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Radio Astronomy Emission Mechanisms

Radio Astronomy Emission Mechanisms. Recipe for Radio Waves. Thermal Continuum Radiation (Black Body Radiation). Thermal or Black Body Emission. Thermal Continuum Radiation. Characteristics: Opaque “Black” Body Isothermal In Equilibrium Planck’s Law:

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Radio Astronomy Emission Mechanisms

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  1. Radio Astronomy Emission Mechanisms

  2. Recipe for Radio Waves Thermal Continuum Radiation (Black Body Radiation) NRAO/AUI/NSF

  3. NRAO/AUI/NSF

  4. NRAO/AUI/NSF

  5. Thermal or Black Body Emission NRAO/AUI/NSF

  6. Thermal Continuum Radiation • Characteristics: • Opaque “Black” Body • Isothermal • In Equilibrium • Planck’s Law: • I = Intrinsic Intensity (ergs/cm2/sec/Hz). • h = Planck’s Constant • k = Boltzman’s Constant • T in K • ν in Hz • Radio Approximation:

  7. Recipe for Radio Waves • Non-Thermal Continuum Radiation • Whenever a charge particle is accelerated • Free-Free Emission • Hot (5000 K) Ionized Gases • Planetary Nebulae • HII Regions NRAO/AUI/NSF

  8. Electron accelerates as it passes near a proton. Electromagnetic waves are released NRAO/AUI/NSF

  9. Planetary Nebula and HII Regions

  10. Free free emission NRAO/AUI/NSF

  11. Recipe for Radio Waves Non-Thermal Continuum Radiation Whenever a charge particle is accelerated • Free-Free Emission • Synchrotron Radiation • Strong magnetic field • Ionized gases moving at relativistic velocities NRAO/AUI/NSF

  12. Electrons accelerate around magnetic field lines Electromagnetic waves are released NRAO/AUI/NSF

  13. NRAO/AUI/NSF

  14. NRAO/AUI/NSF

  15. NRAO/AUI/NSF

  16. ? NRAO/AUI/NSF

  17. Recipe for Radio Waves Spectral Line Radiation Atomic and molecular transitions NRAO/AUI/NSF

  18. Gas Spectra Neon Sodium Hydrogen NRAO/AUI/NSF

  19. Spectral-Line RadiationRecombination Lines • 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)

  20. Hyperfine Transition of Hydrogen • Found in regions where H is atomic (HI). • Spin-flip transition • Electron & protons have “spin” • In a H atoms, spins of proton andelectron may be aligned or anti-aligned. • Aligned state has more energy. • Difference in Energy = h * frequency • Frequency = 1420.4058 MHz • An aligned H atom will take 11 million years to flip • But, 1067 atoms in Milky Way • 1052 H atoms per second emit at 1420 MHz. NRAO/AUI/NSF

  21. NRAO/AUI/NSF

  22. NRAO/AUI/NSF

  23. Doppler Shift NRAO/AUI/NSF

  24. Doppler Shift • c = speed of light = 3 x 105 km/sec • Rest Frequency = 1420.4058 MHz for the hyperfine transition of Hydrogen • If V > 0, object is moving away from us • If V < 0, object is moving toward us. NRAO/AUI/NSF

  25. 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)

  26. Missing Mass

  27. Interstellar Molecules • About 90% of the over 140 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) or Infrared (vibrational) transitions. • Astronomers use CO as a tracer for H2 • A few molecules (OH, H2O, …) maser

  28. Molecules Discovered by the GBT

  29. Discovery of Ethanol

  30. Interstellar Molecule Formation • Need high densities (100 –106 H atoms/cm3) • Lots of dust needed to protect molecules for stellar UV • Form in dust clouds = Molecular Clouds • Associated with stars formation • 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

  31. Grain Chemistry

  32. 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)

  33. Organic Molecules; Seeds of Life NRAO/AUI/NSF

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