ortho / para effects in interstellar chemical models · interstellar gas phase molecular...

! Brief introduction on interstellar physical conditions ! Examples of state to state chemistry in IS conditions ! Deuterium fractionation (see Pagani talk) ! Nitrogen chemistry (see Hily-Blant talk) ! Separation of ortho/para species in chemical networks

! Photon Dominated and Diffuse regions (cf Indriolo talk) ! Revisiting the N+ + H2 (HD) reaction ! Conclusions

T. Grozdanov Institute of Physics, Belgrade, [email protected] "

R. McCarroll LCPMR, [email protected]"

B. Godard, F. Le Petit, J. Le Bourlot, E. Roueff"

Ortho / para effects in interstellar chemical models

Interstellar gas phase species

Reactive species Isomers

Open system No thermal equilibrium achieved

Interstellar gas phase molecular diagnostics

! Density : 10 – 106 cm-3 (excitation conditions)

! Tgaz : 10 – several hundreds K

☞ binary exothermic reactions ☞ possible role of H2 excited levels in overcoming small endothermicities or

activation barriers - state to state chemistry - ortho/para discrimination

! Ionization fraction ☞ Photoionization (ISRF and nearby hot stars in low extinction environments) ☞ X ray sources ☞ cosmic rays (High energy particles from SN explosion)

! Evolution time provided by chemistry?

Astrochemical ISM modelling

Chemical balance: dni / dt = F – Dni = 0

Thermal balance gas : G(T) – L(T) =0

May concern gas + accreted molecules ! Time evolution ni(t) ! Steady state ? ! stochastic effects on grains?

Thermal balance dust: G(Tdust) – H(Tdust) =0 ! temperature fluctuations ! Dependence on the nature of the grains ! Dependence on the size

86 "

H3+ + HD

232

H2D+ H2

000"

101"

111"

110

0"

170.5"104 "

65"

265 10

ortho para

202

Emission Prestellar cores Caselli et al. 2003, AA 403, L37 Caselli et al. 2008, AA 792, 703 Vastel et al. 2006, ApJ 645, 1198

High mass star forming regions Pillai et al. 2012, ApJ 751, 135 Harju et al. 2006, AA 454, L55

212 199.5 "

211 253 "

Absorption SgrB2 Cernicharo et al. 2007, ApJ 657, L21

0"

189 "

K

11

K K

1"

H2D+ + H2

372 GHz

2363 GHz

Observations o-H2D+

86 "

H3+ + HD

232

H2D+ H2

000"

101"

111"

110

0"

170.5"104 "

65"

265 10

ortho para

202

If some H2 (J=1) is present, reverse reaction becomes efficient even at low temperatures, H2D+ is formed less efficiently and deuterium fractionation is reduced.

212 199.5 "

211 253 "

0"

189 "

K

11

K K

1"

H2D+ + H2

372 GHz

2363 GHz

Fractionation further proceeds : H2D+ + HD

H2D+ + HD D2H+"

ortho para

692 GHz

H2

Vastel et al. 2004, ApJ606, L127 Parise et al. 2011, AA 526, A31 Vastel et al. 2012, AA547, A33

K

K K

1477 GHz

Observations p-D2H+

Revisiting o/p chemistry at the light of H2D+ and D2H+ detections:

1. Assumption of complete depletion (Walmsley, Flower & Pineau des Forêts 2004, 2005, 2007 Only 1 level of each nuclear symmetry (ortho, para, meta) H2, H3

+, H2D+, D2H+, D3+, H2

+, D2+

selection rules for H3+ + H2, H3

+ + HD, a lot of discussions! a few measured o/p dependent rates (DR of H3

+, H2D+) linked with theoretical discussion a few theoretical calculations as well : D+ + H2 (Honvault + Scribano JPCA 2013, McCarroll PhyS 2011)

First introduced by Pagani, Salez & Wannier, 1992, AA258, 479

H2 A (s-1) H3+ A (s-1) H2D+ A(s-1)

(0-0) S(0) 2.95(-11) 22 - 11 4.26(-7) 101 - 000 3.59(-3)

(0-0) S(1) 4.77(-10) 21 - 22 5.66(-7) 110 - 111 1.08(-4)

(1-0) O(2) 8.56(-7) (1-0) Q(1) 4.30(-7) (1-0) S(1) 3.48(-7)

(radiative transition >> collisional de-excitation)"

o-H3+

p-H3+

I=3/2

I=1/2

Revisiting o/p chemistry at the light of H2D+ and D2H+ detections:

1. Assumption of complete depletion (Walmsley, Flower & Pineau des Forêts 2004, 2005, 2007 Only 1 level of each nuclear symmetry (ortho, para, meta) H2, H3

