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ISM Lecture 11. Interstellar dust. 11.1 Evidence for interstellar dust. Tielens Chap. 5, 13 Draine 2003, ARAA. Dark clouds: absence of stars on photographic plates, large scale CCD imaging Reflection nebulae Reddening of starlight: continuous extinction Polarization of starlight by ISM

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ism lecture 11

ISM Lecture 11

Interstellar dust

11 1 evidence for interstellar dust
11.1 Evidence for interstellar dust

Tielens Chap. 5, 13

Draine 2003, ARAA

  • Dark clouds: absence of stars on photographic plates, large scale CCD imaging
  • Reflection nebulae
  • Reddening of starlight: continuous extinction
  • Polarization of starlight by ISM
  • Continuous infrared emission from clouds
  • Diffuse galactic light (entire Galaxy as “reflection nebula”)
evidence for interstellar dust cont d
Evidence for interstellar dust (cont’d)
  • Absence of particular elements (e.g., Fe, Si) from gas phase (see Chap. 7) depletion onto grains
  • Existence of H2 in diffuse clouds
  • X-ray haloes
typical size of interstellar grain
Typical size of interstellar grain
  • Reddening and polarization  particles must have size  wavelength of visible light: 2a/  1
  • Much of mass of interstellar dust happens to be in particles with a  0.15 m
  • If a was much larger or smaller, the discovery of interstellar dust would be much more difficult
  • There is some observational bias, of course: how can we rule out the existence of a large population of “interstellar bricks”??
definitions cont d
Definitions (cont’d)
  • g describes the angular redistribution of light
11 2 optical properties of small particles
11.2 Optical properties of small particles
  • How do grains of a given size, shape, and refractive index modify electromagnetic radiation?
  • Use Maxwell’s equations with proper boundary conditions
  • Very complicated problem! Only solved for spheres, spheroids, and infinite cylinders
refractive index and dielectric constant
Refractive index and dielectric constant
  • Refractive index:
  • Dielectric constant:
  • Consider homogeneous sphere with isotrpic refractive index m or dielectrc constant e
mie theory
Mie theory
  • Complete classical theory for homogenous sphere with isotropic index was worked out independently by Mie and Debye
  • Series expansion of EM wave in powers of
  • For small x (large λ): retain only a few terms (easy)
  • For large x (short λ): need to sum many terms (tricky)
mie theory at long wavelengths
Mie theory at long wavelengths
  • For
  • This is the same result as scattering by electric dipole (Rayleigh scattering)
  • Note that at long , absorption by grains depends only on mass in grains, not on size distribution
mie efficiency factors for m 1 27 1 37 i
Mie efficiency factors for m = 1.27 + 1.37i







11 3 the purcell limit
11.3 The Purcell limit

Purcell 1969, ApJ

  • Estimate minimum amount of material in interstellar dust from observed extinction
  • For individual grain with certain shape and zero-frequency dielectric constant e0:
  • It can be shown that F 1, except for strongly non-spherical shapes (“needles”) or conducting grains
  • Thus gives lower limit on grain volume
the purcell limit cont d
The Purcell limit (cont’d)
  • gives info on grain volume per H atom
  • Observations of  / NH from 912 Å  30 m  lower limit


estimate of dust to gas ratio
Estimate of dust-to-gas ratio
  • => Unless grains are extremely elongated / flattened and conducting, there must be considerable mass in interstellar grains
  • Graphite: rgrain = 2.24 g cm–3
  • Silicates: rgrain = 3.2 - 4.1 g cm–3
  • Reasonable estimate:
typical dust model
Typical dust model
  • There cannot be much more dust than 0.01 MH, because that uses up all elements
  • Typical model:
    • 2/3 of carbon used for carbonaceous material
    • Essentially all Mg, Fe, Si and 20% of O in (Mg,Fe)2SiO4
    • Some SiC
  • Recent lowering of elemental abundances may require even larger fraction of carbon in dust
extinction curves
Extinction curves
  • Usually extinction curves are only shown in optical/near-UV
  • 2175 Å bump seen for all lines of sight, but with varying height
  • Standard curve works well on average in diffuse clouds, but many deviations occur for individual lines of sight
  • Extinction curve in dense clouds different in infrared due to presence of ices
modeling of extinction curve
Modeling of extinction curve
  • Observed range of extinction curves can be represented by single-parameter family of functions with RV = AV / E(B–V)
  • Typically RV 3.1
  • Major modeling problem is to “build up” an extinction curve from a collection of particles with scattering / absorption properties as discussed in Sect. 11.2 and composition as described in Sect. 11.3, by choosing appropriate size distribution and refractive index

Solution is not unique!!!

