The Interstellar Medium and Interstellar Molecules Ronald Maddalena National Radio Astronomy Observatory

10/7/20152 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 / cm 3

10/7/20155 Interstellar Medium Properties State of H & CTemperature Densities (H/cm 3 ) Percent Volume HII Regions & Planetary Nebulae H, C Ionized5000 K0.5< 1% Diffuse ISMH, C Ionized1,000,000 K0.0150% Diffuse Atomic H 2 < 0.1 C Ionized K % Diffuse Molecular 0.1 < H 2 < 50% C + > 50% K % Translucent Molecular H 2 ~ 1 C + < 0.5, CO < K ?Small Dense Molecular H 2 ~ 1 CO > K> %

10/7/20156 Interstellar Medium Properties

Interstellar Medium – Life Cycle

10/7/20159 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 (< few x 10 6 years) stars. HII Regions have short lives. Near regions where they formed. Planetary nebulae Evolved (white dwarf) stars

Planetary Nebula and HII Regions

10/7/ Non-Thermal Continuum Radiation Free-Free Emission Ionized regions (HII regions and planetary nebulae) Free electrons accelerated by encounters with free protons

Thermal Continuum Radiation Characteristics: Opaque “Black” Body Isothermal In Equilibrium Planck’s Law: I = Intrinsic Intensity (ergs/cm 2 /sec/Hz). h = Planck’s Constant k = Boltzman’s Constant T in K ν in Hz Radio Approximation:

10/7/ Non-Thermal Continuum Radiation Synchrotron Radiation Free Electrons Magnetic Fields Discrete Sources Supernovae Remnants General Interstellar Medium I  ν  with  between and –1.2

10/7/ 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)

Spectral-Line Radiation Hyperfine transition of Hydrogen Discovered by Ewen and Purcell in 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, atoms in Milky Way H atoms per second emit at 1420 MHz.

Atomic Hydrogen

10/7/ Spectral-Line Radiation What do they tell us? Number of emitting regions in that direction. Frequency of center of line  Object’s 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.

10/7/ 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., H 2 ) form on dust grains Most form via ion-molecular gas-phase reactions Exothermic Charge transfer

10/7/ 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) H 2 most common molecule: No dipole moment so no radio transition. Only observable in UV (rotational) Astronomers use CO as a tracer for H 2

10/7/ Molecular Clouds Discovered 1970 by Penzias, Jefferts, & Wilson and others. Coldest (5-30 K), densest (100 –10 6 H atoms/cm 3 ) 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

10/7/ Molecules Discovered by the GBT

Grain Chemistry

Ion-molecular gas-phase reactions

10/7/ Ion-molecular gas-phase reactions Examples of types of reactions C + + H 2 → CH hν (Radiative Association) H H 2 → H H (Dissociative Charge Transfer) H CO → HCO + + H 2 (Proton Transfer) H Mg → Mg + + H 2 + 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 H 3 +

10/7/ Importance of H Recent results First detected in 1994 in the infrared Creation: H 2 + cr → H e H 2 + H 2 + → H H Destruction H e → H + H 2 or 3H New laboratory measurements for reaction rates Dense Molecular clouds – expected and measured H 3 + agree Diffuse Molecular clouds – measured H 3 + 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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10/7/ Maser Emission

Spectral-Line Radiation Milky 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 V Rotation = V observed + V sun *sin(L) R = R Sun * sin(L) Repeat for a different angle L and cloud B Determine V Rotation (R) From Newton’s law, derive M(R) from V(R)

10/7/ Massive Supernovae

10/7/ Missing Mass

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Prebiotic Molecules

10/7/ The GBT and ALMA

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10/7/ 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”