Radio Astronomy Emission Mechanisms. NRAO/AUI/NSF3 Omega nebula

Download Radio Astronomy Emission Mechanisms. NRAO/AUI/NSF3 Omega nebula

Post on 04-Jan-2016




0 download

Embed Size (px)


<ul><li><p>Radio Astronomy Emission Mechanisms</p></li><li><p>NRAO/AUI/NSF*Omega nebula</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Recipe for Radio Waves</p><p>Thermal Continuum Radiation(Black Body Radiation)</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Thermal or Black Body Emission</p><p>NRAO/AUI/NSF</p></li><li><p>Thermal Continuum RadiationCharacteristics:Opaque Black BodyIsothermalIn EquilibriumPlancks Law:I = Intrinsic Intensity (ergs/cm2/sec/Hz). h = Plancks Constantk = Boltzmans ConstantT in K in HzRadio Approximation:</p></li><li><p>NRAO/AUI/NSF*Recipe for Radio Waves</p><p>Non-Thermal Continuum RadiationWhenever a charge particle is accelerated Free-Free EmissionHot (5000 K) Ionized GasesPlanetary NebulaeHII Regions</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Electron accelerates as it passes near a proton.Electromagnetic waves are released</p><p>NRAO/AUI/NSF</p></li><li><p>Planetary Nebula and HII Regions</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Non-Thermal Continuum RadiationWhenever a charge particle is accelerated Free-Free EmissionSynchrotron RadiationStrong magnetic fieldIonized gases moving at relativistic velocities</p><p>Recipe for Radio Waves</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Electrons accelerate around magnetic field linesElectromagnetic waves are released</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF* Spectral Line RadiationAtomic and molecular transitionsRecipe for Radio Waves</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Gas SpectraNeonSodiumHydrogen</p><p>NRAO/AUI/NSF</p></li><li><p>**Spectral-Line RadiationRecombination LinesIonized regions (HII regions and planetary nebulae)Free electrons temporarily recaptured by a protonAtomic transitions between outer orbital (e.g., N=177 to M = 176)</p><p>NRAO/AUI/NSF</p></li><li><p>Hyperfine Transition of Hydrogen Found in regions where H is atomic (HI).Spin-flip transitionElectron &amp; protons have spinIn a H atoms, spins of proton and electron may be aligned or anti-aligned. Aligned state has more energy.Difference in Energy = h * frequencyFrequency = 1420.4058 MHzAn aligned H atom will take 11 million years to flipBut, 1067 atoms in Milky Way 1052 H atoms per second emit at 1420 MHz.NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Doppler Shift</p><p>NRAO/AUI/NSF</p></li><li><p>NRAO/AUI/NSF*Doppler Shift</p><p> c = speed of light = 3 x 105 km/sec</p><p> Rest Frequency = 1420.4058 MHz for the hyperfine transition of Hydrogen</p><p> If V &gt; 0, object is moving away from us</p><p> If V &lt; 0, object is moving toward us.</p><p>NRAO/AUI/NSF</p></li><li><p>**Spectral-Line RadiationWhat do they tell us?Frequency of center of line Objects velocity along the line of siteDoppler EffectWidth of line Motion of gas within the regionHeight of the line Maybe temperature of the gasArea under the line Maybe number of atoms in that direction.</p></li><li><p>Spectral-Line RadiationMilky Way Rotation and Mass?For any cloudObserved velocity = difference between projected Suns motion and projected cloud motion.For cloud BThe highest observed velocity along the line of siteVRotation = Vobserved + Vsun*sin(L)R = RSun * sin(L)Repeat for a different angle L and cloud BDetermine VRotation(R)From Newtons law, derive M(R) from V(R)</p><p>NRAO/AUI/NSF</p></li><li><p>**Missing Mass</p></li><li><p>**Interstellar MoleculesAbout 90% of the over 140 interstellar molecules discovered with radio telescopes.Rotational (electric dipole) TransitionsUp to thirteen atomsMany 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 H2A few molecules (OH, H2O, ) maser</p></li><li><p>**Molecules Discovered by the GBT</p></li><li><p>Discovery of Ethanol</p></li><li><p>**Interstellar Molecule FormationNeed high densities (100 106 H atoms/cm3)Lots of dust needed to protect molecules for stellar UVForm in dust clouds = Molecular CloudsAssociated with stars formationBut, optically obscured need radio telescopesLow temperatures (&lt; 100 K)Some molecules (e.g., H2) form on dust grainsMost form via ion-molecular gas-phase reactionsExothermicCharge transfer</p></li><li><p>**Molecular CloudsColdest (5-30 K), densest (100 106 H atoms/cm3) parts of the ISM.Where stars are formed50% of the ISM massA few percent of the Galaxys volume.Concentrated in spiral armsDust Clouds = Molecular Clouds</p></li><li><p>Grain Chemistry</p></li><li><p>**Ion-molecular gas-phase reactionsExamples of types of reactionsC+ + H2 CH2+ + h (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 + h (Radiative)Fe+ + grain Fe + h (Grain)</p></li><li><p>H3+ Drives Most Interstellar Chemistry</p></li><li><p>NRAO/AUI/NSF*Organic Molecules;Seeds of Life</p><p>NRAO/AUI/NSF</p></li><li><p>*********Electron is green. Proton is red. Situation where the gas is hot enough to ionize. The protons does not capture the electron to form an atom.***Since circular motion represents acceleration (i.e., a change in velocity), the electrons radiate photons of a characteristic energy, corresponding to the radius of the circle. **Sun, optically*Sun, radio what places on the sun have intense magnetic fields? Sunspots!*Synchrotron emission from the whole Milky Way galaxy! frequency of 408 MHz*A radio galaxy so named because of the huge jets only seen in the radio. This one is Cygnus A about 800 million light years away****This is called a spin flip transition. The atom has less energy after the electron flips the energy equivalent to a 21 cm radio wave!*NGC 628 spiral galaxy, optical image</p><p>*NGC 628 21 cm Hydrogen profileHydrogen atoms emit radio waves at one wavelength: 21 cm. A 21 cm wavelength is equivalent to a frequency of 1420.41 MHz. This is a RADIO spectral line. This spectrum is cool because it tells us how much hydrogen is in this galaxy, AND how fast the galaxy is moving. This line is Doppler shifted!</p><p>******</p></li></ul>