HOT EMISSION SPECTRA FOR ASTRONOMICAL APPLICATIONS: CH 4 & NH 3 R. Hargreaves, L. Michaux, G. Li, C. Beale, M. Irfan and P. F. Bernath 1 Departments of Chemistry and Physics, University of York, York, UK 1 also, Department of Chemistry & Biochemistry, Old Dominion University, Norfolk, VA, USA

NH 3 Overview

CH 4 Overview

Astronomical Temperatures The Sun (5800 K) (e.g., CN, OH, CH, NH) Sunspots (3200 K) (e.g., H 2 O, TiO) Royal Swedish Academy of Science Earth (296 K) Brown Dwarfs Dwarf Stars Stars Exoplanets H+H+ Diatomic Molecules Polyatomic Molecules Temperature / K HITRAN database at 296 K NASASOHO/EIT Consortium H 2 O, NH 3, CH 4

Brown Dwarfs  Not stars because hydrogen fusion does not take place ( 13 M J ).  The L-class is mainly determined by FeH and CrH absorption in near-IR, while the T-class has strong CH 4 and H 2 O overtone absorption.  The proposed Y-class dwarfs are <700 K and are yet to be identified. It is predicted that NH 3 will become a major absorption feature.

Brown Dwarf Spectra Cushing et al., ApJ 648, 614 (2006) see H 2 O, CH 4 and NH 3 absorption with Spitzer Space Telescope. New Y class of sub-brown dwarfs with T<700 K is predicted to be characterized by strong NH 3 bands. NH 3 ν 2 umbrella mode CH 4 ν 4 bend H 2 O ν 2 bend 1400 K 800 K

Experimental Setup (Emission) FT-IR spectrometer under vacuum KBr beam splitter Mirror Scanning mirror MCT detector CaF 2 windows Controllable tube furnace capable of maintaining temperatures of up to 1370 °C. CH 4 or NH 3 in CH 4 or NH 3 out to pump Alumina (Al 2 O 3 ) tube maintained at 1 Torr of pressure Water cooling coils CaF 2 lens

NH 3 Experiments: NH 3 Experiments: 1. R. J. Hargreaves, G. Li and P. F. Bernath, Hot NH 3 for Astrophysical Applications, Astrophys. J. 735, 111 (11 pp) (2011) 2. R. J. Hargreaves, G. Li and P. F. Bernath, Ammonia line lists from 1650 to 4000 cm -1, J. Quant. Spectrosc. Rad. Trans. 113, 670 (2012) 3. N.F. Zobov, S.V. Shirin, R.I. Ovsyannikov, O.L. Polyansky, S.N. Yurchenko, R.J. Barber, J. Tennyson, R. Hargreaves and P.F. Bernath, Analysis of high temperature ammonia spectra from 780 to 2100 cm -1, J. Mol. Spectrosc. 269, 104 (2011) NH 3 Calculations (Tennyson and co-workers): NH spectroscopically-determined potential energy surface: S. N. Yurchenko, R. J. Barber, J. Tennyson, W. Thiel, and P. Jensen, J. Mol. Spectrosc. 268, 123 (2011) Ab initio dipole moment surface: S.N. Yurchenko, R.J. Barber, A. Yachmenev, W. Thiel, P. Jensen, and J. Tennyson, J. Phys. Chem. A 113,11845 (2009) BYTe 1500 K hot ammonia line list: S.N. Yurchenko, R.J. Barber and J. Tennyson, Mon. Not. R. Astron. Soc. 413, 1828 (2011)

NH 3 Emission at 900°C Second paper: R. J. Hargreaves, G. Li and P. F. Bernath, Ammonia line lists from 1650 to 4000 cm -1, JQSRT 113, 670 (2012)

Astronomical Requirements  An ammonia line list that can be used to simulate astronomical spectra for T= K  From Beer-Lambert law: Need a lineshape function g( ν-ν 10 ) (assumed to be Voigt) and a line strength S ’ given by (SI units, from Bernath, Spectra of Atoms and Molecules):  Therefore need a line position, ν 10, partition function, Q T (easily calculated), line intensity, S J′J″ (or S′), and the lower state energy, E low.  Calibrate line positions and line intensities of observed spectra using HITRAN.

Empirical Lower State Energies  From the line strength equation and taking the ratio for two temperatures we get:  Rearranging to give (Plot LHS as a function of 1/T) E low has an accuracy of better than 15% compared to HITRAN E low comparison, NH 3 paper 1 vs paper 2: shaded region is 20%

NH 3 Lower State Energies ν 2 (a 1 ) HITRAN Empirical Lower State Energies HITRAN ν 1 and ν 3 and ν 1 + ν 2 - ν 2, etc. ν 2 + ν 4 and 2ν 2 + ν 4 - ν 2 etc. ν 4 and ν 4 + ν 2 – ν 2 etc.

CH 4 Emission at 1000°C

CH 4 Conditions and Total Number of Lines

CH 4 Lower State Energies Dyad ( ν 4 ) region Pentad ( ν 3 ) region Octad ( ν 3 + ν 4 ) region

HITRAN lines and E low values (1000°C) Observed lines and empirical E low (1000°C)

Accuracy of Line Intensities Nassar & Bernath (2003) used a single calibration factor while Hargreaves et al. (2012) allowed for a wavenumber variation. Accuracy is better than a factor of 2.

Future Work Record near infrared emission spectra (and absorption spectra) of hot CH 4 and NH 3 Assign hot NH 3 and CH 4 spectra using latest variational calculations (Tennyson and co-workers)