Infrared Polarization and Interstellar Grain Models 红外偏振与星际尘埃模型 李祺 （湘潭大学） 导师：李爱根 星际尘埃模型：简介 星际偏振对尘埃模型的限止 结果与讨论 2014年7月15－19日 大连

Interstellar Dust Models Any dust model  composition, size, shape, morphology, elemental depletion; Observational constraints wavelength dependent extinction  dust size; wavelength dependent polarization  dust shape, size; absorption and emission spectra  dust composition; near, mid, far-IR continuum emission  dust size; cosmic abundance (depletion)  dust composition;

Interstellar Polarization Large grains nonspherical and aligned; Small grains spherical and/or not aligned;

Interstellar Dust Models 3 key models Core-mantle model (Li & Greenberg 1997); Composite dust model (Mathis & Whiffen 1989); Silicate/graphite-PAH model (Mathis et al. 1977; Draine & Lee 1984; Li & Draine 2001; Weingartner & Draine 2001); All models consist of amorphous silicates, carbon dust, PAHs; span a wide range of grain sizes (a few Å to a few µm); all satisfy some observational constraints; Key question: What structural forms interstellar grains take?

Core-Mantle Dust Model 硅酸盐 核 ＋ 碳质尘埃壳 Silicate core: 9.7µm Si-O, 18µm O-Si-O features Carbon dust mantle: 3.4µm aliphatic C-H feature Li & Greenberg 1997

Composite Dust Model Mathis (1996) composite collections of small silicates; small graphite, amorphous carbon, HAC; Vacuum (45%);

Silicate-Graphite-PAH Model Silicate dust, graphite dust, PAHs are physically separated! “MRN” (Mathis et al. 1977) Draine & Lee (1984) Weingartnetr & Draine (2001) Li & Draine (2001) Draine & Li (2007)

Spectropolarimetric Constraints: Polarized Silicate Features Becklin-Neugebauer object in OMC-1 (Aitken et al. 1989); 9.7µm Si-O stretching mode, 18µm O-Si-O bending mode;

Silicate Polarization  Core-Mantle Model: Good! Greenberg & Li (1996, A&A, 309, 258)

Silicate Polarization  Composite Model: Not Good! Henning & Stognienko (1993, A&A, 280, 609)

Silicate Polarization  Silicate/Graphite Model: Not Good! Aitken et al. (1989, MNRAS, 236, 919)

Core-Mantle Dust Model 硅酸盐 核 ＋ 碳质尘埃壳 Silicate core: 9.7µm Si-O, 18µm O-Si-O features Carbon dust mantle: 3.4µm aliphatic C-H feature 如果9.7µm, 18µm 光谱特征有偏振  硅酸盐核 非球形，排列！  3.4µm光谱特征也应该有偏振！ Li & Greenberg 1997

Unpolarized 3.4µm Feature (Aliphatic Hydrocarbon C-H Stretching Mode)  Core-Mantle Model: Not Good? (Adamson et al. 1999; Chiar et al. 2004)

Core-Mantle Grains: 9. 7, 18µm silicate features polarized  3 Core-Mantle Grains: 9.7, 18µm silicate features polarized  3.4µm hydrocarbon feature polarized! (Li & Greenberg 2002, ApJ, 577, 789)

Nonspherical Core + Spherical Mantle Dust? 9.7µm Si-O feature polarized  3.4µm C-H feature polarized! (Li, Liang & Li 2014, MNRAS, 440, L56 )

Nonspherical Core + Spherical Mantle Dust? 9.7µm Si-O feature polarized  3.4µm C-H feature polarized! (Li, Liang & Li 2014, MNRAS, 440, L56 )

Composite Dust? 9.7µm Si-O feature polarized  3.4µm C-H feature polarized! (Li, Liang & Li 2014, MNRAS, 440, L56 )

Future Work #1 Nonspherical silicate core with an equal-thickness carbon mantle? Silicate dust and carbon dust are physically separated? The 3.4µm hydrocarbon dust component is just minor in nature?

Future Work #2 What Can We Learn from AFGL 2591? 3.1µm ice feature: not polarized (Kobayashi et al. 1980) ; 9.7,18µm silicate feature polarized; silicates annealed (Aitken et al. 1988);  Non-spherical core-spherical mantle grains? Radiative torques?