+, H2D+, D2H+, D3+, H2

+, D2+

selection rules for H3+ + H2, H3

+ + HD, a lot of discussions! a few measured o/p dependent rates (DR of H3

+, H2D+) linked with theoretical discussion a few theoretical calculations as well : D+ + H2 (Honvault + Scribano JPCA 2013, McCarroll PhyS 2011)

2. Introduction of heavy reactants (Pagani+ 2009, 2011, Wirström+ 2012, Roueff+ 2013, Sipilä+ 2013, Albertsson+ 2014 …) Rules for H2 formation in A + H3

+ : Ex: H3+ (o) + CO → HCO+ + o-H2 kmeas

H3+ (p) + CO → HCO+ + p-H2 2/3 kmeas 1/2 kmeas

→ HCO+ + o-H2 1/3 kmeas 1/2 kmeas A + H2D+ H2D+(p-o) + CO → DCO+ + (p-o)-H2 (conservation of p-o character) 1/3 kmeas + CO → HCO+ + HD 2/3 kmeas NHn

+ + H2(o,p) chemistry (cf Rist et al. JPCA 117, 9800, 2013) H2 formation without H3

+ : Ex: NH+ + H2O → HNO+ + p-H2 only para formation

➥ Inclusion of some reactions displaying different reactivity with o/p H2 : Ex: N+ + H2 → NH+ + H F + H2 → HF + H DR

First introduced by Pagani, Salez & Wannier, 1992, AA258, 479

H2 A (s-1) H3+ A (s-1) H2D+ A(s-1)

(0-0) S(0) 2.95(-11) 22 - 11 4.26(-7) 101 - 000 3.59(-3)

(0-0) S(1) 4.77(-10) 21 - 22 5.66(-7) 110 - 111 1.08(-4)

(1-0) O(2) 8.56(-7) (1-0) Q(1) 4.30(-7) (1-0) S(1) 3.48(-7)

(radiative transition >> collisional de-excitation)"

o-H3+

p-H3+

I=3/2

I=1/2

00

D2H+ + HD

000

111

284

234

305

101

H2

1

0

11

10

22

D3+

46.5

63

0

+

ortho: A1 (10)"para : A2 (1)"meta : E (8)"

123

172 21 20

33

32

188 232

170.5

0

K K K

CH3+ + HD

654

CH2D+ H2

000"101"

111"0

1170

662 11 10

ortho para

202"212" 211"

0

221"220"

303" 313"312"322"321"404"

414"331 330"413 423"422"505, 515"

735"725"818"808" 716 "717 707"625 615"533"616"606"524"514"

817 827"

Not to scale … levels of CH2D+ from CDMS

110"

K

K

862 854"

845 836"826"919 909"

844 927"937 853"

New analysis of ZPEs Roueff+ 2013

Reverse reaction inhibited even at moderate temperature ∆E ≈ 480 K

CH3+ + HD offers a deuteration pathway of HCN and H2CO

at moderate temperature (Roueff, Parise & Herbst 2007, Parise + 2009) negligible effect on ortho/para ratio of H2

CH2D+ tentatively detected

Ori KL "

MonR2"

Courtesy of S. Trevino"

Roueff + 2013"

“Meudon“ PDR code : http://pdr.obspm.fr"

Le Petit et al. 2002, 2006, 2009, "Goicoechea & Le Bourlot 2007, Gonzalez-Garcia et al. 2008, Le Bourlot et al. 2012, Sternberg et al. 2014, AstroPh

UV radiative transfer

Chemistry Thermal balance

Abundances Excitation & Emissivities

Gas and dust temperatures

xi(Av), T, column densities

Intensities / Spectra

output :

UV

UV Equation of state (n, p), vturb, size, Avtot

Uinc, ζ, grain properties, elemental abundances input

Photon Dominated Regions (PDR) and diffuse regions

Benchmark of PDR models: Röllig et al. 2007

Atomic and molecular properties (spectral, collisions,

chemical reactions)

Compare with observations

Coupling between chemistry and excitation of H2

Emission at λ ≈ 1600 A

1. absorption followed by spontaneous emission into the continuum

leads to dissociation : efficient selfshielding depending on pure H2

molecular properties (but also dust shielding…)

fluorescence below H(1s) + H(1s) leads to IR cascades

2. Collisions with H, He, H2, e, H+, H3+

3. gas phase : assumptions of formation in the ground state

some specific rates for H2 (p/o) destruction

solid phase : depends on the formation mechanism

LH

ER,

and on the nature of the surface 0

2

4

6

8

10

12

eV

Coupling between chemistry and excitation of H2

Emission at λ ≈ 1600 A

1.