11 5 grain models and size distribution constraints for diffuse clouds
11.5 Grain models and size distributionConstraints for diffuse clouds
  • Observations of extinction and polarization at visible ll indicate that there are at least two kinds of grains with a 0.1 mm
  • Rapid rise of extinction towards UV suggests presence of small particles with a  100 Å
  • Grains must consist of cosmically abundant elements (C, O, Mg, Si, Fe)
1 mathis rumpl nordsieck dust model
1.Mathis-Rumpl-Nordsieck dust model
  • Power law size distribution of graphite and silicate grains; approximately equal numbers
  • amax 0.25 mm from fit to vis and near-IR curve
  • amin 0.005 mm from fit to UV curve (uncertain)
  • MRN power law has most mass in large particles, most area in small particles:
extension of mrn model
Extension of MRN model

Collect bare carbon + silicates into composites

2 core mantle model
2. Core-Mantle Model

Hong & Greenberg 1980

Chlewicki & Greenberg 1984

Li & Greenberg 1997

  • Three populations:
    • Large grains ( 0.12 m)  VIS extinction
    • Small carbonaceous particles (< 0.01 m)  2175 Å bump
    • Small silicates (< 0.01 m)  VUV extinction
  • Large grains consist of silicate core with radius 0.05 mm surrounded by organic refractory material produced by UV processing of simple ices like H2O, CO, CH3OH, H2CO, …
comparison size distributions
Comparison size distributions

Core mantle model has much flatter distribution

11 6 additional ingredients dust models 1 polycyclic aromatic hydrocarbons pahs
11.6 Additional ingredients dust models1. Polycyclic Aromatic Hydrocarbons (PAHs)
  • The smallest “graphite” particles are molecules known as “PAH”s (= Polycyclic Aromatic Hydrocarbons)
  • These are essentially collection of benzene rings but can also be viewed as fragments of graphite sheets with hydrogen atoms at the edge
  • PAHs show characteristic emission features at 3.3 mm, 6.2 mm, 7.7 mm, … (the so-called Unidentified InfraRed bands), which have been observed in spectra of reflection nebulae, H II regions, AGB stars, local and high-redshift galaxies
pahs are everywhere
PAHs are everywhere!

Peeters et al. 2004

2 diffuse interstellar band carriers dibs
2. Diffuse Interstellar Band carriers (DIBs)
  • 150 DIBs known in range 4,000 … 10,000 Å
  • Lines have broader profiles than those due to simple molecules such as CH, CH+, …
  • Possible carriers (> 100 suggestions in literature):
    • Small dust grains
    • Gas phase molecules
    • Impurities embedded in grains
  • Current favorites: large gas phase molecules (PAHs, carbon chains, C60+)
  • C60 with “Soccer-ball” structure and similar molecules
  • First studied as candidates for interstellar molecules
  • Synthesized in lab as third form of solid carbon (besides diamonds and graphite)  enormous number of applications
  • 1996 Chemistry Nobel prize (Curl, Kroto, Smalley)
structure of c 60
Structure of C60
  • Each C atom connected to other atoms by one double and two single bonds
  • Only rings with five or six members
  • Five-rings completely surrounded by six-rings
  • Closed-shell electronic structure
two dibs possibly associated with c 60
Two DIBs possibly associated with C60+

HD 183143

Foing & Ehrenfreund 1997

circumstellar diamonds
Circumstellar Diamonds
  • ISO spectra of two pre-main-sequence stars
  • Lower traces in each panel are absorption spectra of diamond nanocrystals measured in the laboratory




3 ices in cold dark clouds
3. Ices in cold dark clouds
  • Grains may coagulate  alter size distribution
  • Grains may acquire mantles of molecular ices consisting of mix of H2O, CO, CO2, CH3OH, …
  • This is evidenced by absorption bands due to solid-state features in dense clouds towards embedded and background IR sources
infrared absorption
Infrared: absorption

Background star

Embedded young star


Continuum due to hot dust

Absorption by cold dust


Infrared: vibrational transition of gases and solids

main ice absorption bands
Main ice absorption bands
  • 3.1 mm band: amorphous, impure H2O ice
  • 4.27 mm band: CO2 stretching
  • 4.6 mm band: CN stretch (XCN=OCN– )
  • 4.67 mm band: CO
  • 6.0 mm band: H2O bending
  • 6.8 mm band: unidentified (NH4+?)
  • 15 mm band: CO2 bending
  • + several weaker bands due to CH3OH, H2CO, HCOOH, CH4, NH3, OCS + isotopes
ISO: Full inventory of ices high-mass protostars

Ices as abundant as gas-phase CO (~10-4)=> major component

Gibb et al. 2000

differences between solid state and gas phase
Differences between solid state and 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 and can range from ~1 cm-1 (pure CO ice) to 300 cm-1 (amorphous H2O ice)
  • Line shifting
    • Interactions of molecules with surroundings modify bond force constants  shift of vibrational frequency
      • E.g., CO ice shifted by ~5 cm-1 from CO gas phase
observations gaseous and solid co
Observations gaseous and solid CO

