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Molecular spectroscopy for exoplanets: I - Jonathan Tennyson

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Brave new worlds
May 29-June 03, 2016 – Lake Como School of Advanced Studies

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Molecular spectroscopy for exoplanets: I - Jonathan Tennyson

  1. 1. Jonathan Tennyson University College London Brave New Worlds Lake Como, May 2016 Molecular spectroscopy for exoplanets: I
  2. 2. Published in MNRAS I.BeH, MgH, CaH II.SiO III.HCN/HNC IV.CH4 V.NaCl, KCl VI.PN VII. PH3 VIII. H2CO IX.AlO X.NaH XI.HNO3 XII. CS XIII. CaO XIV. SO2 XV. H2S XVI. HOOH XVII. SO3 (Virtually) Complete •H2 18 O, H2 17 O •H3 + •SH, AlH •H2 16 O •NO, NS •CrH In progress •TiO •C3 •PH, PO, PS •TiH •MnH •NaO Hot line lists Planned •NH3 •MgO •NiH •FeH • C2H4 • SiH • HCCH • CH3Cl • SiH4 • Larger hydrocarbons
  3. 3. www.worldscibooks.com/physics/7574.html About the first edition “The best book for anyone who is embarking on research in astronomical spectroscopy” Contemporary Physics (2006) Published 2011
  4. 4. Molecular spectroscopy for exoplanets Lecture 1 Molecular spectroscopy: basics Lecture 2 for astronomy/exoplanets
  5. 5. Wavelength versus Frequency • Astronomers use wavelengths (µm, Å, nm, etc) except radio use frequency: Hz, km s-1 (!) X-ray, EUV etc use energy: eV • Spectroscopers use frequency /wavenumbers/energies (Hz, cm-1 , eV, etc) Physical understanding/interpretation in energies I will use cm-1 (k = 0.69 cm-1 ) 1 cm-1 = 10000 / λ (in µm) = 30 GHz
  6. 6. What does one learn from an astronomical line spectrum?
  7. 7. What does one learn from an astronomical line spectrum? • What species produces the line(s)? The composition of the object. • What transition is it? Comparison of ground & excited states Physical conditions eg temperature • How strong is the line? Abundance • What is the Doppler shift? Motion of the object or region of the object • What is the line profile? Physical conditions eg temperature (Doppler broadening) pressure (line broadening) •What is the splitting pattern? Local magnetic field
  8. 8. Spectral coverage from the ground
  9. 9. Degrees of freedom Nuclear motion Molecule with N atoms has 3N degrees of freedom 3 are translation (not interesting) 3 are rotation (2 if molecule linear) 3N-6 are vibration (3N-5 if linear) Electronic motion Like atoms but more complicated (lower symmetry)
  10. 10. Rotational motion: Rigid rotor Principal axes: diagonalise moment of inertia tensor Convention: IA IB IC Rotational constant B = 2 /2IB with C B A
  11. 11. Types of rotor 1. Linear molecule: A=0, B=C eg H2, CO, CO2, HCN 2. Spherical top: A=B=C eg CH4 3. Symmetric top: Prolate: A > B=C Oblate: A=B>C eg H3 + , NH3, PH3 4. Asymmetric top: A > B > C eg H2O, H2CO, and lots more
  12. 12. Rotational spectra: require a (permanent) dipole 1. Linear molecule: A=0, B=C eg H2, CO, CO2, HCN 2. Spherical top: A=B=C eg CH4 3. Symmetric top: Prolate: A > B=C Oblate: A=B>C eg H3 + , NH3, PH3 4. Asymmetric top: A > B > C eg H2O, H2CO
  13. 13. Rotational spectra: require a (permanent) dipole 1. Linear molecule: A=0, B=C eg H2, CO, CO2, HCN 2. Spherical top: A=B=C eg CH4 3. Symmetric top: Prolate: A > B=C Oblate: A=B>C eg H3 + , NH3, PH3 4. Asymmetric top: A > B > C eg H2O, H2CO
  14. 14. Rotational spectra: permanent dipole moment, µ Selection rule ∆J = -1, (0), +1 Intensity α µ2 Transitions: radio to far infrared
  15. 15. Rotational spectra: can be very simple Energies = B J(J+1), ∆J = 1, so lines 2B apart eg diatomics such as CO or HCl or linear molecules eg HCN PJ = gn (2J+1) exp (-BJ(J+1)/kT) P0 Thermal population
  16. 16. Rotational spectra: or a mess eg water, methanol etc (asymmetric tops)
  17. 17. The spectra, toward IRC +10o 216 with the IRAM 30m telescope, showing the J=7−6 to 12−11 rotational transitions of NaCl. The HC3N line in the upper spectrum lies in the upper sideband of the receiver.
  18. 18. The interstellar (0,0) band of CN violet system in the spectrum of ζ Oph. T ∼ 2.6 K Notation P(J”) or R(J”) J” is lower level (J’ is upper level) So P(1) means J”=1  J’=0 R(0) means J”=0  J’=1 etc
  19. 19. Vibrations: Displacements from equilibrium Normal modes
  20. 20. Vibrations: Displacements from equilibrium Often considered as harmonic: Normal modes Require change in dipole moment H2, CO, CO2, CH4, H3 + ,NH3, PH3, H2O, H2CO
