Ay 122 - Fall 2004Electromagnetic Radiation And ItsInteractions With Matter(This version has many of the figures missing,in order to keep the pdf file reasonably small)Radiation Processes: An Overview• Physical processes of electromagnetic radiation(EMR) emission and absorption– Kirchoff’s laws– Discrete (quantum) processes– Continuum processes• Next time: radiation transfer, spectral lines, stellaratmospheres and spectral types(Many slides todayfrom P. Armitage)Primary Astrophysical ProcessesEmitting Radio RadiationWhen charged particles change direction(i.e., they are accelerated), they emit radiationEMRDiscrete (quantum transitions)ContinuumThermal (i.e., blackbody)Nonthermal: Synchrotron Free-free CherenkovWhich one(s) will dominate,depends on the physical conditions of the gas/plasma.Thus, EMR is a physical diagnostic.DifferentPhysicalProcessesDominate atDifferentWavelengthsNuclear energylevelsInner shells ofheavier elementsAtomic energylevels(outer shells)MoleculartransitionsHyperfinetransitionsPlasma in typicalmagnetic fieldsKirchoff’s Laws1. CONTINUOUS SPECTRUM: Any hot opaquebody (e.g., hot gas/plasma) produces a continuousspectrum or complete rainbow2. EMISSION SPECTRUM: A hot transparent gaswill produce an emission line spectrum3. ABSORPTION SPECTRUM: A (relatively) cooltransparent gas in front of a source of a continuousspectrum will produce an absorption line spectrumModern atomic/quantum physics provides a readyexplanation for these empirical rulesKirchoff’s Laws in Action:Laboratory spectra Ÿ Line identifications in astro.sourcesAnalysis of spectra Ÿ Chemical abundances + physical conditions (temperature, pressure, gravity,ionizing flux, magnetic fields, etc.)+ VelocitiesAtomic ProcessesRadiation can be emitted or absorbed when electronsmake transitions between different states:Bound-bound: electron moves between two bound states (orbitals) in an atom or ion. Photon is emitted or absorbed.Bound-free: • Bound -> unbound: ionization• Unbound -> bound: recombinationFree-free: free electron gains energy by absorbing aphoton as it passes near an ion, or loses energy by emittinga photon. Also called bremsstrahlung.Energy Transitions: Free-FreeDE(e-) = E(g) = hn = hc/lh = Planck’s constantEnergy Transitions: The Bohr AtomAtoms transition from lower to higher energy levels(excitation / de-excitation) in discrete quantum jumps.The energy exchange can be radiative (involving aphoton) or collisional (2 atoms)† hn= Ei- EjExample of an Atomic Energy Transition:Hydrogen AtomPhoton energy:Hydrogen Energy Levelsn=1n=2n=3GROUND STATEE= -13.6eVE= -3.4eVE= -1.5eVE=0Energy levels are labeledby n - the principalQuantum number.Lowest level, n=1, is theground state.† En= -Rn2where R = 13.6 eV is a Constant (Rydberg)n-th energy level has 2n2 quantum states, whichare degenerate (same E).Energy Levels in a Hydrogen AtomFamilies of Energy Level TransitionsCorrespond to Spectroscopic Line SeriesBalmerSeries Linesin StellarSpectraEven for H - simplest atom - huge number of pairs ofenergy levels with different DE and hence different n.How do we decide which lines we will see?• At particular T, some levels will have a higherprobability of being occupied than others.• Probability of some transitions is greater than others.• Not all transitions are possible (selection rules).Because of conservation laws - e.g. since a photoncarries angular momentum cannot make a transitionbetween two states with zero angular momentum byemitting one photon.Which Energy Levels and Transitions?Computing the Occupation of EnergyLevels… from which we can then compute therelative intensities of spectroscopic linesNeed gas [r,T] and radiation spectrum and intensity (or justTe, if it’s a thermal spectrum). The key question is whetherthe gas and the radiation field are in a thermal equilibriumBoltzmann’s LawCalculating the populations of energy levels is difficult ifThe gas is not in local thermodynamic equilibrium (LTE).In LTE, it is very easy. At temperature T, populations n1and n2 of any two energy levels are: † n2n1=g2g1e-(E2-E1) kTg1 and g2 are the statistical weights of the two levels(allow for the fact that some energy levels are degenerate).For hydrogen:† gn= 2n2Emission or Absorption?It depends on whether the gas (plasma) isOptically thick: short mean free path of photons, getabsorbed and re-emitted many times, only the radiationnear the surface escapes; orOptically thin: most photons escape without beingreabsorbed or scattered(Note that a medium can be optically thick or thin for either line orcontinuum photons. Optical thickness is generally proportionalto density.)And then it depends on the geometry: if a continuum isseen though a cooler, optically thin gas, you will see anabsorption spectrum; but if the gas is hotter, there will bean emission line spectrum superposed on the cont.Spectral Line Emission: MolecularRotational and Vibrational ModesThese transitions (or energysplittings) have generallylower energies (thusprominent in IR/sub-mm),but many more levels (thuscomplex spectra)Example:Orionspectrumfrom CSOAstrophysical Molecular Spectroscopy• Because of the lower energy levels of moleculartransitions, they are a good probe of colder gas (T ~ 10- 100 K), e.g., star forming regions• Commonly observed molecules in space include:hydrogen (H2) carbon monoxide (CO), water (H2O),OH, HCN, HCO+, CS, NH3, formaldehyde (H2CO),etc. Less common molecules include sugar, alcohol,antifreeze (ethylene glycol), …• As a bonus, longer wavelengths are not affected muchby the interstellar extinctionMaser Emission• Stimulated emission from overpopulated energy levels• Sometimes seen in star-forming regions, or cold stellarenvelopes• Produces very sharp emission lines - an excellent tracerof velocity fields (e.g., for central massive black holes)Hydrogen 21cm LineGround state of hydrogen (n=1) has 2 x 12 = 2 states. Correspond to different orientations of the electron spin relative to the proton spin.Very slightly different energies - hyperfine splitting.Energy difference corresponds to a frequency of1.42 GHz, or 21cm wavelength.Very important for radio astronomy, because neutralhydrogen is so abundant in the Universe. This is theprincipal wavelength for studies of ISM in galaxies, andtheir disk structure and rotation curves.Emits photon with awavelength of 21 cm(frequency of 1.42GHz)Spectral Line Emission: Hyperfine Transition ofNeutral