Astrochemistry Les Houches Lectures September 2005 Lecture 2

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AstrochemistryLes Houches LecturesSeptember
2005Lecture 2

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

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Grain Surface Time-scales
Collision time tc vH(pr2nd)-1
109/n(cm-3) years Thermal hopping time th
?0-1exp(Eb/kT) Tunnelling time tt
v0-1exp(4pa/h)(2mEb)1/2 Thermal desorption
time tev ?0-1exp(ED/kT) Here Eb 0.3ED, so
hopping time lt desorption time For H at 10K, ED
300K, tt 2 10-11, th 7 10-9 s Tunnelling
time lt hopping time only for lightest species (H,
D) For O, ED 800K, th 0.025 s. For S, ED
1100K, th 250 s, tt 2 weeks
Heavy atoms are immobile compared to H atoms
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Formation of H2
Gas phase association of H atoms far too slow, k
10-30 cm3 s-1
Gas and dust well-mixed In low-density gas, H
atoms chemisorb and fill all binding sites
(106) per grain Subsequently, H atoms
physisorb Surface mobility of these H atoms is
large, even at 10 K. H atoms scans surface
until it finds another atom with which it
combines to form H2
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Formation of Molecular Hydrogen
Gas-Phase formation H H ? H2 h? very slow,
insignificant in ISM
Grain surface formation
Langmuir-Hinshelwood (surface diffusion)
Eley-Rideal (direct hit)
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Grain Surface Chemistry
Zero-order approximation Since H atoms are much
more mobile than heavy atoms, hydrogenation
dominates if n(H) gt Sn(X), X O, C, N Zero-order
prediction Ices should be dominated by the
hydrogenation of the most abundant species which
can accrete from the gas-phase Accretion
time-scale tac(X) (SXvXsnd)-1, where SX is
the sticking coefficient 1 at 10K tac
(yrs) 109/n(cm-3) 104 105 yrs in a dark
cloud
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Interstellar Ices
Mostly water ice Substantial components - CO,
CO2, CH3OH Minor components - HCOOH, CH4,
H2CO Ices are layered - CO in polar and
non-polar ices Sensitive to f gt 10-6 Solid
H2O, CO gaseous H2O, CO
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Results from a pseudo-time dependent model with
T10K, n(H2)106 cm-3
Fractional abundances varying over time
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Models - History
1950-1972 Grain surface chemistry H2, CH,
CH 1973-1990 Ion-neutral chemistry HD,
DCO 1990-2000 Neutral-neutral chemistry
HC3N 2000-date Gas/Grain interaction D2CO,
ND3 10,000 reactions, 500 species
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Dense Clouds

• H2 forms on dust grains
• Ion-neutral chemistry important
• Time-scales for reaction for molecular ion M
• 109/n(H2) for fast reaction with H2
• 106/n(e) for fast dissociative recombination
  with electrons
• 109/n(X) for fast reaction with X
• Since n(e) 10-8n, dissociative recombination is
  unimportant for ions which react with H2 with k gt
  10-13 cm3 s-1
• Reactions with X are only important if the ion
  does not react, or reacts very slowly, with H2.

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Oxygen Chemistry

• H3 O ? OH H2 M
• OH H2 ? H2O H M
• H2O H2 ? H3O H M
• H3O e ? O, OH, H2O M
• Destruction of H2O He, C, H3, HCO, .. (M)
• Destruction of OH He, C, H3, HCO, .. ,

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Oxygen Chemistry

• O OH ? H O2 M for T gt 160K, fast
• C OH ? H CO
• N OH ? H NO M for T gt 100K, fast
• S OH ? H SO M at T 300K, fast
• Si OH ? H SiO
• C O2 ? CO O M for T gt 15K, fast

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Oxygen Chemistry

• Conclude
• We should be able to explain the abundances of
  H2O (all reactions measured)
• - of OH (no i-n reactions measured, important
  n-n reactions measured)
• - of O2 (all reactions measured)
• But we cannot !!!

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Kinetic Calculation
h.rates
Rate file
hmain.f
hdata.out
h.specs
Species file
hodes.f
File of ODEs
inputhouches.f
Initialises GEAR
Pseudo-time-dependent calculation physical
parameters remain fixed with time
dvode1.f
GEAR codes
subs.f
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hmain.f

• FRAC(I) initial abundances for e,H2,He,O,C,N,Mg
• Rate file I, R1, R2, P1, P2, P3, P4, a, ß, ?
• k(I) a(T/300)ßexp(-?/T) cm3 s-1
• k(I) aexp(-?AV) if R2 PHOTON, AV in mags
• k(I) a?/(1-?) if R2 CRPHOT, ? albedo (
  0.5)
• k(I) a if R2 CRP
• Several k(I) have unphysical values at 10K
  (negative ?), these are reset in hmain.f
• Initial abundances of all species are set in
  hmain.f

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hodes.f

• (Algebraic) conservations are used to determine
  the abundances of e-, H2, and He
• Grain surface rate for H2 formation set in
  hodes.f and included as a loss term in the ODE
  for H atoms
• Term for accretion can be included in hodes.f
• YDOT(I) -SXvXsndn(I) -SXAn(I)/m1/2(I)
• where SX 0 for H, H2, He and their ions, 1
  otherwise
• Some collisions may not lead to sticking, eg X
  with a negatively charged grain, but to new
  gas-phase products
• Grain surface chemistry and physics can lead to
  additional ODEs

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Modelling task

Download gzipped tarfile http//jupiter.phy.umis
t.ac.uk/tjm/tjm.html Unzip (gunzip) and extract
(tar xvf example.tar) Run makefile make Run
job houches Tasks Can you make O2 and H2O
agree with observational abundances (upper
limits) in dark clouds (TMC-1, L134N)? Can you
make NO agree with its abundance in TMC-1? Web
sites www.rate99.co.uk and www.astrochemistry.ne
t
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Modelling task

• Elemental abundance variations
• Vary rate coefficients of key reactions
• Include accretion on to dust grains
• Vary density, temperature, visual magnitude,
  cosmic ray ionisation rate
• Consider abundances at early-time (105 yrs) and
  steady state (if the latter exists)