CO ice

CO gas



Pontoppidan et al 2003

4 crystalline silicates
4. Crystalline silicates
  • ISO satellite has discovered many new solid-state features at 10 … 100 mm in the spectra of AGB-stars, post-AGB stars, PNe, and YSO disks
  • Broader than gas-phase bands, but narrower than amorphous silicate bands at 9.7 and 18 mm
  • Comparison with laboratory  due to mix of crystalline silicates and oxides
    • (Mg,Fe)SiO4: olivines
    • (Mg,Fe)SiO3: pyroxenes
    • Al2O3, FeO: oxides
crystalline vs amorphous solids
Crystalline vs. amorphous solids

Tielens & Allamandola 1987

crystallin e silicates in disks
Crystalline silicates in disks

ISO: Herbig Ae young stars

Malfait, Waelkens et al. 1998

  • Note similarity with spectrum comet Hale-Bopp
11 7 dust formation and destruction a formation
11.7 Dust formation and destructiona. Formation
  • Time scale to grow 0.1 mm grain in dense interstellar clouds very long  formation in outflowing gas from stars
  • n  1013 cm–3, T  1,000 K  thermodynamic equilibrium chemistry  form most stable molecules
  • If partial pressure molecule > vapor pressure  solid condenses out
    • Can be described in first order by classical nucleation theory
  • Once solid particles formed  blown into ISM by radiation pressure from star

Cluster of i


Density of

Single atom

Free energy

sources of dust
Sources of dust

Tielens 1999

b dust destruction
b. Dust destruction
  • Evaporation: if T increased from 10 K  100 K, the icy mantles evaporate, but refractory materials unaltered
  • Sputtering: occurs in shocks; high energy atoms can knock lattice atoms out of lattice  destruction of refractory material of small grains
  • Grain-grain collisions: powerful collisions between grains at high speeds in shocks can destroy large grains
destruction vs formation
Destruction vs. formation
  • Destruction time scale  number of shocks in ISM  supernova rate
    • Destruction time scale estimate ~4x108 yr
    • Formation time scale estimate ~1.5x109 yr
  • Because formation time scale is longer than destruction time scale, it is argued that some refractory material must be made in ISM itself
  • See Tielens ( Chap. 13 and 1999, Erice summer school) for excellent summaries
11 8 grain heating and cooling
11.8 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
radiative heating of grains
Radiative heating of grains
  • Absorption of photon  grain in excited state
  • Probability A  107 s–1 of spontaneously emitting a photon
  • Complex molecules or grains with many energy levels can convert part of electronic energy into vibrational energy on time scale t  10–12 s
  • This energy is quickly distributed over all internal degrees of freedom  heat
  • At  10–5<< 1  every photon absorption results in heating of grain
grain heating rate
Grain heating rate

where ul=radiation density = blackbody x dilution factor W:

grain temperature heating cont d
Grain temperature heating (cont’d)
  • Here Qabs is averaged over Planck function:
  • And use Stefan Boltmann law:
grain cooling rate
Grain cooling rate
  • Cooling rate = rate of heating by blackbody with T=Tgrain
heating and cooling balance
Heating and cooling balance
  • Steady-state:
  • Grain temperature:
grain temperature example
Grain temperature: example
  • a = 0.1 mm graphite grain heated by starlight
  • Qabs(l)  1.5 for 1000 Å  l  8000 Å
  • For l  50 m graphite has
  • For T 100 K:
  • Thus

For W*=1.5x10-13 and T*=5000 K

dust emission diffuse ism
Dust emission diffuse ISM

[C II]


Td=17.5 K

continuous vs impulsive heating
Continuous vs. impulsive heating
  • See Sect. 11.5 for temperature fluctuations small grains/large molecules
  • Large grain in ISM has Tgr~20 K => emission peaks around 145 mm => expect very little emission at <75 mm
  • IRAS observed substantial emission from cool dust clouds at 12, 25 and 60 mm, in addition to 100 mm. This points to the existence of very large numbers of very small grains which are heated impulsively
  • The ratio of 100/60 mm emission is often used to derive grain temperature. If small grains dominate the 60 mm emission, the derived temperature will be larger than the temperature characterizing the large particles only
ultrasmall grains
Ultrasmall grains
  • Observational evidence that very small grains are even more numerous than suggested by MRN size distribution:
    • Strength of diffuse IR emission from reflection nebulae at 2 mm < l < 25 mm is difficult to understand unless grains are much hotter than expected from balancing average heating and cooling
  • Temperature fluctuations are expected for very small grains due to discrete nature of heating by photons (see Sect. 11.10)
temperature fluctuations of a small grain a 5 nm
Temperature fluctuations of a small grain (a = 5 nm)
  • Heating by individual visible photons
  • Cooling by many far-IR photons
  • Each spike is labeled by photon energy in eV
  • Time between spikes is one hour
  • Observed emission suggests at 5% of grain mass must be in very small

particles with a~5-50 Å

11 9 dust masses from fir submm observations
11.9 Dust masses from FIR/submm observations
  • Dust emission
  • Dense clouds:
    • IR: tdust>>1 => info on Tgr
    • FIR/submm: tdust<<1 =>
  • Thus, FIR/submm gives information on dust mass, but need to know Qabs(n)
  • Empirically Qabs(n)nb with b=1-2