Transcript Document
The Interstellar Medium and
Interstellar Molecules
Ronald Maddalena
National Radio Astronomy Observatory
Interstellar Medium
The Material Between the Stars
Constituents:
Gases:
Dust Particles
Hydrogen (92% by number)
Helium (8%)
Oxygen, Carbon, etc. (0.1%)
1% of the mass of the ISM
Average Density: 1 H atom / cm3
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Interstellar Medium
Properties
State of H & C
Temperature
Densities
(H/cm3)
Percent
Volume
HII Regions &
Planetary
Nebulae
H, C Ionized
5000 K
0.5
< 1%
Diffuse ISM
H, C Ionized
1,000,000 K
0.01
50%
Diffuse Atomic
H2 < 0.1
C Ionized
30-100 K
10-100
30%
Diffuse
Molecular
0.1 < H2 < 50%
C+ > 50%
30-100 K
100-500
10%
Translucent
Molecular
H2 ~ 1
C+ < 0.5, CO < 0.9
15-50 K
500-5000?
Small
Dense
Molecular
H2 ~ 1
CO > 0.9
10-50 K
> 104
10%
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Interstellar Medium
Properties
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Interstellar Medium – Life Cycle
Planetary Nebula
and HII Regions
Non-Thermal Continuum Radiation
Free-Free Emission
Ionized regions (HII
regions and planetary
nebulae)
Free electrons
accelerated by
encounters with free
protons
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Spectral-Line Radiation
Recombination Lines
Discovered in 1965 by
Hogburn and Mezger
Ionized regions (HII
regions and planetary
nebulae)
Free electrons
temporarily recaptured
by a proton
Atomic transitions
between outer orbital
(e.g., N=177 to M =
176)
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1
1
3.3 10 2 2
m n
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Spectral-Line Radiation
Hyperfine transition of Hydrogen
Discovered by Ewen and Purcell in 1951.
Found in regions where H is atomic.
Spin-flip (hyperfine) transition
Electron & protons have “spin”
In a H atoms, spins of proton and electron may be aligned or
anti-aligned.
Aligned state has more energy.
Difference in Energy = h v
v = 1420 MHz
An aligned H atom will take 11 million years to flip the spin of
the electron.
But, 1067 atoms in Milky Way
1052 H atoms per second emit at 1420 MHz.
Atomic Hydrogen
Interstellar Molecules
Hydroxyl (OH) first molecule found with radio
telescopes (1964).
Molecule Formation:
Need high densities
Lots of dust needed to protect molecules for stellar UV
But, optically obscured – need radio telescopes
Low temperatures (< 100 K)
Some molecules (e.g., H2) form on dust grains
Most form via ion-molecular gas-phase reactions
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Exothermic
Charge transfer
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Interstellar Molecules
About 90% of the over 130 interstellar
molecules discovered with radio telescopes.
Rotational (electric dipole) Transitions
Up to thirteen atoms
Many carbon-based (organic)
Many cannot exist in normal laboratories (e.g.,
OH)
H2 most common molecule:
No dipole moment so no radio transition.
Only observable in UV (rotational)
Astronomers use CO as a tracer for H2
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Molecular Clouds
Discovered 1970 by Penzias, Jefferts, & Wilson and
others.
Coldest (5-30 K), densest (100 –106 H atoms/cm3)
parts of the ISM.
Where stars are formed
25-50% of the ISM mass
A few percent of the Galaxy’s volume.
Concentrated in spiral arms
Dust Clouds = Molecular Clouds
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Discovery of Ethanol
Molecules Discovered by the GBT
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Grain Chemistry
Ion-molecular gas-phase reactions
Ion-molecular gas-phase reactions
Examples of types of reactions
C+ + H2 → CH2+ + hν
(Radiative Association)
H2+ + H2 → H3+ + H
(Dissociative Charge Transfer)
H3+ + CO → HCO+ + H2
(Proton Transfer)
H3+ + Mg → Mg+ + H2 + H
(Charge Transfer)
He+ + CO → He + C+ + O
(Dissociative Charge Transfer)
HCO+ + e → CO + H
(Dissociative)
C+ + e → C + hν
(Radiative)
Fe+ + grain → Fe + hν
(Grain)
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Importance of H3+
Importance of H3+ -- Recent results
First detected in 1994 in the infrared
Creation:
Destruction
H3+ + e → H + H2 or 3H
New laboratory measurements for reaction rates
H2 + cr → H2+ + e
H2 + H 2+ → H 3+ + H
Dense Molecular clouds – expected and measured H3+ agree
Diffuse Molecular clouds – measured H3+ is 100x higher than expected
Cosmic ray ionization rate has to be higher in diffuse clouds
than in dark clouds. Why?
Confinement of cr in the diffuse molecular clouds
Higher number of low energy cr than in current theory and which can’t
penetrate dark clouds
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Maser Emission
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Spectral-Line Radiation
Milky Way Rotation and Mass?
For any cloud
For cloud B
Observed velocity = difference
between projected Sun’s
motion and projected cloud
motion.
The highest observed velocity
along the line of site
VRotation = Vobserved + Vsun*sin(L)
R = RSun * sin(L)
Repeat for a different angle L
and cloud B
Determine VRotation(R)
From Newton’s law, derive
M(R) from V(R)
Massive Supernovae
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Missing Mass
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Prebiotic Molecules
The GBT and ALMA
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