Toward understanding the 3.4 μm and 9.7 μm extinction feature variations from the local diffuse interstellar medium to the Galactic center
© The Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan 2010
Received: 10 August 2008
Accepted: 4 December 2008
Published: 7 February 2015
Observationally, both the 3.4μm aliphatic hydrocarbon C-H stretching absorption feature and the 9.7μm amorphous silicate Si-O stretching absorption feature show considerable variations from the local diffuse interstellar medium (ISM) to Galactic center (GC): both the ratio of the visual extinction (AV) to the 9.7μm Si-O optical depth (Δτ9.7 μm) and the ratio of AV to the 3.4 μm C-H optical depth (Δτ3.4 μm) of the solar neighborhood local diffuse ISM are about twice as much as that of the GC. In this work, we try to explain these variations in terms of a porous dust model consisting of a mixture of amorphous silicate, carbonaceous organic refractory dust (as well as water ice for the GC dust).
The interstellar extinction law is one of the primary sources of information about the interstellar grain population, and one often obtains direct information on the composition of interstellar dust from spectral features in extinction (Draine, 2003). These spectral features also provide strong constraints on interstellar grain models. With the advent of ground-based and space borne infrared (IR) telescope facilities, the IR extinction continuum and absorption features have been receiving increasing attention and play an essential role in recovering the intrinsic energy distribution of celestial objects and inferring the characteristics of interstellar dust.
In the interstellar extinction curve, the 2175 Å bump is outstanding in the ultraviolet (UV), while in the IR, there are a number of prominent absorption features as well: (1) the ubiquitous 9.7μm and 18μm features respectively due to the Si-O stretching and O-Si-O bending modes of amorphous silicates; (2) the 3.4μm feature due to the CH stretching mode of aliphatic hydrocarbon dust, as ubiquitously present in the ISM of the Milky Way and external galaxies as the 9.7μm and 18μm silicate bands, except this feature is not seen in dense molecular clouds (see Pendleton, 2004 for a review); (3) the 3.3μm and 6.2μm weak features seen in both local sources and GC sources (Schutte et al., 1998; Chiar et al., 2000), respectively due to the C-H stretching and C-C stretching modes of polycyclic aromatic hydrocarbon (PAH) molecules; and (4) in dense clouds the 3.1μm feature due to the O-H stretching mode of water ice as well as a number of weaker features at 4.68μm (CO), 7.68μm (CH4), 4.28μm, 15.2μm (CO2), 3.54μm, 9.75μm (CH3OH).
Along the lines of sight toward the GC, there are dense molecular clouds.3 In cold, dense molecular clouds, interstellar dust is expected to grow through coagulation (as well as accreting an ice mantle) and the dust is likely to be porous (Jura, 1980). In this work, we demonstrate that the observed variations of AV/Δτ9.7 μm and AV/Δτ3.4 μm from the LDISM to the GC could be explained in terms of composite porous dust.
We consider a composite porous dust model consisting of amorphous silicate, carbon, and vacuum (in dense clouds silicate dust and carbon dust are coated with water ice). We take the optical constants of Draine and Lee (1984) for amorphous silicate, of Li and Greenberg (1997) for carbonaceous organic refractory (to represent the carbon dust component), of Li and Greenberg (1998) for water ice. The mass densities of silicate dust, organic refractory dust and ice are taken to be ρsil ≈ 3.5 g cm-3, ρcarb ≈ 1.8 g cm-3 and ρice ≈ 1.2 g cm-3, respectively. We take the mass ratio of organic refractory dust to silicate dust to be mcarb/msil = 0.7 and the mass ratio of water ice to organic refractory dust and silicate dust to be mice/(mcarb + msil) = 0.8, as inferred from the cosmic abundance constraints (see appendix A of Li and Lunine, 2003a).
For the dust in the local diffuse ISM, we assume the dust to be a solid compact mixture of amorphous silicate and organic refractory materials with mcarb/msil = 0.7. We take the dust size to be a = 0.1μm, the typical grain size for the dust in the diffuse ISM (see Draine, 1995). For the dust in the dense molecular clouds along the lines of sight toward the GC, we assume that silicate dust and organic refractory dust are equally coated with an ice layer and then form a porous aggregate (see Li and Lunine, 2003b). For porous dust, a key parameter is the porosity P (or fluffiness; the fractional volume of vacuum in a grain). We will consider a range of porosities. We assume all grains are spherical in shape; the porous grain size a is defined as the radius of the sphere encompassing the entire porous aggregate. In order to find suitable porosity P and dust size a for the dust in the dense molecular clouds to reproduce the observed AV/Δτ9.7 μm and AV/Δτ3.4 μm ratios toward the GC, we leave both P and a adjustable.
