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! 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
Revisiting o/p chemistry in diffuse cloud conditions
Full photodissociation rate, after summation over all J levels
increasing Av increasing Av
Revisiting o/p chemistry in diffuse cloud conditions
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, 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
3 x 10-17 cm3 s-1 H2 formation rate qPAH = 0
LH + ER H2 formation qPAH = 4.6 10-2
Diffuse cloud models with thermal equilibrium, Av,tot = 1, H2O+ 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
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"