SLIDE 1 Ay 102 Physics of the Interstellar Medium
supplemental material Hillenbrand – Winter Term 2019-2020
SLIDE 2 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.
SLIDE 3 Dust Around Other Stars
Circumstellar dust located in “debris disks” that are maintained by planets
SLIDE 4
Dust Emission in the Galaxy
(350 um)
SLIDE 5
Dust in Other Galaxies
SLIDE 6 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.
SLIDE 7 Dust Grain Sizes
λ λ λ π σ
- 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 very large molecules ? PAHs = polycyclic aromatic hydrocarbons σg = π a2
so Qè constant.
SLIDE 8 Dust Absorption and Scattering of Starlight
together, called “extinction”
SLIDE 9
SLIDE 10
A Hole in the Sky ??
SLIDE 11 The thermal emission of dust and the
- bscuration of starlight (by dust) are
anti-correlated. More extinction for colder, denser clouds.
SLIDE 12 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 AV or
“extinction” at optical wavelengths. ISM people prefer τ (optical depth) or N (column density).
SLIDE 13 Dust Extinction with Wavelength (as Observed!)
infrared
ultraviolet
For wavelengths >1 μm, shape is nearly invariant with direction on the sky. For wavelengths <0.5 μm, significant variations in shape for different lines of sight. This tells us about dust grain sizes and some basics of composition.
Aλ α 1/λ
xray
SLIDE 14 Dust Extinction General Features
- Rise in Aλ from infrared to the ultraviolet.
- Several prominent “broad” features:
- 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.
infrared
ultraviolet
Aλ α 1/λ
xray
SLIDE 15 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.
SLIDE 16
Dust Composition
SLIDE 17 Dust Composition
SLIDE 18 Hydrogenated Carbon
Responsible for 5-20% of the entire mid-infrared dust SED!
SLIDE 19
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.
SLIDE 20 Dust Extinction “Law” Details – Variations
0.5 um
emphasizing the near-UV (by plotting 1/lambda) large grains è steeper R and thus shallower Aλ/AV
SLIDE 21 Dust Extinction “Law” Details - Variations
0.5 um
emphasizing the near-UV (by plotting 1/lambda) low metals è shallow R and thus steeper Aλ/AV
RV = 2.7
SLIDE 22
SLIDE 23 Dust Extinction = Absorption Plus Scattering
Qλ= σλ / π a2
SLIDE 24 Examples of Dust Scattering
SLIDE 25 Where Does the Light Go? Absorption + Scattering
Osterbrock& Ferland mid-infrared
ultraviolet xray
SLIDE 26 Dust Extinction “Law” Details
25 Figure 9: Extinction and scattering calculated for Weingartner & Draine (2001a) model for
RV = 3.1 Milky Way dust, but with abundances reduced by factor 0.93 (see text).
emphasizing the xray dust has lots of metals. metals have lots of e- èionization edges in xray directly from the K-shell e- in heavy elements
ultraviolet
xray
SLIDE 27
Do Don’ n’t forge get that hat the here is gas gas absorption/scattering
too, not just
- dust. We are ignoring gas
this week, but it’s there!
SLIDE 28 Scattering Basics
- Scattering can be broadly defined as the redirection of radiation out of the
- riginal 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 29 slide material from S. Carn
SLIDE 30
SLIDE 31
(i.e. small particles)
SLIDE 32 A Generalized Scattering Phase Function
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)
SLIDE 33 slide material from S. Carn x is related to g
SLIDE 34 slide material from S. Carn Define x= 2π a / λ
SLIDE 35 Define x= 2π a / λ
In practice the phase functions are used in a Monte Carlo sense, as probability distributions for which direction the photons go.
SLIDE 36 Mie Theory for Dust
thus Qext = 2
When 2π a / λ is small, scattering: Qλ, scat α a4/ λ4 so cross section σλ α a6/ λ4 When 2π a / λ is small, absorption: Qλ, abs α a/ λ so cross section σλ α a3/ λ
When 2π a / λ is large,
SLIDE 37 Mie Theory for Dust
Recall, by definition Qλ = σλ / π a2
Mie geometric scattering
Both absorption Q_abs and scattering Q_scat components contribute to the total Q_ext(λ, a)
SLIDE 38
What About Grain Size Effects?
Here, we are fixing the particle size a and looking at how Q varies with (inverse) wavelength λ
SLIDE 39
What About Grain Size Distribution Effects?
SLIDE 40
What About Grain Size Distribution Effects?
SLIDE 41
What About Grain Size Distribution and Min/Max Effects?
SLIDE 42
Other Possible Effects?
SLIDE 43 So What Do we Need to Explain the Extinction “Law”?
- Mix of grain sizes
- Mix of grain composition
Desert, Boulanger, & Puget (1990)
SLIDE 44 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.
SLIDE 45
Dust Grain Sizes: Empirical Constraints
SLIDE 46 Desert, Boulanger, & Puget (1990)
Dust Grain Sizes: Range
SLIDE 47 figure from W.-F. Thi
Big grains is the place where chemistry happens!
SLIDE 49
SLIDE 50
SLIDE 51
Thermal Balance è Dust Temperature
for blackbody case of perfect absorber and perfect emitter.
SLIDE 52
Thermal Balance è Dust Temperature
for non-blackbody case of imperfect absorber and/or imperfect emitter.
SLIDE 53
Grain Emissivity
SLIDE 54
NOTE: WE HAVE NOT COVERED POLARIZATION IN CLASS, AND YOU ARE NOT RESPONSIBLE FOR THE TOPIC, BUT FYI SOME BASIC MATERIAL INTRODUCING IT FOLLOWS
SLIDE 55 Dust Polarization
Background Starlight
SLIDE 59 Dust Polarization
SLIDE 60 Dust Polarization
SLIDE 61
SLIDE 62
Dust Scatters And Polarizes (optical)
SLIDE 63 Dust Emits (infrared)
Scatting and polarization in infrared too, but these are much weaker effects compared to absorption.