  21. 21. Vibrations: Displacements from equilibrium Often considered as harmonic: Normal modes Require change in dipole moment H2, CO, CO2, CH4, H3 + ,NH3, PH3, H2O, H2CO
  22. 22. Vibrations: Displacements from equilibrium Often considered as harmonic: Normal modes Require change in dipole moment H2, CO, CO2, CH4, H3 + ,NH3, PH3, H2O, H2CO
  23. 23. Vibrations: Transitions lie (largely) in the infrared. No rigorous selection rule on ∆v (anharmonic) Fundamental: ∆v = 1 change in vibrational quanta strongest Also Overtone: ∆v > 1 Combination: ∆v1 + ∆v2 both change etc Changes accompanied by rotational changes Selection rule ∆J = -1, (0),1 P Q R
  24. 24. Vibration-Rotation spectra: can be very simple eg diatomics such as CO or HCl or linear molecules eg HCN No Q-branch ∆J=0 R-branch ∆J=+1 P-branch ∆J=-1
  25. 25. Contain useful features at low resolution: Acetylene or HCCH
  26. 26. Contain useful features at low resolution: Acetylene or HCCH P-branch ∆J=-1 Q-branch ∆J=0
  27. 27. Vibration-Rotation spectra: or a mess Infrared absorption spectrum due to water
  28. 28. Electronic spectra: • lie largely in ultra-violet except open shell systems eg TiO, VO etc • Involve changes in electronic state • And vibrational state (Any, Franck-Condon approx.) • And rotational state (∆J = -1, (0),1)
  29. 29. Lyman bands Werner bands Electronic transitions in H2
  30. 30. Electronic spectra depends on potential energy curves State labels like atoms spin 2S+1 Eg X 2 Π Angular momentum projection, Λ State label X ground state A, B, C excited states same spin a,b,c excited states different spin Rules are made to be broken!
  31. 31. State labels like atoms spin 2S+1 Eg X 2 Π Angular momentum projection, Λ State label X ground state A, B, C excited states same spin a,b,c excited states different spin Rules are made to be broken!
  32. 32. Electronic spectra: selection rules • Involves changes in electronic state ∆S = 0, ∆Λ = 0,+/-1, (g u for symmetric species) • And vibrational state (Any, Franck-Condon approx.) • And rotational state (∆J = -1, (0),1)
  33. 33. Depends on potential energy curves Allowed transitions?spin 2S+1 CaO X 2 Σ  AlO X 2 Σ  r momentum projection, Λ State label X ground state A, B, C excited states same spin a,b,c excited states different spin Rules are made to be broken!
  34. 34. Depends on potential energy curves Allowed transitions?spin 2S+1 CaO X 1 Σ+ Α 1 Σ+ , Α’ 1 Π AlO X 2 Σ  r momentum projection, Λ State label X ground state A, B, C excited states same spin a,b,c excited states different spin Rules are made to be broken!
  35. 35. Depends on potential energy curves Allowed transitions?spin 2S+1 CaO X 1 Σ+ Α 1 Σ+ , Α’ 1 Π AlO X 2 Σ Α 2 Π, Β 2 Σ r momentum projection, Λ State label X ground state A, B, C excited states same spin a,b,c excited states different spin Rules are made to be broken!
  36. 36. TiO: low-lying electronic states 3
  37. 37. Electronic spectra − ∆S = 0 is weak selection rule Observe ∆S > 0 “intercombination bands” but weak -Band heads: features observed at low resolution
  38. 38. Potential curves and spectra Shayesteh et al 2007 MgH re I = µre 2 Vibrational levels: (nearly) evenly spaced
  39. 39. Spectral coverage from the ground Electronic Vib-Rot Rotations diatomics large molecules Na K CH CN CH+ Lots: Particularly dipolar species H2O CH4 CO2 H3 +
  40. 40. Spectrum of ammonia rotations fundamentals overtones 510 2.5µm20
  41. 41. Spectrum obtained with the Infrared Space Observatory toward the massive young stellar object AFGL 4176 in a dense molecular cloud. The strong, broad absorption at 4.27µm is due to solid CO2, whereas the structure at 4.4- 4.9 µm indicates the presence of warm, gaseous CO along the line of sight. van Dishoeck et al. 1996.
  42. 42. Line profile effects
  43. 43. A spectral line Line position Line shape Line intensity: Probability of absorbing a photon Given by area under the curve Conserved with P,T if optically thin Line profile Import when saturated ie optically thick Combine as Voigt Profile
  44. 44. Aftermath of the collision of comet Shoemaker-Levy 9 with Jupiter: HST spectra of the G impact site taken ∼3-4 hrs after the impact on 18 July 1994 divided by similar spectra taken on 14 July 1994 well before the impact. The principal absorption features are due to S2, CS2 and NH3.

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