1. 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

2. 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;

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

4. 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?

5. 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

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

7. 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)

8. 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;

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

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

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

12. 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

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

14. 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)

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

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

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

18. 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?

19. 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?

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

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

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

23. 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);

24. 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?

25. 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?

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

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

28. 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;

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

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

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

32. 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;
CH µm;
NH µm;

33. 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;

34. 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);

35. 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

36. 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-;

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

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

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

40. 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?