Instructor: Prof. Dong Lai

• Phone: 5-4936. Office: 618 SSB. Email: dong_at_astro.cornell.edu
• Office hours: After class Monday and Wednesday (4:10 pm-). Other times are fine; best contact by email, will usually answer email within 24 hours.

Time & Place: Monday and Wednesday 2:55 pm - 4:10 pm in 622 SSB

Course website: http://astro.cornell.edu/~dong/a6530/a6530.html

Description:

This course will study the physical processes that produce and affect radiation from various astrophysical sources, as well as how we can learn about the astrophysical objects/processes from the detected radiation. We will cover radiative transfer, bremsstrahlung, synchrotron radiation, Compton scattering, gas heating and cooling, dust emission, and atomic and molecular spectroscopy, etc. These topics are discussed in the framework of astrophysical situations, such as stellar/planetary atmospheres, star and planet formation, interstellar and intergalactic gas, pulsars, accreting black holes, active galactic nuclei, clusters of galaxies and cosmology.

Prerequisites: We will apply electrodynamics, special relativity, quantum mechanics (atomic physics) and statistical mechanics to a diverse set of astrophysical radiation processes. It is assumed that you have studied these subjects at the advanced undergraduate level. If you haven't, please see the instructor to figure out how to beef up on these topics. While useful, no astronomy background is required.

Reference Books: (all should be on reserve in the Math Library)

We are not following any particular books. But the following two books are highly recommended:

• G. Rybicki and A. Lightman "Radiative Processes in Astrophysics" (Wiley) .
• F. Shu "Physics of Astrophysics: Vol.1 Radiation" (University Science Books)

In addition, the following books are useful:

• B. Draine "Physics of the Interstellar and Intergalactic Medium" (Princeton University Press)
• Osterbrock & Ferland "The Astrophysics of Gaseous Nebulae and Active Galactic Nuclei" (2nd edition) (University Science Books)
• Jackson "Classical Electrodynamics" (EM text)
• Longair "High Energy Astrophysics" (Cambridge University Press)
• Mihalas "Stellar Atmospheres" (Freeman)

Homeworks, Project and Grade:

There will be about 7-8 homeworks assigned about one every 1.5-2 weeks. Homeworks will account for about 70% of the course grade. Since class time is limited, there will be some topics that you work out on your own in problem sets.

The final exam will take the form of a long homework, and accounts for about 30% of the course grade. There may also be a final oral interview/exam.

Topics covered in class: (suggested reading from RL=Rybicki & Lightman, Shu other sources)

