Grain Alignment_submitted - arXiv

Interstellar Grain Alignment – Observational Status B-G Andersson SOFIA Science Center, Universities Space Research Association, NASA Ames Research Center, M.S. N232-12 Moffett Field, CA 94035

Abstract Interstellar polarization in the optical/infrared has long been known to be due to asymmetrical dust grains aligned with the magnetic field and can potentially provide a resource effective way to probe both the topology and strength of the magnetic-field. However, to do so with confidence, the physics and variability of the alignment mechanisms must be quantitatively understood. The last 15 years has seen major advancements in both the theoretical and observational understanding of this problem. I here review the current state of the observational constraints on the grain alignment physics. While none of the three classes of proposed grain alignment theories: mechanical, paramagnetic relaxation and radiative alignment torque, can be viewed as having been empirically confirmed, the first two have failed some critical observational tests, whereas the latter has recently been given specific observational support and must now be viewed as the leading candidate.

1 Introduction Interstellar optical polarization was discovered in 1949, independently by Hall (1949) and Hiltner (1949a,b). Already in Hiltner's second paper of that year the effect was ascribed to dichroic extinction by asymmetric dust grains aligned with the magnetic field. This is now well established and is supported (Figure 1) by comparing the amount of polarization with the visual extinction, which shows that the upper envelope of the former is linearly correlated with the lat-

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ter, and by comparing the position angle of the optical/near infrared polarization with the position angle of polarization due to synchrotron radiation, for which a well understood theory exists (e.g. Jackson 1975; Burke & Graham-Smith 1997)

Fig. 1. That optical/near-infrared polarization is due to dichroic extinction by asymmetric dust grains aligned with the magnetic field is supported by the fact that (left) the upper envelope of the amount of polarization is correlated with the visual extinction (in this case towards a region around the Southern Coalsack; Andersson & Potter (2005)) and (right) that the position angles of the polarization agree with the position angles observed from synchrotron radiation (with an offset of ~90˚, expected for zero pitch-angle electrons, cf Jackson 1975 ).

We can therefore safely assume that the optical/near-infrared (O/IR) polarization observed from the ISM is due to dichroic extinction by asymmetric dust grains aligned with (if not by!) the magnetic field. In the far-infrared (FIR), these aligned grains give rise to polarized emission which has been observed in a number of sources at wavelengths from the far infrared to mm-waves (cf. reviews by Hildebrand 1996 and summaries by Dotson et al. 2000, Vaillancourt & Matthews 2012, and references therein). Significant progress has been made over the last two decades in both theory and observations and we are now at a point where quantitative comparisons between theory and observations are being made, and promises to reward us with a predictive, observationally supported theory of interstellar grain alignment within the foreseeable future. This review will focus on the observational aspects of interstellar grain alignment physics and will only provide a brief summary of the theoretical aspects of the field. Excellent reviews of the latter can be found in e.g. Roberge (1996) and Lazarian (2007).

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The main observational constraints on the grain alignment mechanisms come from (in the O/IR) the interstellar extinction and polarization curves (e.g. Fitzpatrick & Massa 1990; Cardelli, Clayton, & Mathis 1989, Serkowski 1973; Wilking et al. 1980). Based on the theory for scattering of light by small particles (“Mie theory” and its extensions; Mie 1908; Debye 1909; cf Whittet 2003; Krügel 2003; Draine 2011), complemented by elemental abundance constraints and estimates of the refractive indices for the grain materials, these allow an inversion from the wavelength dependence of the extinction and polarizization, respectively, to grains size distributions (by grain material) of the total grain population and the aligned part (Mathis, Rumple & Nordsieck 1977, Kim, Martin & Hendrey 1994, Clayton et al., 2003; Mathis 1986, Kim & Martin 1995). The dust is usually modeled as consisting of silicates, amorphous carbon and small graphite particles made up of simple shapes (e.g. spheres, spheroids or cylinders). The number density of the dust was found to be satisfactorily (Mathis, Rumple & Nordsieck 1977) modeled a power-law size distribution n(a) ∝ a-q

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with an exponent of q≈3.5, and grains at least in the range 0.0050.25 mm range. Later models extend the upper limit with an exponential decay to larger grains (Kim, Martin & Hendrey 1994, Clayton et al., 2003, Draine & Li 2007; Draine & Fraisse 2009). These grain models are – of course – a simplification, which disregards such complications as possibly composite grains or silicate grains with carbonaceous mantles (Duley, Jones & Williams, 1989; Li & Greenberg 1997) or grains of highly complex shapes or structures. In the dark, cold parts of molecular clouds volatile ice mantles form on the refractory grains (Whittet 2003), which can be of substantial relative volume (Hough et al. 2008). In addition to their influence on the extinction and polarization properties of the material it is becoming increasingly clear that these ice mantles are crucial to the understanding of the chemical evolution of molecular clouds (Herbst & van Dishoeck 2009). The polarized spectrum of MIR absorption and FIR/(sub)mmwave emission from warm dust provide complementary constraints

