Lecture 5. Interstellar Dust: Chemical & Thermal Properties

Transcription

1 Lecture 5. Interstellar Dust: Chemical & Thermal Properties!. Spectral Features 2. Grain populations and Models 3. Thermal Properties 4. Small Grains and Large Molecules Icy Grains

2 1. Features in the Interstellar Extinction Curve The interstellar extinction curve A λ has the potential to reveal the nature of dust. A λ is remarkably smooth, which suggests many components (size & composition): Size distribution: c.f. overall (broad) variation of A λ with λ. Composition: c.f. discrete features in A λ, the 220 nm bump & 9.7 & 18 µm features. a=260 nm

3 Interstellar Extinction Curve The shape of the interstellar extinction curve does not look like a Mie Q ext plot. The overall smoothness implies many components The breadth implies a distribution in sizes, with small grains more abundant than big ones Illustrated here with toy water ice models with 50 nm & 250 nm grains; small grains 90% by number 50nm a=50 nm 250 a=250 nm nm

4 Features in the Extinction Curve 220 nm 9.6 µm 2200 Å feature 10 µm silicate feature The strongest dust spectral features occurs at 220 nm

5 Role of Silicate Minerals Silicates generally have strong absorption resonances near 10 µm due to the Si-O bond stretch. It is virtually certain that the interstellar 9.7 µm feature is produced by interstellar silicates (absorption as well as emission is observed). The 10 µm emission feature is observed in outflows from cool O-rich stars Their atmospheres condense silicates It is absent in the outflows from C-rich stars where O, needed for silicates, is all locked up in CO The broad feature at 18 µm can be identified with the O-Si-O bending mode in silicates

6 Vibrational Modes of Silicate Minerals enstatite fosterite ferrosilite fayalyte Species Mode Wavelength (µm) MgSiO 3 Si-O stretch 9.7 O-Si-O bend 19.0 Mg 2 SiO 4 Si-O stretch 10.0 O-Si-O bend 19.5 FeSiO 3 Si-O stretch 9.5 O-Si-O bend 20.0 Fe 2 SiO 4 Si-O stretch 9.8 O-Si-O bend 20.0 SiC SiC stretch 11.2

7 The 220 nm Feature Ubiquitous in the Milky Way ± 0.5 nm (fixed) width varies (10%) as does strength Graphite has a strong UV resonance due to π-orbital valence electrons Why is the feature so uniform? 220 nm bump is weak in the Small Magellenic Cloud weakness correlated with C/O Hydroxylated Mg 2 SiO 4 (fosterite) also has a 220 nm feature. (Steel & Duley 1986) The actual carrier of the 220 nm feature has not been identified.

8 220 nm Feature in IDPs A. Interstellar 220 nm feature B. Broad & narrow examples C. lab: hydroxylated amorphous silicate D. lab: Mg 3 Si 4 O 10 [OH] 2 E. IDP organic carbon F. IDP silicates JP Bradley et al. Science, 307, 244, 2005: Non-solar isotopic ratios indicate these IDPs are interstellar in origin.

9 2. Grain Populations There are at least three populations: The optical extinction, 220 nm bump, and FUV extinction manifest independent changes 1. A λ rises from the NIR/optical to the near UV requires a ~ 150 nm, but if only 150 nm grains were present, A λ for λ < 200 nm would be approximately constant 2. Steep rise in FUV extinction down to 80 nm requires a ~ λ/2π ~ 15 nm, otherwise Q ext would be flat 3.The 220 nm bump implies a specific carrier symmetry and constancy of λ 0 imply absorption in the small particle limit a 10 nm. small graphite spheroids a 3 nm, b/a = 1.6 might work, except for variation in central wavelength

10 Dust Models: MRN Distribution Grain size distribution is likely to be continuous Mathis, Rumpl & Nordseick (ApJ ) proposed power law distribution of graphite and silicate grains in approximately equal numbers dn da = An Ha 3.5, a min < a < a max a max = 250 nm, set by NIR and visible a min = 5 nm, set by FUV curve MRN power law has most mass in large particles, most area in small particles: M A a 3 dn da da a 2 dn da da a max a min a min a max

11 Draine & Lee Model Drain & Lee 1984 ApJ Two component MRN model: 5 < a/nm < 250 Graphite: 60% of C Astronomical silicate : 90% of Si, 95 % Mg, 94% of Fe & 16% of O

12 Draine & Lee: Silicate

13 Draine & Lee: Graphite

14 Weingartner & Draine Model a 4 times the size distribution Weingartner & Draine, ApJ

15 3. Grain Thermal Properties Heating Processes Absorption of starlight Collisions (warm gas, cosmic rays, other grains) Chemical reactions on grain surface Cooling Processes Radiative (photon emission) Collisions with cool gas Sublimation from grain surface Radiative heating and cooling often dominate.

16 The Galaxy in the Far-Infrared Bulk of emission c.f. 18 K dust (140-µm peak in FIR spectrum of Lec 1) Significant 3-25 µm emission c.f. warmer grains Distinctive features at 3.3, 6.2, 7.7, 8.6 & 11.3 µm Mean spectrum of Milky Way IS Dust (Synthesis of balloon & satellite data)

17 Radiative Heating of Grains On absorption of photon, grain is left in an excited state. The probability for spontaneous emission is large A ~ 10 7 s 1. Complex grains as well molecules with many energy levels can rapidly convert part of this electronic energy into vibrational energy on time scales t s This energy is quickly distributed over all internal degrees of freedom, and the grains are heated. Most photon absorptions heat the grain since A t 10 5 << 1

