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

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

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Aλ ≈ τ λ

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

The densest ISM: Molecular clouds, birthplace of stars

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

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

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

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

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

A diatomic molecule modeled as a “rigid rotor”

Energy is rotational KE

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E = L2

2I

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L = l(l +1)h

Rotational energy levels of a diatomic molecule:

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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

Rotational energy levels for a diatomic molecule

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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

Vibrational energy levels

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En = n + 12

⎛ ⎝ ⎜

⎞ ⎠ ⎟hω n = 0,1,2,...

n = vibrational quantum number

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

Vibrational and rotational energy levels

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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

Rotation-vibration spectrum of HCl

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Radio Spectrum of a molecular cloud