1 / 79

ISM Lecture 11

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

saber
Télécharger la présentation

ISM Lecture 11

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. ISM Lecture 11 Interstellar dust

  2. 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”)

  3. Reflection nebula in the Pleiades cluster

  4. 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

  5. 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”??

  6. Definitions

  7. Definitions (cont’d) • g describes the angular redistribution of light

  8. 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

  9. Refractive index and dielectric constant • Refractive index: • Dielectric constant: • Consider homogeneous sphere with isotrpic refractive index m or dielectrc constant e

  10. 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)

  11. 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

  12. Mie efficiency factors for m = 1.27 + 1.37i extinction absorption Qabs~x scattering Qsca~x4 x=2pa/l

  13. Mie scattering efficiency for m = 1.6 and m = 1.6 – 0.05 i Qext =Qsca Qext Qsca

  14. 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

  15. The Purcell limit (cont’d) • gives info on grain volume per H atom • Observations of  / NH from 912 Å  30 m  lower limit Observed

  16. 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:

  17. Candidate constituents for interstellar dust

  18. 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

  19. 11.4 Extinction curve

  20. A ‘standard’ curve?

  21. 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

  22. 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!!!

  23. Varying RV

  24. The Horsehead nebula

  25. 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)

  26. 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:

  27. Decomposition of extinction curve in MRN model Draine & Lee 1984

  28. Extension of MRN model Collect bare carbon + silicates into composites

  29. 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, …

  30. Core-mantle model (cont’d)

  31. Comparison size distributions Core mantle model has much flatter distribution

  32. Summary dust models

  33. 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

  34. PAHs are everywhere! Peeters et al. 2004

  35. Comparison of PAH lab and astronomical spectra Obs Lab Allamandola et al. 1999

  36. Auto exhaust along the Milky (High-?) Way Tielens et al. 1985

  37. Carriers UIR bands

  38. Carbonaceous material

  39. Size distribution

  40. PAHs vs long carbon chains

  41. 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+)

  42. Fullerenes • 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)

  43. 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

  44. Two DIBs possibly associated with C60+ HD 183143 Foing & Ehrenfreund 1997

  45. 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 ‘Diamond’ PAH ‘Diamond’

  46. Forms of carbon likely present in the ISM

  47. 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

  48. Infrared: absorption Background star Embedded young star Flux Continuum due to hot dust Absorption by cold dust Wavelength Infrared: vibrational transition of gases and solids

  49. 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

More Related