astronomical spectroscopy lecture 1: hydrogen and the early universe jonathan tennyson department of...
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Astronomical spectroscopyLecture 1: Hydrogen and the Early Universe
Jonathan TennysonDepartment of Physics and Astronomy Helsinki
University College London December 2006
Astronomical Spectroscopy
Lecture 1: Hydrogen and the Early Universe
Lecture 2: Molecules in harsh environments
Lecture 3: The molecular opacity problem
Layers in a star: the Sun
Spectrum of a hot star: black body-like
Infra red spectrum of an M-dwarf star
Cool stellar atmospheres: dominated by molecular absorption
BrownDwarf
M-dwarf
The molecular opacity problem
(m)
Cool stars: T = 2000 – 4000 KThermodynamics equilibrium, 3-body chemistryC and O combine rapidly to form CO.
M-Dwarfs: Oxygen rich, n(O) > n(C)H2, H2O, TiO, ZrO, etc also grains at lower T
C-stars: Carbon rich, n(C) > n(O) H2, CH4, HCN, C3, HCCH, CS, etc
S-Dwarfs: n(O) = n(C) Rare. H2, FeH, MgH, no polyatomics
Also (primordeal) ‘metal-free’ starsH, H2, He, H, H3
+ only at low T
Also sub-stellar objects:CO less important
Brown Dwarfs: T ~ 1500 KH2, H2O, CH4
T-Dwarfs: T ~ 1000K‘methane stars’
How common are these?Deuterium burning test using HDO?
Burn D only
No nuclear synthesis
Modeling the spectra of cool stars
• Spectra very dense – cannot get T from black-body fit.• Synthetic spectra require huge databases > 106 vibration-rotation transitions per triatomic molecule• Sophisticated opacity sampling techniques.• Partition functions also important
Data distributed by R L Kururz (Harvard), seekurucz.harvard.edu
Physics of molecular opacities:Closed Shell diatomics
CO, H2, CS, etc
Vibration-rotation transitions.
Sparse: ~10,000 transitions
Generally well characterized by lab data and/or theory
(H2 transitions quadrupole only)
HeH+
Physics of molecular opacities:Open Shell diatomics
TiO, ZrO, FeH, etc
Low-lying excited states.
Electronic-vibration-rotation transitions
Dense: ~10,000,000 transitions (?)
TiO now well understood using mixture of
lab data and theory
Physics of molecular opacities:Polyatomic molecules
H2O, HCN, H3+, C3, CH4, HCCH, NH3, etc
Vibration-rotation transitions
Very dense: 10,000,000 – 100,000,000
Impossible to characterize in the lab
Detailed theoretical calculations
Computed opacities exist for: H2O, HCN, H3+
Ab initio calculationof rotation-vibrationspectra
The DVR3D program suite: triatomic vibration-rotation spectraPotential energy
Surface,V(r1,r2,)
Dipole function (r1,r2,)
J Tennyson, MA Kostin, P Barletta, GJ Harris
OL Polyansky, J Ramanlal & NF Zobov
Computer Phys. Comm. 163, 85 (2004).
www.tampa.phys.ucl.ac.uk/ftp/vr/cpc03
Potentials: Ab initio or Spectroscopically determined
H3+
H2O (HDO)H2S
HCN/HNC HeH+
Molecule considered at high accuracy
Partition functions are important
Model of cool, metal-free magnetic white dwarf WD1247+550 by Pierre Bergeron (Montreal)
Is the partition function of H3+ correct?
Partition functions are important
Model of WD1247+550 using ab initio H3+ partition function
of Neale & Tennyson (1996)
HCN opacity, Greg Harris
High accuracy ab initio potential and dipole surfaces Simultaneous treatment of HCN and HNC Vibrational levels up to 18 000 cm-1
Rotational levels up to J=60 Calculations used SG Origin 2000 machine 200,000,000 lines computed Took 16 months
Partition function estimates suggest 93% recovery of opacity at 3000 K
2006 edition uses observed energy levels
Ab initio vs. laboratory
HNC bend fundamental (462.7 cm-1).
•Q and R branches visible.
•Slight displacement of vibrational band centre (2.5 cm-1).
•Good agreement between rotational spacing.
•Good agreement in Intensity distribution.
Q branches of hot bands visible.Burkholder et al., J. Mol. Spectrosc. 126, 72 (1987)
GJ Harris, YV Pavlenko, HRA Jones & J Tennyson, MNRAS, 344, 1107 (2003).
