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射电天文基础

This course covers the basic principles of spectral lines in radio astronomy, including Einstein coefficients, radiative transfer, dipole transition probabilities, and more.

nicholasnelson
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射电天文基础

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  1. 射电天文基础 姜碧沩 北京师范大学天文系 2009/08/24-28日，贵州大学

  2. Spectral Line Fundamentals • The Einstein Coefficients • Radiative Transfer with Einstein Coefficients • Dipole Transition Probabilities • Simple Solution of the Rate Equation 射电天文暑期学校

  3. The Einstein Coefficients • Line profile function 射电天文暑期学校

  4. Transition between two states and the Einstein coefficients 射电天文暑期学校

  5. Radiative Transfer with Einstein Coefficients 射电天文暑期学校

  6. Stimulated Emission 射电天文暑期学校

  7. Dipole Transition Probabilities--- Electric Dipole 射电天文暑期学校

  8. Magnetic Dipole 射电天文暑期学校

  9. Allowed Transition and Forbidden Transition • Electric dipole transition probability • Mean electric dipole moment for hydrogen:μmn=ea0/2=4.24×10-19 • Transition probability for atomic hydrogen close to the Lyman limit: Amn=109/s • Magnetic dipole transition probability • Mean magnetic dipole moment for hydrogen:μmn=9.27×10-21 erg Gauss-1 • Transition probability for the lowest Bohr : Amn=104/s • Electric dipole transitions are referred to as “allowed” transition and magnetic dipole transitions are termed “forbidden”. 射电天文暑期学校

  10. Simple Solutions of the Rate Equation • The Rate Equation • Transition j→k • Process y • Spontaneous emission • Simulated absorption and emission • Collisional absorption and emission 射电天文暑期学校

  11. Radiative Processes Only 射电天文暑期学校

  12. Collision Probability 射电天文暑期学校

  13. Collisional Processes Involved 射电天文暑期学校

  14. Excitation Temperature 射电天文暑期学校

  15. Exercise • We now investigate the variation of Tex with the collision rate C21 and the spontaneous decay rate A21 for a two-level system. From ‘Tools’ equation(11.41), the dependence on the kinetic temperature TK and the temperature of the radiation field is shown. Suppose that the collision rate C21 is given by n<σ v> where the value of n<σ v> is 10-10. When n<σ v> =A21 for the transition involved, this is referred to as the critical density, n*. For the 21cm line, A21=2.85×10-15s-1. Find n* for this transition. For neutral hydrogen, in most cases, only two levels are involved in the formation and excitation of the 21cm line since the N=2 level is 9eV higher. Less secure is any result for a multi-level systems, However, to obtain an order of magnitude estimate, repeat this calculation for the J=1-0 transition for the HCO+, modelling the molecule as a two-level system in which the Einstein A coefficient is A21=3×10-5S-1. What is the value of n*? Compare this to the value for the 21cm line. For HCO+, take TK=100K, find the value of the local density for which Tex=3.5K. For the same density, calculate n* for the J=1-0 transition of the CO molecule, modelling this as a two-level system with A21=7.4×10-8s-1. 射电天文暑期学校

  16. 射电天文暑期学校

  17. Line Radiation of Neutral Hydrogen • The 21cm Line of Neutral Hydrogen • The Zeeman Effect • Spin Temperature • Emission and Absorption Lines • The Physical State of the Diffuse Interstellar Gas • Differential Velocity Fields and the Shape of Spectral Lines • The Galactic Velocity Field in the Interstellar Gas • Atomic Lines in External Galaxies 射电天文暑期学校

  18. Radio Atomic Lines 射电天文暑期学校

  19. 射电天文暑期学校

  20. The 21cm Line of Neutral H • Transition between the hyperfine structure levels 12S1/2, F=0 and F=1 • Magnetic dipole transition • Frequency:ν10 =1.420 405 751 786 × 109 Hz • Spontaneous transition probability • A10 = 2.86888 × 10-15 s-1 • A factor of about 1023 smaller than that of an allowed optical transition • Mean half-life time of the F=1 state t1/2=1.11×107a • Collision changes the spin of electron in about 400 a 射电天文暑期学校

  21. 射电天文暑期学校

  22. Spin Temperature • The relative population of the hyperfine structure levels is determined by collision in practically all astronomical situations • The excitation temperature in this case is usually called the spin temperature 射电天文暑期学校

  23. Column Density 射电天文暑期学校

  24. Optical Depth 射电天文暑期学校

  25. The Zeeman Effect 射电天文暑期学校

  26. Spin Temperatures 射电天文暑期学校

  27. 射电天文暑期学校

  28. Emission and Absorption Lines • Solution of the radiation transfer equation in terms of the brightness temperature • For positions without a background source • Tc=0 • Pure emission profile • In optically thin cases 射电天文暑期学校

  29. 射电天文暑期学校

  30. The Influence of Beam Filling Factors and Source Geometry 射电天文暑期学校

  31. Optical Depth 射电天文暑期学校

  32. Degree to Which the Continuum Source is Covered 射电天文暑期学校

  33. Source Size 射电天文暑期学校

  34. The Physical State of the Diffuse Interstellar Gas • The value of the local kinetic gas temperature is determined by a balance between energy gain and loss • Four classes medium • Cold neutral medium: T<50K • Warm neutral medium: T>200K • Warm ionized medium: T～104K • Hot ionized medium: T～106K 射电天文暑期学校

  35. Differential Velocity Fields and the Shape of Spectral Lines 射电天文暑期学校

  36. 射电天文暑期学校

  37. 射电天文暑期学校

  38. The Galactic Velocity Field in the Interstellar Gas 射电天文暑期学校

  39. 射电天文暑期学校

  40. Terminal Velocity 射电天文暑期学校

  41. Atomic Lines in External Galaxies 射电天文暑期学校

  42. Virial Masses 射电天文暑期学校

  43. The Tully-Fisher Relation 射电天文暑期学校

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