Radio Astronomy Emission Mechanisms

NRAO/AUI/NSF3 Omega nebula

NRAO/AUI/NSF4 Recipe for Radio Waves Thermal Continuum Radiation (Black Body Radiation)

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7 Thermal or Black Body Emission

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:

NRAO/AUI/NSF9 Recipe for Radio Waves Non-Thermal Continuum Radiation Whenever a charge particle is accelerated 1.Free-Free Emission Hot (5000 K) Ionized GasesHot (5000 K) Ionized Gases Planetary NebulaePlanetary Nebulae HII RegionsHII Regions

NRAO/AUI/NSF10 Electron accelerates as it passes near a proton. Electromagnetic waves are released

Planetary Nebula and HII Regions

NRAO/AUI/NSF12 Free free emission

NRAO/AUI/NSF13 Non-Thermal Continuum Radiation Whenever a charge particle is accelerated 1.Free-Free Emission 2.Synchrotron Radiation Strong magnetic fieldStrong magnetic field Ionized gases moving at relativistic velocitiesIonized gases moving at relativistic velocities Recipe for Radio Waves

NRAO/AUI/NSF14 Electrons accelerate around magnetic field lines Electromagnetic waves are released

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NRAO/AUI/NSF21 Spectral Line Radiation Atomic and molecular transitions Recipe for Radio Waves

NRAO/AUI/NSF22 Gas Spectra Neon Sodium Hydrogen

11/4/ Spectral-Line Radiation Recombination 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)

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 and electron may be aligned or anti-aligned. –Aligned state has more energy. –Difference in Energy = h * frequency Frequency = MHz –An aligned H atom will take 11 million years to flip –But, atoms in Milky Way H atoms per second emit at 1420 MHz. NRAO/AUI/NSF24

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NRAO/AUI/NSF27 Doppler Shift

NRAO/AUI/NSF28 Doppler Shift c = speed of light = 3 x 10 5 km/sec Rest Frequency = MHz for the hyperfine transition of Hydrogen If V > 0, object is moving away from us If V < 0, object is moving toward us.

11/4/ Spectral-Line Radiation What do they tell us? Frequency of center of line  Object’s velocity along the line of site –Doppler Effect 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.

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)

11/4/ Missing Mass

11/4/ 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) H 2 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 H 2 A few molecules (OH, H 2 O, …) maser

11/4/ Molecules Discovered by the GBT

Discovery of Ethanol

11/4/ Interstellar Molecule Formation Need high densities (100 –10 6 H atoms/cm 3 ) –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., H 2 ) form on dust grains Most form via ion-molecular gas-phase reactions –Exothermic –Charge transfer

11/4/ Molecular Clouds Coldest (5-30 K), densest (100 –10 6 H atoms/cm 3 ) parts of the ISM. Where stars are formed 50% of the ISM mass A few percent of the Galaxy’s volume. Concentrated in spiral arms Dust Clouds = Molecular Clouds

Grain Chemistry

11/4/ 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)

H 3 + Drives Most Interstellar Chemistry

NRAO/AUI/NSF43 Organic Molecules; Seeds of Life Organic Molecules; Seeds of Life