Ay 102 Physics of the Interstellar Medium supplemental material Hillenbrand – Winter Term 2019-2020
Dust in the Interstellar Medium and in Circumstellar Environments Dust grains can be collected from the upper atmosphere and analyzed for chemical composition. Most samples have been processed in the solar system over the past 4.5 Gyr. But some are inferred to be pre-solar , i.e. interstellar in origin.
Dust Around Other Stars Circumstellar dust located in “debris disks” that are maintained by planets
Dust Emission in the Galaxy (350 um)
Dust in Other Galaxies
Dust Emission Spectrum Strong spectral lines caused by UV heating of small grains, which then radiate like large molecules. Dominant cool continuum component. Also superposed in this figure are WNM gas lines.
Dust Grain Sizes Range of particle sizes with n(a) ~ a -3.5 - <a> is 0.05 um - Most of mass is in large grains; most of surface area in small grains. “very small grains” ? OR σ g = π a 2 σ π very large molecules ? λ λ λ so Q è constant. PAHs = polycyclic aromatic hydrocarbons
Dust Absorption and Scattering of Starlight together, called “extinction”
A Hole in the Sky ??
The thermal emission of dust and the obscuration of starlight (by dust) are anti-correlated. More extinction for colder, denser clouds.
Extinction, Optical Depth, Column Density Dust absorbs , heats up, and then re- • radiates photons at longer λ , and it scatters them out of the line of sight. Extinction = absorption + scattering • Exact relationship between extinction • and wavelength depends on grain size distribution and composition. Stars people quote in terms of A V or • “extinction” at optical wavelengths. ISM people prefer τ (optical depth) or N (column density) .
Dust Extinction with Wavelength (as Observed!) xray For wavelengths >1 μm, ultraviolet A λ α 1/ λ shape is nearly invariant with direction on the sky. optical For wavelengths <0.5 μm, significant variations in infrared shape for different lines of sight. This tells us about dust grain sizes and some basics of composition.
xray Dust Extinction General Features ultraviolet A λ α 1/ λ optical • Rise in A λ from infrared to the ultraviolet. • Several prominent “broad” features: infrared • 0.2175 μm: (a.k.a. “the 2200A°bump”) attributed graphite or PAH grains. • optical and near-infrared bands called DIBS = Diffuse Interstellar Bands, whose exact carriers are still largely unidentified. • 3.4 μm: C-H bond stretching in hydrocarbons (weak). • 18, 9.7 μm: O-Si-O bond bending and Si-O stretching within amorphous silicate grains. • All of the above features can be in absorption depending on the radiative transfer.
Dust Extinction “Law” Details mid-infrared CH, SiO, SiO2 These are stretching and bending (e.g. vibrational motions) of specific molecular bonds within dust grains. optical “DIB” features
A. Glassgold Dust Composition - Silicates
Dust Composition - Carbons
Responsible for 5-20% of the entire mid-infrared dust SED! Hydrogenated Carbon
Dust è Ices In high extinction cold environments such as dense molecular clouds and circumstellar disks, ices solids can be present -- often as mantles to silicate/graphite dust interior (recall the movie!). CO ice implies T < 17 K.
Dust Extinction “Law” Details – Variations emphasizing the near-UV (by plotting 1/lambda) large grains è steeper R and thus shallower A λ /A V 0.5 um B. Draine
Dust Extinction “Law” Details - Variations emphasizing the near-UV (by plotting 1/lambda) R V = 2.7 low metals è shallow R and thus steeper A λ /A V 0.5 um R. Mushotzky
Dust Extinction = Absorption Plus Scattering Q λ = σ λ / π a 2 J. Williams
Examples of Dust Scattering B. Draine
Where Does the Light Go? ultraviolet xray optical Absorption + mid-infrared Scattering Osterbrock& Ferland
Dust Extinction “Law” Details 25 ultraviolet xray optical emphasizing the xray dust has lots of metals. metals have lots of e - è ionization edges in xray directly from the K-shell e - in heavy elements Figure 9: Extinction and scattering calculated for Weingartner & Draine (2001a) model for R V = 3 . 1 Milky Way dust, but with abundances reduced by factor 0.93 (see text).
Do Don’ n’t forge get that hat the here is gas absorption/scattering gas of photons to too, not just dust. We are ignoring gas this week, but it’s there! T. Montmerle
Scattering Basics • Scattering can be broadly defined as the redirection of radiation out of the original direction of propagation, usually due to interactions with solid or gaseous particles. • Reflection, refraction, diffraction etc. are just different forms of scattering. • Matter is composed of discrete electrical charges (atoms, molecules, charged grains – dipoles). • Light is an oscillating electromagnetic field – excites charges, which radiate EM waves. • The radiated EM waves are scattered waves, excited by a source external to the scatterer. • What is observed is the superposition of incident and scattered EM waves. • Types of scattering: slide material from S. Carn
slide material from S. Carn
K. Wood (i.e. small particles)
A Generalized Scattering Phase Function K. Wood Henyey & Greenstein defined the most commonly used phase function: Just a single parameter: g! A typical assumption is that g = <cos θ > = 0.6 (forward scattering)
x is related to g slide material from S. Carn
Define x = 2 π a / λ slide material from S. Carn
In practice the phase functions are used in a Monte Carlo sense, as probability distributions for which direction the photons go. Define x = 2 π a / λ
Mie Theory for Dust When 2 π a / λ is small, When 2 π a / λ is small, absorption: Q λ, abs α a / λ scattering: Q λ, scat α a 4 / λ 4 so cross section σ λ α a 3 / λ so cross section σ λ α a 6 / λ 4 When 2 π a / λ is large, thus Q ext = 2
Mie Theory for Dust Both absorption Q_abs and scattering Q_scat components contribute to the total Q_ext(λ, a) Recall, by definition Q λ = σ λ / π a 2 scattering geometric Mie
J. Williams What About Grain Size Effects? Here, we are fixing the particle size a and looking at how Q varies with (inverse) wavelength λ
J. Williams What About Grain Size Distribution Effects?
J. Williams What About Grain Size Distribution Effects?
J. Williams What About Grain Size Distribution and Min/Max Effects?
J. Williams Other Possible Effects?
So What Do we Need to Explain the Extinction “Law”? • Mix of grain sizes • Mix of grain composition Desert, Boulanger, & Puget (1990)
Dust Grain Sizes <a> is 0.05 microns, but there is a range of particle sizes from 0.005 to 0.25 micron in the diffuse ISM (can grow to bigger mm grains and cm size “pebbles” in denser circumstellar disks). n(a) ~ a -3.5 so most of mass in large grains while most of surface area in small grains.
Dust Grain Sizes: Empirical Constraints
Dust Grain Sizes: Range Desert, Boulanger, & Puget (1990)
Big grains is the place where chemistry happens! figure from W.-F. Thi
B. Draine
Thermal Balance è Dust Temperature for blackbody case of perfect absorber and perfect emitter. J. Williams
Thermal Balance è Dust Temperature for non-blackbody case of imperfect absorber and/or imperfect emitter. J. Williams
Grain Emissivity
NOTE: WE HAVE NOT COVERED POLARIZATION IN CLASS, AND YOU ARE NOT RESPONSIBLE FOR THE TOPIC, BUT FYI SOME BASIC MATERIAL INTRODUCING IT FOLLOWS
Dust Polarization of Background Starlight
A. Goodman
A. Goodman
A. Goodman
Dust Polarization of Starlight B. Draine
Dust Polarization B. Draine
Dust Scatters And Polarizes (optical)
Dust Emits (infrared) Scatting and polarization in infrared too, but these are much weaker effects compared to absorption.
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