8: Composition and Physical state of Interstellar Dust

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1 8: Composition and Physical state of Interstellar Dust James Graham UC, Berkeley 1

2 Reading Tielens, Interstellar Medium, Ch. 5 Mathis, J. S. 1990, AARA, 28, 37 Draine, B. T., 2003, AARA, 41, 241 2

3 Nature of Interstellar Dust Grain heating and cooling Grain size distribution Tiny grains, temperature fluctuations and PAH Carbon in the ISM Interstellar ices 3

4 The Galaxy in the Near-IR The sky in the near-ir COBE maps the sky between 1.3 µm and 4 mm The near-ir (J, K & L) shows mostly stars & reduced ISM absorption The disk-like nature of our Galaxy with its bulge is evident 4

5 The Galaxy in the Far-Infrared COBE 100, 140 & 240 µm No ordinary stars, only a few with circumstellar dust shells are weakly detected The bulk majority of emission is from clouds of cool dust ( 20 K) 5

6 Grain Heating and Cooling Possible sources of grain heating: Absorption of starlight Collisions with atoms, e, cosmic rays, other grains Chemical reactions on grain surface Possible mechanisms of grain cooling: Radiative cooling (emission of photons) Collisions with cold atoms and molecules Sublimation of atoms/molecules from grain surface Under many circumstances, radiative heating and cooling dominate 6

7 Radiative Heating of Grains Absorption of photon Grain left in excited state Probability A ~ 10 7 s 1 for spontaneous emission Complex molecules with many energy levels can convert part of electronic energy into vibrational energy on time scale Δt s This energy is quickly distributed over all internal degrees of freedom Grains are heated A Δt 10 5 << 1 Most photon absorptions heat the grain 7

8 Equilibrium Grain Temperature Large grains Heating is by the IS radiation field Flux on a grain surface is!j λ F λ = cosθi λ dω dω = 2π sinθdθ = 2π dµ surface 1 = 2π I λ µdµ = π I λ = π J λ 0 for an isotropic radiation field Heating rate for one grain of radius a is 4π a 2 0 π J λ Q abs (a,λ)dλ 8

9 Grain Heating Most of the heating is by UV photons where Q abs ~ 1 Define J UV J UV weakly dependent on a for large grains The heating rate 0 J λ Q abs (a,λ)dλ 4π a 2 0 π J λ Q abs (a,λ)dλ = 4π a 2 J UV 9

10 Grain Emission The emissivity of a grain is given by Kirchoff s law In thermodynamic equilibrium absorption at λ per unit grain area π B λ (T)Q a bs(a,λ) hence this must be the emission rate The total power radiated by a grain is 4π a 2 0 π B λ (T gr )Q abs (a,λ)dλ 10

11 Equilibrium Balance between absorption and radiation is expressed at 4π a 2 J UV = 4π a 2 J UV = 0 0 π B λ (T gr )Q abs (a,λ)dλ B λ (T gr )Q abs (a,λ)dλ = Q abs (a,t gr ) σt 4 gr π Q abs is the Planck-average emissivity Q abs (a,t) = 0 B λ (T)Q abs (a,λ)dλ B λ (T) dλ 0 11

12 Equilibrium In the diffuse ISM grains are cold (~ 20 K) Need Q abs in the far-ir For constant m =n-ik we have Q abs ~ a/λ, but m=m(λ) Typically for real materials Q abs ~ 1/λ 2 at long wavelength More generally Q abs ~ a/λ 1+β J UV 0 2hυ 1 λ 2 ah kt gr h e hυ / kt gr 5+β 0 a 1 λ x 4 +β e x 1 dx 1+β dυ Thus Q abs ~ T 1+β and the equilibrium dust temperature is T gr J UV a 1/(5+β ) 12

13 Draine & Lee Graphite Draine & Lee 1984 ApJ

14 Draine & Lee Silicate λ -2, β =1 14

15 Planck Average Emissivity T 2, β =1 15

16 Equilibrium Temperatures Grains heated by the mean IS radiation field T * 5000K W 1.5 x Equilibrium temperature for grains 0.1 µm is about 20 K Graphite grains are hotter because of stronger UV absorption 16

17 Absorption & Grain Size Q ext =Q sca Q ext Q sca 17

18 Interpretation of the Continuum Absorption Continuum opacity shows absorption over a broad range of wavelengths (Mathis 1990 AARA 28 37) Mie curves show a steep rise then flattening 18

19 Decomposing Interstellar Extinction The shape of the interstellar extinction curve Does not look like a Mie efficiency plot Overall smoothness of A λ implies multicomponent Size distribution of grain Breadth of curve imples particle size distribution Small grains more abundant than big ones Toy water ice model 50nm & 250nm grains 90% by number are small grains a=50 nm a=250 nm 19

20 Grain Populations There are at least three populations The optical extinction, 220 nm bump, and the FUV extinction each change without affecting the other Steep rise in FUV extinction up λ 80 nm Requires a ~ λ/2$ = 15 nm Otherwise Q ext would be flat 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 A(λ) rises through the near-ir/optical near UV a ~ 150 nm If only 150 nm grains were present A(λ) at λ < 200 nm would be approximately constant 20

21 Dust Models: MRN Grain size distribution is likely continuous Mathis Rumpl & Nordseik (1977 ApJ ) proposed a power law size distribution of graphite and silicate grains; approximately equal numbers dn da = An H a 3.5, a min < a < a max a max = 250 nm, set by fit to near-ir and visible a min = 5 nm, set by fit to 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 21