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of the characteristics of the dust in terms of grain shape and refractive indices (e.g. Hildebrand & Dragaovan 1995, Draine & Fraisse 2009) for high column density line of sight. The most direct probe of the grain alignment is through the fractional polarization (p/AV). Although this, to first order, measures the fraction of aligned, asymmetric grains, it will, in detail, depend on a number of, often poorly constrained, parameters, including 1. The size distribution of the grains 2. The chemical composition and refractive index of the grains 3. The fraction of asymmetric grains 4. The alignment efficiency of the grains 5. The thermal and radiation field environment of the grains 6. The general orientation of the ordered magnetic field 7. The magnetic field line of sight topology, and level of turbulence Therefore these factors, their uncertainties and – possibly – variations along each line of sight, need to be taken into account when comparing observations to theoretical predictions. Polarization due to dichroic extinction by asymmetric grains aligned with the magnetic field, will only be sensitive to the planeof-the-sky component component of the field. Hence variations in the line-of-sight to plane-of-the-sky ratio of the magnetic field will affect the O/IR fractional polarization. Since both Zeeman splitting (Verchuur 1969; Crutcher et al. 1975; Troland & Crutcher 2008) and Faraday rotation observations (Wolleben & Reich 2004 and ref. therein) probe the line-of-sight component of the field, direct “cross calibration” between these, nominally more quantitative, methods and O/IR polarimetry are inherently difficult (Andersson & Potter 2009) To account for the maximum observed polarization at small to moderate opacities of p/EB-V ≤ 9 % mag-1 (Serkowski, Mathewson & Ford 1975) efficient alignment of the larger silicate grains is required (Jones 1996 and refs therein). Mathis (1986) could reproduce the wavelength dependence of the polarization assuming perfect alignment of the large silicate grains. Kim & Martin (1995), using a combination of extinction and polarization data, showed that even

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for small axis-ratios, oblate spheroidal silicate grains, combined with a population of spherical (silicate and carbonaceous) grains can reproduce the observed general maximum polarization fraction seen with a high alignment fraction. Comparisons between the linear and circular polarization observed in the ISM indicates that the dust grains giving rise to the polarization are good dielectrics (Martin 1974, Martin & Angel 1976, Mathis 1986), consistent with silicate, but not with carbonaceous grains. Strong polarization has been detected in the 9.7 µm and 18 µm silicate spectral features (Smith et al. 2000), whereas the 3.4 µm aliphatic C-H stretch feature, usually associated with carbonaceous dust, does not show detectable polarization (Chiar et al. 2006). For these reasons it is often assumed that large carbonaceous are either absent, symmetrical or unaligned. The 2175 Å extinction "bump" shows associated polarization in 2 out of 30 lines of sight observed (Anderson et al., 1996; Martin, Clayton & Wolff, 1999). Since this extinction feature is usually thought to be due to small graphite grains it is possible that these can be aligned, at least in some environments, whereas the larger amorphous carbon grains required in some models (Clayton et al. 2003; Draine & Li 2007) might not be alignable.

Fig. 2 The fractional polarization in the K-band seen towards embedded sources and background field stars (Whittet et al 2008) are compared with the predictions from a model of polarimetric radiative transfer through a cloud with a turbulent magnetic field. Only at very large opacities without embedded sources does the fractional polarization drop below what can be explained by depolarization due to the random field component (addapted from Jones et al. 2011).

At large opacities the fractional polarization (p/AV) drops systematically with opacity (Jones 1989, Jones, Klebe & Dickey 1992 (JKD); Goodman et al. 1992, 1995; Gerakines et al. 1995; Whittet et

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al., 2008), which is usually interpreted in terms of decreasing polarization efficiency. Some of this drop-off is, however, likely due to line-of-sight depolarization where different gas parcels in the cloud have intrinsic magnetic field directions (close to) perpendicular to each other and hence cancel out the polarization from each other. This phenomenon has been studied by Meyers & Goodman (1991), Jones, Klebe & Dickey (1992), Ostriker, Stone & Gammie (2001) and Wiebe & Watson (2001). In general it is found that for multiple "decorrelation zones" and a large random field component a limiting behavior of p~AV1/2 is predicted, as is appropriate for a random walk process. Observationally, the level of turbulence, and hence the decrease in the fractional polarization that can be ascribed to a turbulent magnetic field, can be constrained by the fact that even for clouds with large opacities the observed polarization angles are not random (Jones 1996). For the Taurus cloud (observed by Whittet et al 2008; Figure 3) and even in the deep star-less cores (e.g. Crutcher et al. 2004) ordered magnetic fields are seen.

Fig. 3 (Left) The distribution in polarization position angles for the “Field star” sample from Whittet et al (2008; Figure 2) shows a broad, but non-random distribution, indicating that the decrease in fractional polarization could not be fully ascribed to turbulence in the cloud. The dispersion in the position angles (right) shows no systematic variation with opacity.

Figure 2 shows the fractional polarization seen towards embedded and field star background sources (Whittet et al. 2008; Jones et al 2011) overlaid with the predictions from JKD using an equal mix of ordered and random components for the magnetic field with a decor-

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relation length for the random component of τK=0.1 (Jones et al. 2011). For the lines of sight towards embedded sources, and for AV≤10 for the field stars, the drop in fractional polarization can be accounted for by the random field’s depolarizing effects, while towards even deeper sightline (with these field parameters, additional effects – likely a decrease in the grain alignment efficiency are required. Figure 3 shows the distribution of polarization angles observed for the field star sample of Whittet et al (2008). While the distribution is broad, it does not allow a depolarization fully dominated by magnetic field turbulence. Note, however, that this result should not be interpreted to mean that grain alignment variations are not present for AV