Astrochemistry Les Houches Lectures September 2005 Lecture 3

1. AstrochemistryLes Houches LecturesSeptember 2005Lecture 3 T J Millar School of Physics and Astronomy University of Manchester PO Box88, Manchester M60 1QD

2. Dissociative Recombination H3+: CRYING measurement at Trot = 30 K a = 6.7 10-8(T/300)-0.52 (McCall et al., Phys Rev A, 70, 057216, 2004) N2H+: CRYING measurement a = 1.0 10-7(T/300)-0.51 N2H+ + e  NH + N 0.64 N2H+ + e  N2 + H 0.36 Consequences: N2H+ is depleted at high density. ( (Geppert et al., ApJ, 609, 459, 2004)

3. Dissociative Recombination CH3OH2+ Branching ratio to methanol is 5% - most models assume 50% (Geppert et al. 2005) Observed fractional abundance in dark clouds ~ 10-8 – 10-9

4. Chemical Databases UMIST Database for Astrochemistry: Rate99:4000 reactions, 400 species, 12 elements www.rate99.co.uk Rate04:4500 reactions, 413 species, 12 elements www.udfa.net -Improved n-n rate coefficients (Smith et al. 2004, M Agundez) - Improved cosmic-ray-induced photoreactions (Doty) - Improved i-n reactions (Anicich) - Additional photorates (Herbst & Leung, van Dishoeck) - Improved dissociative recombination rates and branching ratios (Geppert)

5. Chemical Databases Rate04 Oxygen Chemistry: Extremely low abundance of CH3OH Implication – Methanol is made by grain surface reactions in dense IS clouds k(CH3+ + H2O) = 2.0 10-12 cm3 s-1 (Experiment at low T – Luca, Voulot & Gerlich)

6. Chemical Databases Ohio State University (OSU): Gas Phase:4300 reactions, 430 species, 12 elements - 3 basic reaction sets available NIST Chemical Kinetics Database: Gas Phase neutral-neutral: 27,000 reactions, theory and experiment, generate best fit JPL Anicich Database: Gas Phase ion-neutral:‘all’ reactions in 1936-2003, products, 1200 pages, 2300 references Huebner Photo-Cross-Section Database: About 60 atoms/molecules listed

7. Water in Cold Clouds SWAS: o-H2O at 557 GHz in B68 and ρ Oph D: Bergin & Snell, ApJ, 581, L105 (2002) Non-detection of water with fractional abundances relative to H2 of 3 10-8 (B68) and 6 10-9 (ρ Oph D)

8. Solution – Accretion?

9. Solution ? (Bergin et al.)

10. Solution ? (Spaans & Van Dishoeck) Clumpy interstellar clouds: Allows for greater penetration of UV photons which can destroy H2O and O2 very effectively Dashed lines – homogeneous models Solid lines – clumpy model In the end, solutions depend on physics not on chemistry

11. Water formation in shocks Supersonic shock waves: Sound speed ~ 1 km s-1 Shocks compress and heat the gas Hydrodynamic (J-type) shocks: immediately post-shock, density jumps by 4-6, gas temperature ~ 3000(VS/10 km s-1)2 Gas cools quickly (~ few tens, hundred years) and increases its density further as it cools. Importance for chemistry: Endothermic neutral-neutral reactions can occur.

12. Water formation in shocks EA/k 3150 1740 OOHH2O EA/k 1950 9610 Water formation requires high temperature to overcome activation energy barriers, and the balance between O/OH/H2O depends on the H/H2 ratio – but because of the large barrier to the H + H2O reaction, it is easy to convert O to H2O for moderate shock velocities, 5-15 km s-1. The rate coefficients are well-determined experimentally over temperature ranges from 300-3000K, typically.

13. Water formation in shocks Hydrodynamic shock: Shock speed VS ~ 10 km s-1 Pre-shock O atom abundance n0(O), cooling time tc T(t) = Tps(0)exp(-t/tc) In a cooling time, the shock front sweeps up a column density: N(O) = VSn0(O)tc If a fraction f is converted to water then N(H2O) = fVSn0(O)tc With typical parameters, VS = 10 km s-1, tc = 100 yrs, n0(O) = 0.1 cm-3, and if f = 1, then N(H2O) = 3 1014 cm-2, a small column density

14. Water formation in MHD shocks MHD (C-type) shocks: Magnetic fields mediate the effect of the shock wave. A magnetic precursor allows the pre-shock gas to respond to the arrival of the shock Consequences: Ion flow and the neutral flow are de-coupled Ion and neutral temperatures are different Tn < Ti, and Tn (C) << Tn (J) Ion and neutral velocities are different (ion-neutral drift), typically VS/2 Chemical path-length is much larger

