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The Interstellar Medium and Interstellar Molecules. Ronald Maddalena National Radio Astronomy Observatory. 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
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The Interstellar Medium and Interstellar Molecules Ronald Maddalena National Radio Astronomy Observatory
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
Non-Thermal Continuum RadiationFree-Free Emission • Ionized regions (HII regions and planetary nebulae) • Free electrons accelerated by encounters with free protons
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)
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.
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
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
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
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+ -- 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
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)