• 8/26: Description of radiation field: geometric optics limit, specific intensity, flux density etc. Constancy of specific intensity along ray. Flux amplification in lensing. Ralationship between intensity and photon occupation number, Liouville theorem. Reading: RL 1.1-1.3, 1.5. Shu: Chap.1
• 8/31: Black-body radiation. Radiative transfer equation, stimulated emission, Kirchhoff's law, concept of LTE. When LTE breaks down. Transfer equation including scattering. Reading: RL: 1.4, 1.7. Shu: Chap.1 Homework # 1 out.
• 9/2: Formal solution of transfer equation. optical depth, absorption/emission line formation, stellar/planetary atmosphere. Eddington-Barbier relation.
• 9/7: Labor day: no class
• 9/9: Scattering-dominated atmosphere (effective mean free path, diluted BB). Atmosphere modeling (radiative equilibrium). Moment equations of the transfer equation (closure, Eddington appriximation). Reading: RL 1.7; Shu Chap.2-3. Mihalas. Homework # 2 out.
• 9/14: (extended class to make up for travel) Two-stream approximation. Greenhouse effects. Einstein coefficients and relations, LTE vs non-LTE. Relations between opacity/emissivity and Einstein A, B's, net absorption vs pure absorption opacity, astrophysical Maser. Reading: RL 1.6-1.7
• 9/16: Review of EM theory (Maxwell eqn, Poynting theorem, EM energy density and flux). EM waves: monochromatic wave, spectral decomposition. Polarization of EM waves: three descriptions, Stokes parameters. Reading: RL 2.1-2.4; Shu Chap.11-12
• 9/21: Review of classical radiation theory (scalar/vector potential, Lienard-Wiechart potential, radiation from a non-relativitic charge particle, Larmor formula, multiple expansion). Multiple radiation (E1, M1, E2). Reading: RL 2.4-2.6, 3.1-3.3. Shu Chap.11-13
• 9/23: Thomson scattering: cross-section and polarization dependence. CMB polarization (anisotropy + scattering ==> polarization). Radiative damping, natural line shape of atomic emission. Reading: RL 3.3-3.5; Shu Chap.14
• 9/28: Scattering of EM waves by bound electrons (Rayleigh scattering, resonance scatterings, oscillator strength concept). Bremsstrahlung: impluse approximation, power spectrum from a single event. Reading: RL 5.1-5.3, skim 5.5.
• 9/30: Free-free emission, Gaunt factor (qualitative), thermal bremstrahlung, free-free absorption. Continuum emission spectrum of ionized cloud (HII region) (thin-thick transition). Interaction of dust with radiation. Gran opacity, Mie's theory, analytic result for lambda>>a limit (Rayleight scattering, 1/lambda law, reddening). Reading: RL 5.1-5.3, skim 5.5. RL 4.1-4.5 (to beef up special relativity), 4.7-4.8
• 10/5 (extended): Grain heating and radiation, IR emissivity. Beamed radiation: Radiation power relativistic charge. Angular distribution: aberration of light, beaming effect, angular distribution for the two cases (linear vs circular acceleration). Reading: RL 4.1-4.5 (to beef up special relativity), 4.7-4.8. Homework #4 out
• 10/7: Synchrotron radiation: total power, Spectrum of produced by electron with a given gamma (qualitative discussion, peak frequency, power spectrum), radiation from a power-law distribution of electrons; origin of power-law distribution (basic of Fermi acceleration process). Reading: RL 6.1-6.5.
• 10/12: Fall break
• 10/14: (Travel, no class)
• 10/19: Synchrotron radiation: Polarization property (qualitative and quantitative). Synchrotron self-absorption (qualitative discussion using effective T), synchrotron absorption opacity (derived using Einstein relation). spectrum in optical thick regime (elementry derivation). Reading: RL 6.5, 6.8
• 10/21 (extended): Curvature radiation (fast damping of gyro-motion, characteristic frequency, pulsar radiation, bigh brightness temperature, coherence). Inverse Compton scattering (Thomson regime, Klein-Nishina), energy boost in e-photon collision. Reading: RL 7.1. Shu p.179-181.
• 10/26 (extended): Inverse Compton power, inverse Compton spectrum: from single-gamma electron, from power-law distribution electrons and arbitrary background radiation field. Synchrotron self-Compton radiation. Comptonization: energy trasnfer in a single scattering. Reading: RL 7.1-7.4.
• 10/28: Compton y-parameter, collisional Boltzmann equation, Kompeneets equation. Kompeneets equations: derived from Master equation; properties (conservation of photon numbers, steady state: saturated Compton spectrum)). Reading: RL 7.4, 7.6-7.7
• 11/2: Thermal SZ effect (estimate and derivation). Wave propagation in plasmas: dielectric tensor vs wave propagation. Wave in cold zero-field plasma, plasma frequency, pulse dispersion. General property of wave propagation in inhomogeneous medium (Snell's law and generalization). Wave in magnetized plasmas. Faraday rotation. Reading: RL 8.1-8.2
• 11/4: Review of atomic structure: H atom electronic, fine structure (LS coupling, Darwin term, Relativistic correction). Hyperfine structrue; 21 cm line estimate and calculation outline. Multi-electron atoms: Perturbative approaches. Reading: RL, Chap.9
• 11/9 (extended): Multi-electron atoms, Hund's rule (from Pauli), spin-spin correlation. spectroscopic notation. Radiative transition (interaction Hamiltonian). Radiative absorption (B-B transition). Fermi's Golden rule, crosss section. Reading: RL 10.1-10.5. Homework #7 out
• 11/11: Oscillator strength, relation to Einstein coefficients, dipole approximation, classical "derivation" of A coefficent. Section rule for allowed transition (single-e atom and multiple-e atoms). Reading: RL 10.1-10.5
• 11/16: Strickly forbidden radiative transition (2s to 1s in H atom). Photoionization (bound-free), cross-section derived in Bonn approx., general result (Gaunt factor). Reading: RL 10.5. Shu: Chap.2 (or corresponding chapter in Osterbrock).
• 11/18: Radiative recombination, Milne relation, radiative recombination rate in a plasma (recombination coefficient derived, fitting formula). Photoionization equilibrium (pure H), ionization parameter. Collisional (de)excitation, collision strength, rates. Collsional ionization (fitting formula) and 3-body recombination (mentioned). Reading: RL 10.5. Shu: Chap.2 (or corresponding chapter in Osterbrock). Reading: Osterbrock-Ferland: 2.1-2.3, 3.1-3.5. CLOUDY code can be found here.
• 11/23: Example: Level populations in the presence of collsion and radiation. Molecular levels, estimates, Born-Oppenheimer approximation, radiative transitions (rotational selection rule derived, ro-vibrational transitions). Emissio/cooling of optically thin thermal plasma (ff at high T, line cooling at low T, etc). Reading: RL Chap.11. Osterbrock-Ferland: 8.1-8.4. Final Homework #8 out
• 11/25: Thanksgiving

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