The Interstellar Medium and Interstellar Molecules

Ronald MaddalenaNational 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 TemperatureDensities (H/cm3)

Percent Volume

HII Regions & Planetary

NebulaeH, 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 Ionized30-100 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 K500-

5000?Small

Dense Molecular

H2 ~ 1

CO > 0.910-50 K > 104 10%

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Interstellar Medium Properties

Interstellar Medium – Life Cycle

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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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., N=177 to M = 176)

3 3 1 01 11 5

2 2.m n

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.

Atomic Hydrogen

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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 (< 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 Galaxy’s volume. Concentrated in spiral arms Dust Clouds = Molecular Clouds

Discovery of Ethanol

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Molecules Discovered by the GBT

Grain Chemistry

Ion-molecular gas-phase reactions

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Ion-molecular gas-phase reactionsExamples 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)

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 can’t

penetrate dark clouds

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Maser Emission

Spectral-Line RadiationMilky Way Rotation and Mass? For any cloud

Observed velocity = difference between projected Sun’s motion and projected cloud motion.

For cloud B 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)

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Massive Supernovae

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Missing Mass

Prebiotic Molecules

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The GBT and ALMA

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