Importance of water spectra
Other• Models of the Earth’s atmosphere• Major combustion product (remote detection of forest fires,
gas turbine engines)
• Rocket exhaust gases: H2 + ½ O2 H2O (hot) • Lab laser and maser spectra
Astrophysics• Third most abundant molecule in the Universe (after H2 & CO)
• Atmospheres of cool stars• Sunspots• Water masers• Ortho-para interchange timescales
Sunspots Image from SOHO : 29 March 2001
Molecules on the Sun
T=5760KDiatomicsH2, CO, CH, OH,CN, etc
SunspotsT=3200KH2, H2O,CO, SiO
Sunspot
lab
Sunspot: N-band spectrum
L Wallace, P Bernath et al, Science, 268, 1155 (1995)
Assigning a spectrum with 50 lines per cm-1
1. Make ‘trivial’ assignments (ones for which both upper and lower level known experimentally)
2. Unzip spectrum by intensity 6 – 8 % absorption strong lines 4 – 6 % absorption medium 2 – 4 % absorption weak < 2 % absorption grass (but not noise)
3. Variational calculations using ab initio potential Partridge & Schwenke, J. Chem. Phys., 106, 4618 (1997) + adiabatic & non-adiabatic corrections for Born-Oppenheimer approximation
4. Follow branches using ab initio predictions branches are similar transitions defined by
J – Ka = na or J – Kc = nc, n constant
Only strong/medium lines assigned so far
OL Polyansky, NF Zobov, S Viti, J Tennyson, PF Bernath & L Wallace, Science, 277, 346 (1997).
Sunspot
lab
Assignm
entsSunspot: N-band spectrum
L-band, K-band & H-band spectra also assignedZobov et al, Astrophys. J., 489, L205 (1998); 520, 994 (2000); 577, 496 (2002).
Assignments using branches
Ab initio potentialLess accurate but extrapolate well
J
Err
or /
cm-1
Determined potentialSpectroscopically
Variational calculations:
Accurate but extrapolate poorly
Spectroscopically determined water potentials
Reference Year vib/cm-1 Nvib Emax /cm-1
Hoy, Mills & Strey 1972 214 25 13000
Carter & Handy 1987 2.42 25 13000
Halonen & Carrington 1988 5.35 54 18000
Jensen 1989 3.22 55 18000
Polyansky et al (PJT1) 1994 0.6 40 18000
Polyansky et al (PJT2) 1996 0.94 63 25000
Partridge & Schwenke 1997 0.33 42 18000
Shirin et al 2003 0.10 106 25000
mportant to treat vibrations and rotations
Viti & Tennyson computed VT2 linelist:Partridge & Schwenke (PS), NASA AmesNew study by Barber & Tennyson (BT2)
Computed Water opacity• Variational nuclear motion calculations
• High accuracy potential energy surface
• Ab initio dipole surface
• 50,000 processor hours.
• Wavefunctions > 0.8 terabites
• 221,100 energy levels (all to J=50, E = 30,000 cm) 14,889 experimentally known
• 506 million transitions (PS list has 308m) >100,000 experimentally known with intensities
Partition function 99.9915% of Vidler & Tennyson’s value at 3,000K
New BT2 linelistBarber et al, Mon. Not. R. astr. Soc. 368, 1087 (2006).
http://www.tampa.phys.ucl.ac.uk/ftp/astrodata/water/BT2/
Comparison with Experimental Levels
BT2 AMES
Agreement: % %
Within 0.10 cm-1 48.7 59.2
Within 0.33 cm-1 91.4 85.6
Within 1 cm-1 99.2 92.6
Within 3 cm-1 99.9 96.5
Within 5 cm-1 100.0 97.0
Within 10 cm-1 100.0 98.1
Number of Experimental Levels: 14,889
1 7 1 54 7 0 33 9003.892 7003.799 2000.092 4.01E-03 2.78E-22 3.89E-04 6.71E-01
1 3 0 38 3 1 17 9098.530 7098.116 2000.415 1.56E-03 1.01E-22 1.41E-04 5.59E-01
1 7 0 84 6 0 47 10486.138 8485.481 2000.657 4.69E-02 1.12E-21 1.56E-03 7.84E+00
1 6 0 77 6 1 45 10939.532 8938.685 2000.848 4.83E-03 8.33E-23 1.16E-04 9.34E-01
1 6 1 11 5 1 5 4407.221 2406.299 2000.922 2.77E-02 5.25E-20 7.34E-02 5.35E+00
0 6 0 16 5 0 5 4407.355 2406.297 2001.058 3.26E-02 2.06E-20 2.88E-02 6.30E+00
1 4 1 60 4 0 46 11384.245 9383.183 2001.062 6.66E-03 8.35E-23 1.17E-04 1.86E+00
1 6 0 78 7 0 60 10955.914 8954.726 2001.188 1.69E-02 2.88E-22 4.03E-04 3.27E+00
0 7 1 19 7 0 9 6034.992 4033.695 2001.297 7.29E-04 1.43E-22 2.00E-04 1.22E-01
1 5 1 104 5 0 75 12912.871 10911.526 2001.344 3.36E-02 1.40E-22 1.96E-04 7.68E+00
Raw spectra from DVR3D program suite
A B C D E F G H I J K