18 Heating of Large Grains Heating by IS radiation, whose flux for an isotropic radiation field, is πj λ F λ = µi λ dω = 2π I λ µdµ = π I λ = π J λ surface 1 0 Heating rate for one grain of radius a is 4π a 2 0 π J λ Q a (λ)dλ = 4π a 2 J UV where J UV is defined as J UV 0 J λ Q a (λ)dλ J uv is insensitive to a for large grains. Most of the heating of large grains c.f. UV photons for which Q a ~ 1

19 Steady Thermal Balance Using Kirchoff s Law, j ν (T) = B ν (T) κ ν (T), the radiative cooling rate is 4π a π BλQ ( λ) dλ 2 0 The balance between absorption and radiation is 4π a 2 J UV = 4π a 2 J UV = where Q a is the Planck-averaged emissivity a 0 π B λ Q a (λ)dλ B λ Q a (λ)dλ = Q 0 a (T) σt 4 π Q a (a,t) = 0 B λ Q a (a,λ)dλ B λ dλ 0

20 Planck Average Emissivity Q a (a,t) = 0 B λ Q a (a,λ)dλ B λ dλ 0 Based on the Draine & Lee dust model

21 The Temperature of Large IS Grains Grains in the diffuse ISM are cold, ~ 20 K. To calculate Td, we need Q a in the far-ir Recall that for constant m =n-ik, Q a ~ a/λ, but in fact m=m(λ) and typically for real materials Q a ~ 1/λ 2 at long wavelength. More generally we parameterize the efficiency as Q a ~ a/λ 1+β J UV 0 2hυ 1 λ 2 e hυ / kt d 1 a λ 1+β dυ ah kt d h 5+β The equilibrium dust temperature is 0 x 4 +β e x 1 dx T d J UV a 1/(5+β )

22 Calculated Grain Temperatures Specify the mean IS radiation field by a BB color temperature T * 5000K and a dilution fatcor W 1.5 x For 0.1 µm grains, T d ~ 20 K Graphite grains are hotter because they are more efficient UV absorbers.

23 4. Small Grains and Large Particles Small grains have small heat capacity and small radiating area. Absorption of starlight photons leads to temperature spikes. A 10 nm grain at 20 K has 1.7 ev of internal energy. Since C v ~ m gr T 3, the grain compensates for its small size by getting hot before cooling down. Where do these statements come from?

24 Temperature Fluctuations ev ev Heating of a small 5-nm grain by individual photons absorbed from the mean IS radiation field (Purcell 1976 ApJ ) Cooling by many IR photons Time between spikes is ~ 1 hr

25 Temperature Fluctuations IR emission from tiny grains occurs at shorter wavelengths than expected from equilibrium. For a grain rising to T max, and the emitted radiation peaks at hv 5 T max emission at 60 µm requires T max 50 K, or a 10 ev photon absorbed by a 7 nm grain emission at 12 µm requires T max 250 K or a 10 ev photon absorbed by a 1.5 nm grain

26 Tiny Grains Very small grains are more abundant than suggested by MRN size distribution: The diffuse IR emission of reflection nebulae from 2-25 µm is hard to understand unless grains are hotter than expected from equilibrium considerations (temperature fluctuations of very small grains!). PAHs Polycyclic Aromatic Hydrocarbons PAH molecules are fragments of graphite sheets with edge H atoms; they show characteristic emission at 3.3, 6.2, 7.7, 8.6 & 11.3 µm observed in warm dust.

27 PAHs & Astronomical Spectra Orion Bar

28 Mid-IR ISO Spectral Features Courtesy of AGGM Tielens

29 Early Composite Dust Model Desert, Boulanger, & Puget AA Big silicate grains 15 < a/nm < 110 ρ dust /ρ gas = Very small graphitic grains 1.2 < a/nm < 15 ρ dust /ρ gas = PAHs 0.4 < a/nm < 1.2 ρ dust /ρ gas = See Draine ARAA for update.

30 Possible Forms of Carbon in the ISM Too many possibilities?

31 Structure of C 60 Each C atom connected by one double and two single bonds Soccer ball Closed-shell electronic structure Introduced by Kroto as proposed origin of polyacetylene chains observed in C-rich AGB stars; 1996 Nobel Prize awarded for lab discovery,

32 Diffuse Interstellar Bands ~ 200 DIBs known Most DIBs are unidentified Some DIBs may be due to large carbon-bearing molecules C 60 + was a candidate for λλ 9577, 9632 bands BD+63 o 1964

33 DIBs Associated with C 60 + HD

34 Circumstellar Diamonds ISO spectra of two pre main-sequence stars Lab spectra of nanodiamond crystals resemble astrophysical sources

35 5. Icy Grains in Cold Regions In addition to radiative processes, grains can interact with one another and with the gas, especially in dense regions. Examples are: Coagulation leading to grain growth and changes in the size distribution, manifested by variation of R V along different lines of sight & especially its increase in dense regions. Cold grains acquire mantles of molecular ices, consisting of mix of H 2 O, CO 2, CO2, CH 3 OH, etc.

36 Interstellar Ices 3.1 µm: amorphous, dirty H 2 O ice 4.27 µm: CO 2 stretching 4.6 µm: CN stretch (XCN, OCN?) 4.67 µm: CO 6.0 µm: H 2 O bending 6.8 µm:? 15 µm: CO 2 bending Absorption bands due to solid-state features in dense clouds towards embedded IR sources.

37 Spectroscopic Differences Between Solid & Gas Phase Suppression of rotational structure Molecules cannot rotate freely in ice P, Q, R branches collapse into one broad vibrational band Line shifting Interaction of molecules with surroundings modifies bond force constants Line broadening Interact with ice environment: each molecule is located at a slightly different site Broadening depends on species

38 Gas-Phase and Solid CO

39 Amorphous & Crystalline Solids