etal. (2003), Natta etal. (2004),Furlanc/ al. (2006), Rodmann et al. (2006), and Lommen el al. (2007). Typical errors are 10-30% in both /3 and silicate band strength. Note also that differences in how the silicate band strengths were derived may introduce slight systematic offsets for the different data sets. The circle symbols represent dust opacity models calculated for the interstellar medium at a range of densities. From top to bottom the circles are Ry = 3.1 and Ry = 5.5 from Weingartner Draine (2001), a Spitzer-constrained dust opacity for dense clouds from Pontoppidan et al. (in preparation) and the particle growth simulation for protostellar envelopes [thin ice mantles, Ossenkopf Henning (1994)].

The earliest detailed studies of silicate dust in protoplanetary disks targeted those brightest in the mid-infrared, where high quality spectra could be obtained even by severely flux-limited observations. Cohen Wittebom (1985) reported the earliest detection of crystalline silicate emission from the environment of young stars and interpreted it as evidence for dust having been transformed from its pristine state in the interstellar medium to the material known to be contained in the comets and perhaps primitive meteorites. Interestingly, this observation and explanation pre-dated the evidence that young stars are surrounded by disks and not by spherical envelopes. 

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