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