Silicate Features: Polarized AFGL 2591: annealed silicates (Aitken et al. 1988); Smith et al. 2000; Wright et al. 2001;

BN Object: Ice Feature Polarized (Kobayashi et al. 1980)

W33A: CO (4.67µm), XCN (4.62µm) Features Polarized (Chrysostomou et al. 1996)

Bare Silicate/Graphite Model? The 3.4µm Feature Carrier? Henning & Schnaiter (1998): Nano-sized hydrogenated carbon grains  2175Å extinction hump and 3.4µm feature common origin? HD204827, Taurus cloud: with 3.4µm feature, no 2175Å extinction hump (Valencic et al. 2003, Whittet et al. 2004);

Summary Polarimetric data provide powerful constraints on dust models grain geometry; grain size; grain composition; Existing polarimetric data  bare silicate/graphite model: 9.7, 18µm silicate polarization? composite model: 9.7, 18µm silicate polarization core-mantle model: 3.4µm hydrocarbon polarization?

Composite Grain Models vs. Far-IR Polarization Polarized thermal far-IR emission reveals magnetic field geometry; grain geometry; grain dielectric function ε(λ) = ε1 + iε2; No polarization when ε(λ)  1; can very “fluffy” composite grains produce large far-IR polarization?

Large Far-IR Polarizations Are Observed! SCUBA 850µm: 9.1% polariz. on M82 (Greaves et al. 2000);

Large Far-IR Polarizations Are Observed! SCUBA 850µm: 11.9% polariz. on OMC-3 (Matthews & Wilson 2000);

Interstellar Extinction 2 grain populations: a < 100 Å; a>0.1 µm; Characterized by RV=AV/E(B-V); 2175 Å hump Stecher 1965; stable λ0 ; variable FWHM;

Interstellar Polarization Large grains nonspherical and aligned; Small grains spherical and/or not aligned;

Interstellar Polarization DCB Whittet (2003); λmax ~ RV ~ dust size; Pmax/AV ≤3%/mag; HD 204827 (top) λmax =0.42µm; HD 99872 (bottom) λmax =0.58µm; Martin et al. (1999)

Interstellar Polarization Features 2175 Å hump polarization HD 197770 (top) λmax=0.51µm; (Clayton et al. 1992); ρ Oph (bottom) λmax =0.68µm (Wolff et al. 1997); (ΔP2175Å/ΔA2175Å)/ (PV/AV) ≤ 0.09;

Absorption Features Silicate Aliphatic carbon Ices 9.7 µm: Si-O stretching; 18 µm: O-Si-O bending; amorphous; Aliphatic carbon 3.4 µm C-H band not seen in dense clouds; Ices H2O 3.1, 6.0 µm; CO 4.68 µm; CO2 4.28, 15.2 µm; CH3OH 3.54,9.75 µm; H2CO 5.81 µm; CH4 7.68 µm; NH3 2.97 µm;

Absorption Features Aliphatic hydrocarbon Aromatic hydrocarbon 3.4 µm C-H stretching band; Diffuse ISM; PPN CRL 618; Other galaxies; Aromatic hydrocarbon 3.3 µm C-H stretching; 6.2 µm C-C stretching;

Infrared Emission Classic grains Ultrasmall grains: Td ~ 20K; emit at λ>60 µm; ~ 2/3 of total emitted power; Ultrasmall grains: PAHs (~10% C); a<100 Å; emit at λ<60 µm; stochastic heating; ~ 1/3 of total power; Ultrasmall silicate grains: ≤5% (Li & Draine 2001a);

Elemental Depletions [X/H]gas <[X/H]๏; ≥90% of Si, Mg, Fe and ≥60% of C, O are locked up in dust; Dust composition: Silicate, carbon

Interstellar Polarization Features 9.7,18µm silicate: polariz. Becklin-Neugebauer object; Aitken et al. (1989); Greenberg & Li (1996); 3.4µm hydrocarbon: one obs: unpolarized; PAHs bands: unpolariz.; Far-IR emission: polariz.; Ice bands: polarized; 3.1µm H2O; 4.67µm CO; 4.62µm XCN-;

Core-Mantle Model Li & Greenberg (1997) silicate core; carbon mantle; PAHs + small graphitic grains; dust destruction τdes≈1-5 108yrs; dust injection τpro≈2.5 109yrs; mantle (accretion; protection);

Core-Mantle Model 3.4µm C-H carbon (Greenberg, Li, et al. 1995) 9.7,18µm silicate polarizat. (Greenberg & Li 1996)

Silicate Polarization  Silicate/graphite Model: Not Good Silicate Polarization  Silicate/graphite Model: Not Good! (Greenberg & Li, 1996, A&A, 309, 258)

Future Work Obtain spectropolarimetric 3.4µm and 9.7, 18µm data for the same lines of sight; Are there regional variations in the interstellar silicate absorption and polarization profiles?  synthesize dielectric functions for “astronomical silicates” in different regions; Study mid and far-IR polarization properties of different grains using DDA) non-spherical core-spherical mantle grains; can fluffy composite grains produce large far-IR polarization?