Transcript Document

The Interstellar Medium and
Interstellar Molecules
Ronald Maddalena
National Radio Astronomy Observatory
Interstellar Medium
The Material Between the Stars
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Constituents:
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Gases:
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Dust Particles
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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
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Ionized regions (HII
regions and planetary
nebulae)
Free electrons
accelerated by
encounters with free
protons
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Spectral-Line Radiation
Recombination Lines
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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
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Discovered by Ewen and Purcell in 1951.
Found in regions where H is atomic.
Spin-flip (hyperfine) transition
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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
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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
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1052 H atoms per second emit at 1420 MHz.
Atomic Hydrogen
Interstellar Molecules
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Hydroxyl (OH) first molecule found with radio
telescopes (1964).
Molecule Formation:
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Need high densities
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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
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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:
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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
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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
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First detected in 1994 in the infrared
Creation:
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Destruction
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H3+ + e → H + H2 or 3H
New laboratory measurements for reaction rates
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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?
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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?
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For any cloud
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For cloud B
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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
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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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