No ortho/para transfer 2. Collisions with H, He, H2, e, H+, H3

+ (see talk of F. Lique)

3. gas phase : assumptions of formation in the ground state

some specific rates for H2 (p/o) destruction

solid phase : depends on the formation mechanism

LH

ER,

and on the nature of the surface 0

2

4

6

8

10

12

eV

Lyman-Werner radiative transfer: line + dust"Sternberg et al. 2014, Le Petit et al. 2006 "

Numerical radiative transfer on a fine frequency grid with a spectral resolution ≈ 105"

partial overlap of H2 LW transitions is displayed;"case of beamed radiation, NH = 3.74 1020 cm-2, n=103 cm-3, IUV= 35.5, Z’=1 (Sternberg+ 2014) "

Revisiting o/p chemistry in diffuse cloud conditions

“Meudon“ PDR model 1 side nH = 10 cm-3

T = 100 K “Draine“ radiation field

The H2 photodissociation rate is attenuated by a combination of dust opacity and line-self-shielding + H2 overlap (not included here).

computed photodissociation rate for the first J levels of H2

➜ Similar photodissociation rates at the edge

➜ strong dependence with Av ➜ line self-shielding effect ➜ role of dust opacity

➜ to be weighted by the corresponding H2 level populations

increasing Av


Full photodissociation rate, after summation over all J levels

increasing Av increasing Av


Comparison with Albertsson et al. 2014, ApJ787, 44 : Pd(p-H2,o-H2) = 3.4 10-11 exp(-2.5Av) * fselfshielding (s-1)

H2 ortho / H2 para photodissociation rates equivalent ?

nH = 10 cm-3, ζ = 10-16 s-1, Draine Radiation field

increasing Av increasing Av increasing Av

fselfshielding (s-1) depends on N(H2), b (Draine+Bertoldi 1996 formula) tested to be OK

Diffuse cloud models nH/T, role of H3+ + H2 collisions

H3+- H2 collision

rates of Gomez-Carrasco et al 2012 JCP 137, 4303 with selection rules and different rates for p and o-H2

H3+- H2 collision

rates of Oka + Epp 2004, no selection rules

Observational constraints

(3,3) / (2,2) > 5 0.7 < (3,3) / (1,1) < 1.5

Diffuse cloud models with thermal equilibrium, Av,tot = 1, H3+

nH = 50 cm-3

100 cm-3

1000 cm-3

LH + ER H2 formation qPAH = 4.6 10-2

3 x 10-17 cm3 s-1 H2 formation rate qPAH = 0

Diffuse cloud models with thermal equilibrium, Av,tot = 1, OH+ nH = 50 cm-3

100 cm-3

1000 cm-3



Diffuse cloud models with thermal equilibrium, Av,tot = 1, H2O+ nH = 50 cm-3

100 cm-3

1000 cm-3



Two main effects

Enhancement of H2 formation through LH + ER formation ➥ increase of heating through H2 formation

Formation on PAHs

Reduction of electrons due to recombination on PAHs ➥ Increase of H3

+, OH+, … by about 1 order of magnitude ➥ Increase of temperature

∆E Reaction (meV) AS85 M88 SA 94 T94 G93 Z13

N+ + H2 ⟶ NH+ + H - 11± 3 -18 ± 2 -18 ± 2 - 11 - 17 - 19

N+ + D2 ⟶ ND+ + D - 33 ± 4 -46 ± 2 - 29 - 35

N+ + HD ⟶ NH+ + D - 43 ± 6 - 54 ± 2

N+ + HD ⟶ ND+ + H 10 ± 4 -1.4 - 4 ± 2

Experimental endothermicities/activation barriers of N+ (3P) + H2, D2 and HD reactions.

AS85: Adams & Smith 1985, CPL117, 67 M88: Marquette et al 1988, JCP 89, 2041 G93: Gerlich 1993, J. C.Soc.Faraday Trans. 89, 2199 SA94: Sunderlin & Armentrout 1994, JCP 100, 5639 T94: Tosi et al. 1994, JCP 100, 4300 Z13: Zymak et al. 2013, ApJ 768, 86

Endothermicities are interrelated ZPE of H2, HD, D2, NH+, ND+ known from spectroscopy Dissociation energy of NH+ and ND+ still under discussion

An additional look on the reactive collisions of N+ (3P) with H2, D2 and HD.

1meV = 11.6 K

Look on the N+ (3P) + HD reaction.