15. Water formation in MHD shocks Flower et al. 1987, MNRAS, 227, 993 Shock velocity = 15 km s-1, T(ps, HD) ~ 5000K; here it is ~ 500K. Ion-neutral rather than neutral-neutral chemistry may dominate – water can be difficult to form – but path-length over which shock acts is 5 1017 cm – HD case, it is VStc = 5 1015 cm

16. Water formation in MHD shocks Water has a low abundance per unit volume but a long path length Flower et al. 1987, MNRAS, 227, 993

17. Water in shocks • SWAS observations of IC443: Snell et al. ApJ, 620, 758 (2005) o-H2O/CO ~ 2 10-4 – 3 10-3 Or o-H2O/H2 ~ 10-8 Again, seemingly a big discrepancy between observation ands theory Fast J shocks: too little H2 IR, ok for H2O Slow J shocks: cannot produce H2 and OI emission, too much water Fast C shock: cannot produce H2 and OI emission, too much water Slow C shock: too little H2 IR, ok for H2O, too little CII

18. Water in shocks • SWAS observations of IC443: Fundamental problem: H2 IR emission requires T ~ 1000 K At these temperatures all O not in CO is converted to H2O Solutions(?): (1) Large H abundance – doesn’t work (2) Freeeze H2O when gas cools – doesn’t work (3) Freeze all free O as H2O before the shock arrives (4) Photodissociative H2O with UV photons produced in fast shock (5) Shocks are not in steady-state (6) Several types of shock are present

19. Grain Surface Chemistry • Deterministic (Rate Coefficient) Approach: Basics: Define an effective rate coefficient based on mobility (velocity) and mean free path before interaction (cross-section). Let ns(j) be surface abundance (per unit volume) of species i which has a gas phase abundance n(i). Then we can write the usual differential terms ofr formation and loss of grain species allowing for surface reaction, accretion from the gas phased and desorption from the grain. Technique: Solve the set of coupled ODEs which describe grain surface and gas phase abundances (approximately doubles the no. of ODEs) Problem: Rate equations depend on an average being a physically meaningful quantity – ok for gas but not for grains 4 grains + 2 H atoms – average = 0.5 H atoms per grain BUT reaction cannot occur unless both H atoms are actually on the same grain

20. Grain Surface Chemistry • Stochastic (Accretion Limit) Approach: Basics: Reaction on the surface can only occur if a particle arrives while one is already on the surface – the rate of accretion limits chemistry Technique: Monte-Carlo method – attach probabilities to arrival of individual particles and fire randomly at surface according to these probabilities Caselli et al. 1998, ApJ, 495, 309 Agreement between rate and MC poor for low values of n(H) – as expected

21. Grain Surface Chemistry • Stochastic (Accretion Limit) Approach: Solution?: Improve method of calculating surface rate coefficients Problem: Modifications cannot be a priori – you need a MC calculation – and these are ‘impossible’ for large numbers of species Caselli et al. 1998, ApJ, 495, 309 Fully modified rate approach

22. Grain Surface Chemistry • Stochastic (Accretion Limit) Approach: Solution?: Master Equation Reaction depends on the probabilities of a particular number of species being on the grains e.g. PH(0), PH(1), PH(2), … PH(N), PO(0), PO(1), … Biham et al. 2001, ApJ, 553, 595 Green et al. 2001, A&A, 375, 1111 Technique: Integrate the rates of change of probabilities, eg dPH(i)/dt Problem: Formally, one has to integrate an infinite number of equations For a system of H only: dP(i)/dt = kfr[P(i-1) - P(i)] + kev[(i+1)P(i+1) – iP(i)] +0.5kHH[(i+2)(i+1)P(i+2) –i(i-1)P(i)] for all I = 0 to infinity For larger systems, eg O, OH, H2O, H, H2, the ODEs get very complex – even the steady state solution is difficult to solve

23. What have I missed ? • Protoplanetary Accretion Disks: H2CO distribution in the inner 10 AU of a PPD

24. What have I missed ? • Hot Molecular Cores: Detailed spatial (and temporal) distributions depend on details of surface binding energies, the detailed process by which species evaporate, and the grain temperature Can induce lots of small scale structure amenable to interferometers (particularly ALMA).

25. What have I missed ? • Diffuse Interstellar Clouds • Circumstellar Envelopes • Protoplanetary Nebulae • Comets • The Early Universe • Protostellar Chemistry • Deuterium Fractionation

26. IRAS 16293-2422 N2D+ 3-2 D2CO 5-4 13CS 5-4 OCS 9-8