Chapter 14
The Interstellar Medium
ISM
• 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
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Evidence for Interstellar Dust:
Interstellar extinction: scattering, absorption, reflection
Dust grains scatter and
absorb background
starlight
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 d 
m  M   5log 
 A
10pc 

A   
Evidence for dust:
Interstellar reddening
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•
•
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Longer wavelengths pass through
Shorter (bluer) light is more easily scattered
Interstellar extinction curve
o
2200 A bump
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The densest
ISM:
Molecular
clouds,
birthplace of
stars
Gas and dust collect into clouds:
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Dust radiates in the thermal infrared
Phases of the ISM
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Compare the pressures (P=nkT) of each of these phases!
Comparing the pressures of different phases of ISM
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The Milky Way at 100 microns
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Warm dust traces out the ISM
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
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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 !!
Compounds are identified by their rotational spectrum
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Sgr B2
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 al
Snyder L E PNAS 2006;103:12243-12248
©2006 by National Acadey of Sciences
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. Rotational
• Lowest E photons
•  ~ radio, microwave (mm to m)
• Cold gas-phase molecules
Rotational spectra
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Diatomic molecules
A diatomic molecule
modeled as a “rigid rotor”
Energy is rotational KE
2
L
E
2I
L  l(l 1)

Rotational energy levels
of a diatomic molecule:

l = (rotational) angular
momentum quantum #.
Our book uses J
E l  l(l  1)
2
2I
(l  0,1,2,...)
How to find momentum of inertia, I
m1m2
mr 
m1  m2


I  mr r0
2
Rotational energy levels for a diatomic molecule
E l  l(l  1)
2
2I
(l  0,1,2,...)

**Our book uses J instead of l
B
2
J  1
2I

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Vibrational energy levels
 1 
E n  n    n  0,1,2,...
 2 
n = vibrational
quantum number

Vibrational levels for a diatomic molecule
(harmonic oscillator):
•
Equally spaced
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Formation of interstellar molecules
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Hydrogenated (H2O, CH4, NH3)
CO, CO2, N2, etc
Vibrational and rotational
energy levels
2
1
E nl  l(l  1)  (n  ) 
2I
2
Selection rules:


l  1
If vibrational level changes,
• n must increase by 1 if a photon
is absorbed
• n must decrease by 1 if a photon
is emitted
Rotation-vibration spectrum of HCl
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Rotation-vibration spectrum of HCl
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Radio Spectrum of a molecular cloud