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EXPERIMENTAL LINE LISTS OF HOT METHANE Image credit: Mark Garlick MONDAY 22 nd JUNE 2015 ROBERT J. HARGREAVES rhargrea@odu.edu MICHAEL DULICK PETER F. BERNATH Department of Chemistry Old Dominion University Norfolk, VA 23529
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TEMPERATURE RANGES 1000 2000 30004000 5000 6000 7000 8000 The Sun - 5800 K (e.g., CN, OH, CH, NH) Sunspots - 3200 K (e.g., H 2 O, TiO) Brown Dwarfs Stars Exoplanets/ H+H+ Diatomic Molecules Polyatomic Molecules Temperature / K H 2 O NH 3 CH 4 0 Planets M Dwarf Stars EARTH – 296 K HITRAN database HITEMPA
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BROWN DWARFS Characterized by their spectra Not stars No hydrogen fusion occurs CH 4 Major source of opacity in brown dwarfs Most abundant molecule in Jupiter and Saturn Characterizes T dwarfs ~1400 K ~800 K Cushing et al. (2006), ApJ 648, 614 ML T Y JupiterG ~3500 K~2000 K ~1000 K ~300 K 160 K5700 K Stars Brown dwarfs Planets Sun Adapted from R. Hurt (Caltech/IPAC)
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EXOPLANETS CH 4 also a major feature of exoplanet transit spectra Mainly have hot atmospheres Also CO 2, H 2 O, CO Adapted from: Hand (2011), Nature 480, 302 Swain et al., (2008), Nature 452, 329 HD 189733b (T ~ 1100 K)
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CH 4 POLYADS Adapted from: S. Albert et al. 2009, Chem. Phys. 356, 131 ModeDegeneracy Band Origin (cm -1 ) Type ν 1 (a 1 )12914 Symmetric C-H stretch ν 2 (e)21526Bend ν 3 (t 2 )33020 a Asymmetric C-H stretch ν 4 (t 2 )31306 a Bend T d symmetry a infrared active ν 1 ≈ ν 3 ≈ 2ν 2 ≈ 2ν 4
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PREVIOUS WORK Line parameters mainly at room temperature or below: HITRAN 2012 database (Rothman et al. 2013) Based on the line list from Brown et al. (2013) HITEMP does not include CH 4 Campargue et al. (2013) - WKLMC empirical line lists Hot experimental (emission) work: Nassar & Bernath (2003) Thiévin et al. (2008) Hargreaves et al. (2013) Ab initio theoretical line lists: Rey, Nikitin & Tyuterev (2014) - RNT 0-5000 cm -1 Contains up to 11.5 billion transitions T = 500, 1000, 1500 and 2000 K Yurchenko & Tennyson (2014) - 10to10 Part of ExoMol project Almost 10 billion transitions Valid up to 1500 K
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ASTRONOMICAL REQUIREMENTS Square of transition dipole moment, S J’J’’ Partition function, Q T retrieved
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FOUR SPECTRAL METHOD Experimental method Emission and absorption Bruker FTS 0.02 cm -1 resolution Tube furnace Quartz cell Record temperatures Temperatures 24 – 900°C Spectral range: 2500 – 5000 cm -1 Pentad and Octad 60 Torr methane 600 scans Based on previous experience Need 4 spectra per temperature
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FOUR SPECTRAL METHOD 700°C 1: Hot CH 4 + Lamp (+ background T) 3: No sample + Lamp (+ background T) 2: Hot CH 4 + no lamp (+ background T) 4: No sample + no lamp (+ background T) 700°C
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EQUILIBRIUM 1: Hot CH 4 + Lamp (+ background T) 2: Hot CH 4 + no lamp (+ background T) Optically Thin Optically Thick Demonstrates Kirchoff’s Law: ε = 1 – α
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EMPIRICAL LOWER STATE ENERGIES From line strength equation ( S’ ): Rearranging to give: Typically contain 25,000 lines Calibrated to HITRAN PentadOctad
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LINE LISTS AND CONTINUA Line lists contain Temperature Position (cm -1 ) Intensity (cm molecule -1 ) Lower state energy (cm -1 ) But, also require cross section cm 2 molecule -1 Harrison et al. (2010) PentadOctad
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BROWN DWARF COMPARISON T4.5 dwarf 2MASS 0559-14 1200 K VSTAR Versatile Software for Transfer of Atmospheric Radiation Contains new CH 4 data Low resolution Continuum needed Line lists ~ 25,000 lines Compares well with 10to10 10 billion lines
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OVERVIEW AT 700°C Observed Transmission at 700°C Simulated spectrum (new line list + continuum) HITRAN 2012 RNT 10to10 5000
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HIGH RESOLUTION AT 700°C <0.1 ~ 0.5 x ? 0.15 0.1 >1.0 Simulated spectrum (new line list + continuum) HITRAN 2012 RNT 10to10 Observed Transmission at 700°C
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SUMMARY Hot empirical line lists and continua Accounts for emission and absorption Assignments are not necessarily needed for astronomical applications Low resolution Can be used to accurately model brown dwarf atmospheres Continuum is crucial Compares well with theoretical line lists High resolution Line positions more accurate than state-of-the-art theoretical data Can be used to refine theory High resolution data needed for latest observations CRIRES observations R ~ 100,000 e.g., Snellen et al. (2010) Only data presented here performs well at both high and low resolution
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Acknowledgements The work on the spectra of hot molecules has been funded by a NASA laboratory astrophysical grant. Thanks to the group of Peter Bernath at Old Dominion University and Jeremy Bailey (VSTAR model) at the University of New South Wales. THANKS FOR LISTENING
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