We use Mie theory in combination with the Maxwell- Garnett and Bruggeman effective medium theories (Bohren and Huffman, 1983; see eqs. 7-9 of Li and Lunine, 2003b and Kimura et al., 2008b) to calculate the optical properties of composite porous grains. This approach is valid for computing the integral scattering characteristics (e.g. extinction, scattering, absorption cross sections, albedo and asymmetry parameter; see Hage and Greenberg, 1990; Wolff et al., 1994).
3. Results and Discussion
For AV/Δτ3.4 μm, our model with mcarb/msil = 0.7 (and P = 0.8, a = 0.5–1.5μm) is consistent with the observed factor-of-two variations in the local ISM and toward the GC (see Tables 1, 2). However, the model values of AV/Δτ9.7 μm for both the diffuse ISM dust (AV/Δτ9.7 μm ≈ 38.2) and the GC dust (AV/Δτ9.7 μm ≈ 16.3) are higher by a factor of ∼1.5–2 than that observed in the local diffuse ISM (AV/Δτ9.7 μm ≈ 18.2) and the GC (AV/Δτ9.7 μm ≈ 8.4). This discrepancy may result from the underestimation of the silicate mass fraction. With an increased silicate mass fraction, say, mcarb/msil = 0.5 which is consistent with the in situ measurements of comet Halley (Jessberger and Kissel, 1991) and widely adopted in cometary dust modeling (Greenberg, 1998; Greenberg and Li, 1999; Kolokolova et al., 2004; Kimura et al., 2006, 2008a; Mann et al., 2006; Kolokolova and Kimura, 2010), we obtain AV/Δτ3.4 μm ≈ 252 and AV/Δτ9.7 μm ≈ 27.1 for the local ISM (assuming compact dust), and AV/Δτ3.4 μm ≈ 154 and AV/Δτ9.7 μm ≈ 11.3 for the GC (assuming porous dust). These values are closer to that observed. It is expected that with a smaller mcarb/msil (i.e. a larger silicate mass fraction), one would obtain a smaller AV/Δτ9.7 μm while AV/Δτ3.4 μm does not change much. Thus the observed variations of AV/Δτ3.4 μm and AV/Δτ9.7 μm from the local ISM to the GC could be explained. It is worth noting that, based on a detailed analysis of the GC 5–8μm absorption spectra obtained from the Kuiper Airborne Observatory, Tielens et al. (1996) argued that silicate dust may contribute as much as 60% of the interstellar dust volume. This would translate to mcarb/msil ≈ 0.34 if we assume that the remaining 40% of the interstellar dust volume is all from the 3.4μm C-H feature carrier (which is indeed a very generous assumption).
Admittedly, the proposed explanation is oversimplifed. In the future we will consider more realistic models in which more dust species (e.g. hydrogenated amorphous carbon with a range of C/H ratios), the distribution of dust along the line of sight toward the GC (e.g. see Sandford et al., 1995), a distribution of dust sizes, and the possible porous nature of the diffuse ISM dust (e.g. see Mathis and Whiffen, 1989) will be considered.
Cohen et al. (1989) argued that the observed 9.7μm dip on which Δτ9.7 μm was measured may be partly contributed by PAHs (i.e. the red tails of the PAH 7.7μm and 8.6μm bands and the blue tail of the 11.3μm band could form an “artificial” 10μm dip). But this would result in a smaller Δτ9.7 μm for the GC and a larger Δτ9.7 μm for the LDISM (since the PAH emission is more likely to present in the LDISM while toward the GC PAHs are seen in absorption), quite on the opposite.
They also pointed out that the C-H and Si-O carriers may be coupled, perhaps in the form of silicate-core organic-mantle grains. This idea is challenged by the nondetection of the 3.4μm feature polarization along sightlines where the 9.7μm feature polarization is detected (Adamson et al., 1999; Chiar et al., 2006; also see Li and Greenberg, 2002). It is possible that the 3.4μm feature may be not produced by a carrier residing in a mantle on a silicate core but by very small (unaligned) grains (Chiar et al., 2006).
We thank the anonymous referees for their very helpful comments. We thank Dr. H. Kimura for sharing with us his work prior to publication. This work is supported in part by NSFC grant No. 10603001, SRFDP grant No. 20060027013 and NCET-05-0144. AL is supported in part by NASA/HST Cycle-15 Theory Program and NSF grant AST 07-07866.
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