Chapter 14 The Interstellar Medium. All of the material other than stars, planets, and degenerate...
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Transcript of Chapter 14 The Interstellar Medium. All of the material other than stars, planets, and degenerate...
![Page 1: Chapter 14 The Interstellar Medium. All of the material other than stars, planets, and degenerate objects Composed of gas and dust ~1% of the mass of.](https://reader036.fdocuments.net/reader036/viewer/2022062401/5a4d1b627f8b9ab0599ae1da/html5/thumbnails/1.jpg)
Chapter 14
The Interstellar Medium
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• All of the material other than stars, planets, and degenerate objects
• Composed of gas and dust
• ~1% of the mass of the galaxy
• Site of star formation
ISM
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Evidence for Interstellar Dust:
Interstellar extinction: scattering, absorption, reflection
Dust grains scatter and absorb background starlight
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mλ = Mλ + 5log d10pc ⎛ ⎝ ⎜
⎞ ⎠ ⎟+ Aλ
€
Aλ ≈ τ λ
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Evidence for dust:Interstellar reddening
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• Longer wavelengths pass through• Shorter (bluer) light is more easily scattered
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Interstellar extinction curve
2200 A bumpo
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The densest ISM: Molecular clouds, birthplace of stars
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Gas and dust collect into clouds:
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Dust radiates in the thermal infrared
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Phases of the ISM
Compare the pressures (P=nkT) of each of these phases!
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Comparing the pressures of different phases of ISM
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The Milky Way at 100 microns
Warm dust traces out the ISM
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Observing cold neutral hydrogen, HI (“H-one”)
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Interaction between electron spin and nuclear spin
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21-cm emission from the entire Milky Way
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Spectroscopy of molecular clouds
• First detected in 1937 !!!• 146 compounds have been identified in the ISM as of
June 2006• Most are organic (contain C and at least one atom
other than O)• List does not include deuterated species or ions• Good reference:
http://www.cv.nrao.edu/~awootten/allmols.html• More than half originally detected in Sgr B2 (massive
star forming region near galactic center)• My personal faves: glycine, ethanol, acetic acid !!
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Compounds are identified by their rotational spectrum
Sgr B2
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The rich molecular spectrum of Sgr B2
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Beam-averaged column densities 〈 NT 〉 determined from Sgr B2(N-LMH) interferometric measurements: acetic acid (CH3COOH); formic acid (HCOOH); acetone ((CH3)2CO); ethyl
cyanide (CH3CH2CN); and methyl formate (HCOOCH3).
[Reproduced with permission from Snyder et alSnyder L E PNAS 2006;103:12243-12248
©2006 by National Acadey of Sciences
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Molecular spectra
Three main types of transitions emit photons (corresponding to specific spectral lines):
1. Electronic • Hot gases• highest E photons: ~ few eV• ~ visible, UV
2. Vibrational • For gas phase molecules, always comes with
rotation• Solids have pure vibrational spectra• ~ IR
3. Rotational1. Lowest E photons2. ~ radio, microwave (mm to m)3. Cold gas-phase molecules
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Rotational spectra
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A diatomic molecule modeled as a “rigid rotor”
Energy is rotational KE
€
E = L2
2I
€
L = l(l +1)h
Rotational energy levels of a diatomic molecule:
€
E l = l(l +1) h2
2I (l = 0,1,2,...)
Diatomic molecules
l = (rotational) angular momentum quantum #. Our book uses J
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mr = m1m2
m1 + m2
How to find momentum of inertia, I
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I = mrr02
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Rotational energy levels for a diatomic molecule
€
E l = l(l +1) h2
2I (l = 0,1,2,...)
**Our book uses J instead of l
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B = h2
2I
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ΔJ = ±1
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Vibrational energy levels
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En = n + 12
⎛ ⎝ ⎜
⎞ ⎠ ⎟hω n = 0,1,2,...
n = vibrational quantum number
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Vibrational levels for a diatomic molecule (harmonic oscillator):
• Equally spaced
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Formation of interstellar molecules
• Hydrogenated (H2O, CH4, NH3)• CO, CO2, N2, etc
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Vibrational and rotational energy levels
€
Δl = ±1
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Enl = l(l +1) h2
2I+ (n + 1
2)hω
Selection rules:
If vibrational level changes, • n must increase by 1 if a photon
is absorbed • n must decrease by 1 if a photon
is emitted
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Rotation-vibration spectrum of HCl
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Rotation-vibration spectrum of HCl
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Radio Spectrum of a molecular cloud