43432 11 1 50 8730.136998 0 2 1 11 3 8
43433 11 1 51 8819.773962 0 4 0 11 6 6
43434 11 1 52 8918.536215 0 0 2 11 2 10
43435 11 1 53 8965.496130 0 2 1 11 5 6
43436 11 1 54 8975.145175 2 0 0 11 4 8
43437 11 1 55 9007.868894 1 0 1 11 3 8
43438 11 1 56 9082.413891 1 2 0 11 6 6
43439 11 1 57 9170.343871 1 0 1 11 5 6
43440 11 1 58 9223.444158 0 0 2 11 4 8
43441 11 1 59 9264.489815 2 0 0 11 6 6
43442 11 1 60 9267.088316 0 5 0 11 2 10
43443 11 1 61 9369.887722 0 2 1 11 7 4
43444 11 1 62 9434.002547 0 4 0 11 8 4
43445 11 1 63 9457.272655 1 0 1 11 7 4
43446 11 1 64 9498.012728 0 0 2 11 6 6
43447 11 1 65 9565.890023 1 2 0 11 8 4
Energy file: N J sym n E/cm-1 v1 v2 v3 J Ka Kc
144848 146183 3.46E-04
115309 108520 7.42E-04
196018 198413 1.95E-04
7031 7703 1.13E-02
149176 150123 1.69E-04
81528 78734 2.30E-01
80829 78237 8.83E-04
209672 210876 2.51E-01
207026 203241 2.72E-04
188972 184971 1.25E-01
152471 153399 1.12E-02
39749 37479 1.46E-07
10579 15882 6.90E-05
34458 35617 1.15E-03
Transitions file: Nf Ni Aif
12.8 GbDivided into 16 files by frequencyFor downloading
S.A. Tashkun, HiRus conference (2006)
Astronomical Spectroscopy
Lecture 1: Hydrogen and the Early Universe
Lecture 2: Molecules in harsh environments
Lecture 3: The molecular opacity problem
Merry Christmas
Master file strategy:Inclusion of Experimental (+ other theoretical) data
Added to record. Data classified:
Property of level Energy File• Experimental levels (already included)• Alternative quantum numbers (local modes)
Property of transition Transition File• Measured intensities or A coefficients • Line profile parameters
Line mixing as a third file? Location of partition sums?
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 4.27m is due to solid CO2, whereas the structure at 4.4-4.9 m indicates the presence of warm, gaseous CO along the line of sight.
van Dishoeck et al. 1996.
Photon dominated region (PDR)
Photon dominated regions (PDRs)
• Photoionisation important• Molecular ions • Hot (T ~ 1000 K) but • Not thermodynamic equilibrium• Electron collisions• Optical pumping
Planetary nebula NGC3132
Cernicharo, Liu et al, Astrophys. J., 483, L65 (1997).
Rotational excitation of molecular ions:Astrophysical importance
Photon dominated regions (PDRs)Electron density, ne ~ 104 n(H2)
Rotational excitation cross sectionelectron > 105 molecule
Radiative lifetime < mean time between collisionsTherefore:
Observed emissions proportional toelectron x column density
Similar arguments hold for vibrational excitation
Rotational excitation of molecular ions:Theoretical models
Standard modelDipole Coulomb-Born approximationOnly considers (long-range) dipole interactionsOnly J = 1 excitations possibleOnly J = 1 emissions should be observed
No experimental data available forelectron impact rotational excitation of molecular ions
Tests of this model performed with R-matrix calculationswhich explicitly include short-range electron-molecular ion interactions
Have considered HeH+, CH+, NO+, CO+, H2
+, HCO+
A. Faure and J. Tennyson, Mon. Not. R. astr. Soc., 325, 443 (2001)
Working on H3+ and H3O+
Find J=2-1 emissions should be observablefor HeH+ and others
Rotational excitation of molecular ions
Summary of resultsJ = 1
c Coulomb-Born model satisfactoryc Short range interactions important
Find c ~ 2 Debye
J = 2Dominated by short range interactions
Always important, can be bigger than J = 1
J > 2Determined by short-range interactions
Usually small, but J = 3 can be significant
For light molecules (H containing diatomics),cross-sections need to energy modified near threshold
CometsDirty snowballs which link our solar system with theISM
Comet Hale-Bopp
Molecules identified in comet Hale-Bopp
Simple speciesH2O HDO CO CO2 H2S SO SO2 OCS CS NH3
Molecular ionsH2O+ H3O+ HCO+ CO+
Organic and similarHCN DCN CH3CN HNC HC3N HNCO C2H2 CH3OCHOC2H6 CH4 NH2CHO CH3OH H2CO HCOOH H2CS
RadicalsOH CN NH2 NH C3 C2