AstrochemistryLes Houches Lectures

September 2005Lecture 3

T J MillarSchool of Physics and Astronomy

University of ManchesterPO Box88, Manchester M60 1QD

Dissociative Recombination

H3+:

CRYING measurement at Trot = 30 K = 6.7 10-8(T/300)-0.52

(McCall et al., Phys Rev A, 70, 057216, 2004)

N2H+:CRYING measurement

= 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)

Dissociative RecombinationCH3OH2

+

Branching ratio to methanol is 5% - most models assume 50%

(Geppert et al. 2005)

Observed fractional abundance in dark clouds ~ 10-8 – 10-9

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)

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)

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

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)

Solution – Accretion?

Solution ? (Bergin et al.)

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

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.

Water formation in shocks

O OH H2OEA/k 3150 1740

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.

Water formation in shocksHydrodynamic 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

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

Water formation in MHD shocks

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

Flower et al. 1987, MNRAS, 227, 993

Water formation in MHD shocks

Flower et al. 1987, MNRAS, 227, 993

Water has a low abundance per unit volume but a long path length

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

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

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

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

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

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

What have I missed ?

• Protoplanetary Accretion Disks:

H2CO distribution in the inner 10 AU of a PPD

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).

What have I missed ?

• Diffuse Interstellar Clouds

• Circumstellar Envelopes

• Protoplanetary Nebulae

• Comets

• The Early Universe

• Protostellar Chemistry

• Deuterium Fractionation

IRAS 16293-2422

OCS 9-813CS 5-4

N2D+ 3-2 D2CO 5-4