SA94: Measurements of x-sections as a function of energy PST calculations unsuccessful

Reaction N+ + HD particularly interesting ! very different endothermicities ! ND+ x-sections independent of T

➜ exothermic? ! NH+ x-sections highly dependent on T

➜ endothermic ! ratio of x-sections allows to eliminate some experimental uncertainties

N+ + HD ⟶ NH+ + D (a) ⟶ ND+ + H (b)

Main uncertainty: energy transfer between internal (rota5on + fine structure) and transla5onal

E(K) %@105 K %@305 K N+ 3P0 0 0.30 0.17

3P1 70 0.45 0.39 3P2 188 0.25 0.44

HD J=0 0 0.505 0.2 J=1 128.4 0.434 0.391 J=2 384.3 0.059 0.275

microcanonical statistical theory with conservation of energy, motional angular momentum, nuclear spin and parity can be used to calculate state to state reaction probabilities (Park & Light 2007, Grozdanov & McCarroll (2011, 2012). This method allows to compute the branching ratios between all of the different product channels (both reactive and non-reactive). To obtain correct absolute cross sections, all channels (reactive and non-reactive) must be considered. (Note that in the experimental measurements , the non-reactive channels are not measured explicitly.)

The general allure of the experiments is reproduced with a cross over of the NH+ and ND+ cross sections at 105 meV. But the NH+ cross section is much lower than the experimental value in the 10-40 meV range. On the other hand the ND+ cross section seems high. Clearly the statistical mixing method is successful in the energy range greater than 40 meV, it should also be a valid procedure at lower energies. So there must be some other reason. One possibility could be that while the relative endothermicities are OK, the absolute error could be much greater.

• Our first set of calculations using the endothermicities of 54 meV for (a) and 4 meV for (b), when both reactants are in their ground state, namely In the 3P0 state of N+ and in the j=0 state of HD.

N+ (3Pjs ) + HD(j) → NH+ (j’) + D (a) N+ (3Pjs ) + HD(j) → ND+ (j’’)+ H (b) • The calculations are performed for all js and j states which are thermally populated at some given

temperature (TN for the N+ temperature and THD for HD) . A sum is then made over all possible j’ and j’’ states of the reaction products. Typical results are compared with the experimental cross sections in the figure

" Reducing the endothermicity of (a) improves the results, but there is still a problem of reconciling the low meV results with expt.

" If the reactions are endothermic, the values of the 4 reactions are all related. Then, adopting 44meV for reaction (a) leads to 8 meV for the endothermicity of N+ + H2(J=0) and an exothermic reaction for N+ + H2(J=1)

????

Reducing the endothermicity of (b) to 44 meV

This figure illustrates the results of a typical calculation. The red and green curves correspond to the ND+ cross section for a HD temperature of 305 and 105 K. The blue and cyan curves correspond to the NH+ cross section for 305 and 105 K The black curves give the total non-reactive cross section (elastic, excitation or de-excitation of the vibration-rotation levels of HD). Solid curves for THD=305K, dotted curve for THD=105 K. The magenta curve is the total reactive and non-reactive cross section. Note that this curve is identical to the Langevin cross section. TN+

= 300K

N+ + HD ⟶ NH+ + D (a) ⟶ ND+ + H (b)

Preliminary conclusions on the N+ + H2 reaction.

# Work still in progress

# To obtain better agreement with the experimental measurements, it looks necessary to reduce previous estimates of the endothermicities by about 10-15 meV. This has the following consequences for reactions occurring at very low temperatures, where only the ground state of the reactants is populated.

• The reaction (b) becomes exothermic and has a large Langevin type reaction rate for ND+ formation

• The reaction N+ + H2 is endothermic for para H2 but may be exothermic for ortho-H2

# disturbing discrepancy at low kinetic energy. Introduction of a barrier taking into account thermal motion of targets in defining relative impact energy;

32

Recap and conclusions

H2D+, D2H+ detections have allowed to recognize potential importance of state to state"chemistry in the ISM"Reciprocally, introduction of state to state chemistry (thanks to fundamental experimental"/ theoretical studies) in astrochemical models may have significant effects"

Deuteration via D+ transfer in H2D+ + X reactions : high dependence on H2 J=1/J=0 " role of temperature, density, time evolution"N+ + H2 : is the story completely settled? May be not"" " " " ➜ influence on D and N fractionation "Diffuse cloud conditions: " ➜ H2 photodissociation dependent on o/p state in partly molecular dusty" environments" ➜ Role of PAHs on H2 formation and ion neutralization" ➜ efficient heating of cosmic rays" ➜ Ratio of H3

+ level populations reproduced for a range of diffuse conditions"

ortho / para effects in interstellar chemical models · interstellar gas phase molecular...

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