1 / 38

Radio Astronomy Emission Mechanisms

Delve into the fascinating world of radio astronomy, exploring emission mechanisms and recipes for radio waves. Discover the mysteries behind thermal continuum radiation and non-thermal continuum radiation through detailed characteristics and processes explained using Planck's Law and free-free emission concepts. Unveil the spectral line radiation and Doppler Shift phenomenon in the vast expanse of space. Learn about Milky Way rotation, missing mass in interstellar molecules, and intriguing discoveries of interstellar molecules like ethanol. Dive into the complex formation processes of interstellar molecules, grain chemistry, and the significance of organic molecules as potential "seeds of life" in the universe.

heathc
Download Presentation

Radio Astronomy Emission Mechanisms

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related