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Ortho and para H2 and nitrogen astrochemistry
Pierre Hily-Blant
C. Rist, A. Faure, F. Daniel, L. Bonal, E. Quirico, R. Le GalG. Pineau des Forets‡
Universite Joseph Fourier/IUF/IPAG, Grenoble,‡ Universite Paris Sud/Orsay, IAS
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 1 / 27
Introduction
Chemistry of nitrogen in dark and dense clouds
Herbst & Klemperer (1973); Pineau des Forets et al. (1990); Hily-Blant et al. (2013)
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 2 / 27
Introduction
Interstellar nitrogen chemistry
Gas-phase vs grain chemistry ?
Problematic
• Reservoirs of gaseous nitrogen in diffuse to dense ISM
• Nitrogen isotopic ratios to track nitrogen reservoirs
Strategy: observational astrochemistry
• identify pivotal reactions
• determine ratios which depend on a small number of species
• observation of abundance ratios
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 3 / 27
Effects of o/p(H2)
Effects of the o/p(H2)
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 4 / 27
Effects of o/p(H2)
Formation of Nitrogen Hydrides
N+ + H2 −−→ NH+ + H
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 5 / 27
Effects of o/p(H2)
Formation of Nitrogen Hydrides
Dislaire et al. (2012)
At 10 K, new rate is ∼ 10− 100 times lower than previously assumed
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 6 / 27
Effects of o/p(H2)
Formation of Nitrogen Hydrides
N2H+ + e – 7%−−→ NHVigren et al. (2012)
Dislaire et al. (2012)
NH/NH2 sensitive to o/p(H2) (noted $)
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 7 / 27
Effects of o/p(H2)
Spin modification ratios of N-hydrides
Thermalization of H2: Grain formation vs gas-phase conversionNon-LTE ratios below ∼ 20 K
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 8 / 27
Effects of o/p(H2)
Spin modification ratios of N-hydrides
Initial o/p from grains partially lost after ∼ few 106 yr
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 9 / 27
Effects of o/p(H2)
Spin modification ratios of Nitrogen Hydrides
H2 NH+4 NH2 NH3
JKaKc JK
p I=0 0 o I=1 0 o 000 0 o 00 0o I=1 170.5 m I=2 17 p 101 30 p 11 23
m I=2 51 o 111 46 o 10 27p I=0 51 p 110 53 p 22 64
o 202 89 o 20 86
Reassessment of branching ratios for NH+n + H2 reactions, valid up to
∼ 30 K.Rist et al. (2013); Le Gal et al. (2014)
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 10 / 27
Effects of o/p(H2)
Spin modification ratios of NH3
NH+4 + e – −−→ NH3 + H
Faure et al. (2013); Le Gal et al. (2014)
Non-LTE ratio below ∼ 20 K driven by non-LTE H2.
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 11 / 27
Effects of o/p(H2)
Spin modification ratios of NH2
NH+4 + e – −−→ NH2 + 2 H
Faure et al. (2013); Le Gal et al. (2014)
Non-LTE ratio below ∼ 20 K driven by non-LTE H2
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 12 / 27
Effects of o/p(H2)
Effects of o/p(H2) on deuterated species
• Gas-phase reactions of H+3 , H2D+, D2H+, and D+
3 with H2, HD, andD2
• Gerlich et al 2002, 2006, Hugo et al 2009
• Widely used in the astrochemistry community
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 13 / 27
Effects of o/p(H2)
Effects of o/p(H2) on deuterated species
• Species-to-species conversion rates computed from Hugo et al 2009
• LTE ratios above ∼ 20 K
• Strongly non-LTE ratio below ∼ 20 K driven by non-LTE H2
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 14 / 27
Effects on isotopic ratios
Linking the ISM and Solar System
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 15 / 27
Effects on isotopic ratios
Linking the ISM and Solar System
• Memory of ISM chemistry in protosolar relics ?
• Isotopic ratios as tools ?
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 16 / 27
Effects on isotopic ratios
Deuterium fractionation in the Solar System
Hartogh et al 2012
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 17 / 27
Effects on isotopic ratios
Nitrogen Isotopic Ratios
• Solar System: wide variations
• ISM: close to Solar and cometary
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 18 / 27
Effects on isotopic ratios
Interstellar fractionation
• Chemical fractionation: gas colder than ZPE difference betweenisotopologues
• ZPE ∼ 20− 30 K for nitrogen; ∼ 200 K for D.
