The Interstellar Medium and Interstellar Molecules

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The Interstellar Medium and Interstellar
Molecules

• Ronald Maddalena
• National Radio Astronomy Observatory

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Interstellar Medium The Material Between the
Stars

• Constituents
• Gases
• Hydrogen (92 by number)
• Helium (8)
• Oxygen, Carbon, etc. (0.1)
• Dust Particles
• 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 lt 1
Diffuse ISM H, C Ionized 1,000,000 K 0.01 50
Diffuse Atomic H2 lt 0.1 C Ionized 30-100 K 10-100 30
Diffuse Molecular 0.1 lt H2 lt 50 C gt 50 30-100 K 100-500 10
Translucent Molecular H2 1 C lt 0.5, CO lt 0.9 15-50 K 500-5000? Small
Dense Molecular H2 1 CO gt 0.9 10-50 K gt 104 10
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Interstellar Medium Properties
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Interstellar Medium Life Cycle
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HII Regions Planetary Nebulae

• Isolated regions where H is ionized.
• UV from hot (20,000 50,000 K), blue stars
  produces ionization.
• HII Regions
• Young, massive, short-lived (lt few x 106 years)
  stars.
• HII Regions have short lives.
• Near regions where they formed.
• Planetary nebulae
• Evolved (white dwarf) stars

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Planetary Nebula and HII Regions
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Non-Thermal Continuum RadiationFree-Free Emission

• Ionized regions (HII regions and planetary
  nebulae)
• Free electrons accelerated by encounters with
  free protons

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Thermal Continuum Radiation

• Characteristics
• Opaque Black Body
• Isothermal
• In Equilibrium
• Plancks Law
• I Intrinsic Intensity (ergs/cm2/sec/Hz).
• h Plancks Constant
• k Boltzmans Constant
• T in K
• ? in Hz
• Radio Approximation

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Non-Thermal Continuum Radiation

• Synchrotron Radiation
• Free Electrons
• Magnetic Fields
• Discrete Sources
• Supernovae Remnants
• General Interstellar Medium
• I ? ?? with ? between -0.2 and 1.2

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Spectral-Line RadiationRecombination 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.,
  N177 to M 176)

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Spectral-Line RadiationHyperfine 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.

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Atomic Hydrogen
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Spectral-Line RadiationWhat do they tell us?

• Number of emitting regions in that direction.
• Frequency of center of line ? Objects velocity
• Doppler Effect
• Frequency Observed Frequency Emitted / (1
  V/c)
• Width of line ? Motion of gas within the region
• Height of the line ? Maybe temperature of the gas
• Area under the line ? Maybe number of atoms in
  that direction.

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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 (lt 100 K)
• Some molecules (e.g., H2) form on dust grains
• Most form via ion-molecular gas-phase reactions
• 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 Galaxys volume.
• Concentrated in spiral arms
• Dust Clouds Molecular Clouds

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Discovery of Ethanol
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Molecules Discovered by the GBT
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Grain Chemistry
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Ion-molecular gas-phase reactions
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Ion-molecular gas-phase reactionsExamples of
types of reactions
C H2 ? CH2 h?
(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 h?

(Radiative) Fe grain ? Fe h?
(Grain)
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Importance of H3
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Importance of H3 -- Recent results

• First detected in 1994 in the infrared
• Creation
• H2 cr ? H2 e
• H2 H2 ? H3 H
• Destruction
• H3 e ? H H2 or 3H
• New laboratory measurements for reaction rates
• 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 cant penetrate dark clouds

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Maser Emission
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Spectral-Line RadiationMilky Way Rotation and
Mass?

• For any cloud
• Observed velocity difference between projected
  Suns motion and projected cloud motion.
• For cloud B
• The highest observed velocity along the line of
  site
• VRotation Vobserved Vsunsin(L)
• R RSun sin(L)
• Repeat for a different angle L and cloud B
• Determine VRotation(R)
• From Newtons law, derive M(R) from V(R)

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Massive Supernovae
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Missing Mass
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Prebiotic Molecules
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The GBT and ALMA
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Where to Get More Information

• Harwitt Astrophysical Concepts
• Verschuur and Kellerman Galactic and
  Extra-Galactic Radio Astronomy
• Verschuur Invisible Universe Revealed
• Kraus Radio Astronomy
• Burke and Graham Smith Radio Astronomy