22 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 22

23 Grain Size Determines Spectral Properties Mie calculation for a = 0.1, 1, & 10 µm spherical grains Optical constants from 23

24 PAHs & Tiny Grains Many nebulae HII regions Planetary nebulae Reflection nebulae show emission in the 3 15 µm region far stronger than expected from grains in thermal equilibrium with the ambient radiation field 24

25 NGC 7023 NGC 7023 is a reflection nebulae excited by a B3 star 25

26 PAHs & Astronomical Spectra Orion Bar 26

27 The Galaxy in the Infrared Mean spectrum of the Galactic ISM dust Synthesized from balloon & satellite observatories Bulk of emission from 18 K dust Significant 3-25 µm emission from from hotter grains Distinctive features at 3.3, 6.2, 7.7, 8.6 & 11.3 µm

28 Polycyclic Aromatic Hydrocarbons Small (< 1.5 nm) graphitic particles may occur as large molecules known as PAH (Polycyclic Aromatic Hydrocarbons) Fragments of graphite sheets with hydrogen atoms at the edge Lab spectra of PAHs show characteristic emission at 3.3, 6.2, 7.7, 8.6 & 11.3 µm observed in spectra of reflection nebulae etc. 28

29 Small PAHs 29

30 PAH Modes C-H stretching at 3.3 µm C-C stretching at 6.2 µm C-C stretching at 7.7 µm C-H in-plane bending at 8.6 µm C-H out-of-plane bending wavelength depends on the number of neighboring H atoms: 11.3 µm for mono no adjacent H 12.0 µm for 2 contiguous H 12.7 µm for 3 contiguous H µm for 4 contiguous H Mid-IR spectrum depends size spectrum and degree of hydrogenation 30

31 Tiny Grains Tiny grains have small heat capacity A 10 nm grain at 20 K has ~ 1.7 ev of internal energy Heat capacity is small Grains are small Grains are cold c v ~ T 3 Absorption of starlight photons leads to temperature spikes 31

32 Temperature Fluctuations ev ev Heating of a small grain (5 nm) by individual photons absorbed form the mean IS radiation field (Purcell 1976 ApJ ) Cooling by many IR photons Time between spikes is ~ 1 hr 32

33 A Day in the Life of a C Grain A day in the life of carbonaceous grains, heated by the local interstellar radiation field τ abs is the mean time between photon absorptions (Draine & Li 2001 ApJ ) 33

34 Temperature Fluctuations IR emission from tiny grains occurs at shorter wavelengths than expected from equilibrium For grains achieving T max Radiation peaks at hv 5 T max Emission at 60 µm needs T max 50 K grain 10 ev photon absorbed by a 7 nm grain Emission at 12 µm needs T max 250 K grain 10 ev photon absorbed by a 1.5 nm grain 34

35 The 220 nm Feature The 220 nm feature is ubiquitous in the Milky Way Strongest discrete feature in the extinction curve Only C, O, Mg, Si or Fe is abundant enough to give such a strong feature Central wavelength is almost constant ± 0.5 nm Significant variation in the width (10%) & strength Weakness correlated with metal abundance Weak 2175 Å in the LMC Missing in the SMC 35

36 The 220 nm Feature C atoms four valence electrons 2s 2 2p 2 Three make up a σ orbital The remaining p electron is shared or delocalized among all the C-C bonds Individual graphite sheets are held together by weak van der Waals forces Graphite has a strong UV resonance due to these $-orbital valence electrons Need 25% of the cosmic C abundance in small graphite spheres to explain the strength All six p orbitals are parallel to one another, and each contains one electron. Therefore there are three $ bonds. Since there is no reason to prefer one form of $ interaction over the other those three $ bonds are delocalized over the whole molecule. 36

37 The 220 nm Feature Why is the feature so uniform? The width of the feature depends on the shape of the particles but tuning the shape shifts the central wavelength Polycyclic aromatic hydrocarbons (PAHs) have similar structures to graphite sheets, with similar electronic wavefunctions PAHs generally have strong $ $ * absorption at nm PAHs are seen in emission in the IR Large (up to 10 5 C atoms) PAH molecules may be the carrier of the interstellar 2175 Å (Weingartner & Draine 2001 ApJ ) Laboratory spectra are unavailable for PAH molecules of the sizes characteristic of the ISM 37

38 Desert Boulanger & Puget (1990) Desert Boulanger & Puget (1990 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 =

39 Forms of Carbon in the ISM 39

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

41 DIBs Associated with C 60 + HD ? Foing & Ehrenfreund 1994, Nature, 369, 296; 1997, A&A, 317, L59 41

42 Circumstellar Diamonds ISO spectra of two pre-main-sequence stars Lab spectra of nano-diamond crystals resemble astrophysical source 42

43 Grains in Cold, Dark Clouds Grains may coagulate and alter size distribution Variation of R along different lines of sight If A λ ~ λ -β β 0 R Cardelli et al ApJ

44 Grains in Cold, Dark Clouds Grains may acquire mantles of molecular ices consisting of mix of H 2 O, CO 2, CO2, CH 3 OH, etc. Absorption bands due to solid-state features in dense clouds towards embedded IR sources 44

45 Solid State vs. Gas Phase Suppression of rotational structure Molecules cannot rotate freely in ices P, Q, R branches collapse into one broad vibrational band Line broadening Molecules in ice interact with environment; each is located at slightly different site Band is broadened Amount of broadening depends on species Line shifting Interaction of molecules with surroundings modify bond force constants Shift vibrational frequency 45

46 Gas-Phase and Solid CO 46

47 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 Interstellar Ices 47