• Cold interstellar conditions are favourable
• Fractionation of nitrogen and deuterium should correlate (Alexanderet al 2008, Bonal et al 2009, Aleon 2010)
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 19 / 27
Effects on isotopic ratios
Effect of the o/p(H2)
• Rate of N+ + H2 −−→ NH+ + H sensitive to $ (o/p of H2)
• $ decreases: less 15N+ eventually incorporated into ammonia, butmore is recycled back into 15NN; fractionation of ammonia is lowered.
• provided $ . 10−2, deuteration is insensitive to $.
• Consequence: depending on $, nitrogen and deuterationfractionations may decorrelate, even in cold ISM gas (Wirstrom et al2012).
• Strongly time-dependent effect
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 20 / 27
Effects on isotopic ratios
Gas-phase fractionation
Hily-Blant et al 2013
• 104 cm−3, 10 K
• gas-phase only
• Three 14N/15N values• deprived: N, NO, CN• unaltered: N2, NH3, NH2
• enriched: HCN, NH, N2H+
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 21 / 27
Effects on isotopic ratios
Influence of o/p(H2)
Only a weak effect on steady-state fractionation levelsP. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 22 / 27
Conclusions / Perspectives
Conclusions
• Non-LTE o/p ratio of H2 is expected over a wide range of parametersin cold interstellar gas
• o/p(H2) ∼ 10−4, constant below critical temperature of ∼ 20 K
• p-H2 rich gas drives o/p ratios of NH3 and NH2 out of LTE
• Non-LTE o/p(H2) also drives non-LTE ratios of deuterated species
• Sensitivity of N+ + H2 to o/p(H2) crucial to explain the observedNH:NH2 abundance ratio
• Can explain uncorrelated D- and 15N- enrichments in early-SS objects
• Effects on gas-phase nitrogen fractionation are small.
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 23 / 27
Conclusions / Perspectives
Perspectives
Theory / Models
• Explore consequences of o/p(H2) on other groups of chemical species:C-bearing, O-bearing, S-bearing
• Include a full o/p chemistry in our 15N chemical network
• Propagate branching ratios (Rist et al 2013, Hugo et al 2009) todeuterated nitrogen hydrides (e.g. ND, NHD, NH2D, etc)
• Derive observational diagnostics of o/p(H2)
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 24 / 27
Conclusions / Perspectives
Perspectives
?ALMA
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 25 / 27
Conclusions / Perspectives
Perspectives
Theory / Models
• Explore consequences of o/p(H2) on other groups of chemical species:C-bearing, O-bearing, S-bearing
• Include a full o/p chemistry in our 15N chemical network
• Propagate branching ratios (Rist et al 2013, Hugo et al 2009) todeuterated nitrogen hydrides (e.g. ND, NHD, NH2D, etc)
• Inlfuence of the total gas-phase abundances (e.g. C, O, N, S)
Observations
• Derive observational diagnostics of o/p(H2)
• Nitrogen fractionation in TWHya disk: CN, HCN
• Nitrogen fractionation in dark clouds: ammonia, HCN
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 26 / 27
Conclusions / Perspectives
Dislaire, V., Hily-Blant, P., Faure, A., et al. 2012, A&A, 537, A20
Faure, A., Hily-Blant, P., Le Gal, R., Rist, C., & Pineau des Forets, G. 2013, ApJ, 770, L2
Herbst, E. & Klemperer, W. 1973, ApJ, 185, 505
Hily-Blant, P., Bonal, L., Faure, A., & Quirico, E. 2013, Icarus, 223, 582
Le Gal, R., Hily-Blant, P., Faure, A., et al. 2014, A&A, 562, A83
Pineau des Forets, G., Roueff, E., & Flower, D. R. 1990, MNRAS, 244, 668
Rist, C., Faure, A., Hily-Blant, P., & Le Gal, R. 2013, Journal of Physical Chemistry A, 117, 9800
Vigren, E., Zhaunerchyk, V., Hamberg, M., et al. 2012, ApJ, 757, 34
P. Hily-Blant (IPAG/IUF) Nuclear Spin Effects in Astrochemistry Goteborg, June 9-11th 2014 27 / 27