Astrochemistry Les Houches Lectures September 2005 Lecture 3

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AstrochemistryLes Houches LecturesSeptember
2005Lecture 3

• T J Millar
• School of Physics and Astronomy
• University of Manchester
• PO Box88, Manchester M60 1QD

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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)

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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
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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)

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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)
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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

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Water in Cold Clouds

• SWAS
• o-H2O at 557 GHz in B68 and ? Oph D

Bergin Snell, ApJ, 581, L105 (2002) Non-detecti
on of water with fractional abundances relative
to H2 of 3 10-8 (B68) and 6 10-9 (? Oph D)
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Solution Accretion?

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Solution ? (Bergin et al.)

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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
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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.
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Water formation in shocks
EA/k 3150 1740
O OH H2O
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.
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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

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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 lt Ti, and Tn (C)
ltlt Tn (J) Ion and neutral velocities are
different (ion-neutral drift), typically
VS/2 Chemical path-length is much larger
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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
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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
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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
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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 doesnt work (2) Freeeze H2O when
gas cools doesnt 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
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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
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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
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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
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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, AA, 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 kfrP(i-1) -
P(i) kev(i1)P(i1) iP(i) 0.5kHH(i2)(i
1)P(i2) 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
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What have I missed ?

• Protoplanetary Accretion Disks

H2CO distribution in the inner 10 AU of a PPD

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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).

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What have I missed ?

• Diffuse Interstellar Clouds
• Circumstellar Envelopes
• Protoplanetary Nebulae
• Comets
• The Early Universe
• Protostellar Chemistry
• Deuterium Fractionation

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IRAS 16293-2422
N2D 3-2
D2CO 5-4

OCS 9-8
13CS 5-4