The Chemistry of Interstellar Molecular Clouds

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The Chemistry of Interstellar Molecular Clouds Powered By Docstoc
					The Chemistry of Interstellar Molecular Clouds
T J Millar School of Mathematics and Physics Queen’s University Belfast

Dark Interstellar Clouds
Masses: 10-500 solar masses Sizes: 1-5 pc Temperature: 10 K Density: 104 cm-3 Form few low mass stars

H2 formed on dust grains
Carbon chains dominate Molecular D/H ratios ~ 0.01-0.05 f(e-) ~ 10-7 - 10-8 Icy grain mantles of H2O, CO, ..

Barnard 68

Star-Forming Hot Cores
Density: 106 - 108 cm-3 Temperature: 100-300 K Very small UV field Small saturated molecules: NH3, H2O, H2S, CH4 Large saturated molecules: CH3OH, C2H5OH, CH3OCH3 Large deuterium fractionation Few molecular ions - low ionisation ? f(CH3OH) ~ 10-6

Formation of Molecules
Ion-neutral reactions: Activation energy barriers rare if exothermic Temperature independent (or inversely dependent on T) Neutral-neutral reactions: Often have activation energy barriers Often rate coefficient is proportional to temperature

Dense Clouds
• H2 forms on dust grains • Ion-neutral chemistry important • Time-scales for reaction for molecular ion M+
– 109/n(H2) – 106/n(e) – 109/n(X) for fast reaction with H2 for fast dissociative recombination with electrons for fast reaction with X

Since n(e) ~ 10-8n, dissociative recombination is unimportant for ions which react with H2 with k > 10-13 cm3 s-1; Reactions with X are only important if the ion does not react, or reacts very slowly, with H2.

The UMIST Database for Astrochemistry 2005

4566 binary reactions among 420 species 13 elements – H, He, C, N. O, F, Na, Mg, Si, P, S, Cl, Fe

Fluorine included for first time
540 neutral-neutral reactions - 292 measured 2940 ion-neutral reactions - 1160 measured 510 dissociative recombination reactions/product channels – 100 measured

25 radiative recombinations
90 radiative associations 11 cosmic-ray ionisation reactions 155 cosmic-ray-induced photoreactions 216 photoreactions 32 ternary reactions Database including ion-dipole enhanced rate coefficients also available

Source of data


Oxygen Chemistry
H3+ + O OH+ + H2 H2O+ + H2 H3O+ + e  OH+ + H  H2O+ + H  H3O+ + H  O, OH, H2O M M M M

Destruction of H2O: He+, C+, H3+, HCO+, .. (M)
Destruction of OH: He+, C+, H3+, HCO+, .. ,

Oxygen Chemistry
O + OH  H + O2 C + OH  H + CO N + OH  H + NO S + OH  H + SO Si + OH  H + SiO C + O2  CO + O M for T > 160K, fast M for T > 100K, fast M at T = 300K, fast

M for T > 15K, fast

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 !!!

Oxygen Chemistry

Sulphur Chemistry

Formation of Organics
Starts with proton transfer from H3+
C + H3+  CH+ + H2 CH+ + H2  CH2+ + H CH2+ + H2  CH3+ + H CH3+ + H2  CH5+ + hυ CH5+ + CO  CH4 + HCO+

C+ + CH4  C2H2+ + H2
C+ + CH4  C2H3+ + H

Formation of Hydrocarbon Chains
C insertion:
C + CmHn+  Cm+1Hn-1+ + H C+ + CmHn  Cm+1Hn-1+ + H C + CmHn  Cm+1Hn-1 + H Binary reactions: C2H + C2H2+  C4H2+ + H

C2H + C2H2  C4H2 + H
CN + C2H2  HC3N + H


Formation of Organics
Radiative association:

CH3+ + H2O  CH3OH2+ + hυ
CH3+ + HCN  CH3CNH+ + hυ CH3+ + CH3OH  CH3OCH4+ + hυ Dissociative recombination: C2H3+ + e-  C2H2 + H CH3OH2+ + e-  CH3OH + H CH3OCH4+ + e-  CH3OCH3 + H

Formation of Organics

Most complex molecules are difficult to form at low temperature in gas-phase reactions

Dissociative Recombination
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; NH is abundant and ND/NH ~ N2D+/ N2H+ is large – ND observable at 492 GHz (Geppert et al., ApJ, 609, 459, 2004)

Some key rate coefficients
Methanol Formation: Radiative association CH3+ + H2O  CH3OH2+ + h k = 5.5 10-12 (T/300)-1.7 cm3 s-1 (Theory based on 3-body association) k = 2.0 10-12 cm3 s-1 (Experiment at low T, rate coefficient independent of T down to 80K – Luca, Voulot & Gerlich)

Dissociative Recombination
Branching ratio to methanol is 5% - most models assume 50% (Geppert et al. 2005) Steady state abundance of methanol decreases by over 100

Early time abundance decreases by over factor of 10
All results involving new rate coefficients are at least 10 times less than those observed in dark clouds.

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



OCS 9-8

Complex molecules around low mass protostars
Recent observations at the JCMT and IRAM show evidence of complex molecules in a low-mass protostar for the first time

Detected species include methanol, acetaldehyde, methyl formate, ethyl cyanide, as well as evidence for large abundances of D-bearing molecules such as D2CO

Chemical modelling of IRAS 16293
• Depth dependence of temperature and density
• Deuterium fractionation • Accretion – ion-grain recombination

• Limited grain surface chemistry – CO → H2CO → CH3OH
• Follow frozen molecule and grain-produced molecules • Desorption by cosmic rays and by thermal heating

• Follow gas and mantle compositions

Chemical modelling of IRAS 16293
Pre-stellar core at 105 yr:

Mantles contain a layered history of the cloud composition.
Water and methane dominate the inner layers of the mantle, formed the hydrogenation of O and C.

CO and CH3OH and H2CO, both formed by the hydrogenation of CO, dominate the outer layers. T = 10K

Chemical modelling of IRAS 16293
Pre-stellar core at 105 yrs:

Because the outer layers form from highly-depleted gas, molecular D/H ratios can be large e.g.
HDCO/H2CO ~ 0.1 D2CO/H2CO ~ 0.005

Not sufficient to explain the observations.

Chemical modelling of IRAS 16293
Assume that gas is heated over time by central protostar (Viti & Williams)

Gas undergoes a ‘desorption wave’ as grains warm to observed temperature

Grains undergo differential desorption as most weakly bound material desorbs first

Chemical modelling of IRAS 16293
Following warming of grains evaporate mantle materials can form other complex species

Time-scales are dependent on the arrival of the ‘desorption wave’

Column densities are strong functions of time

.. and therefore of the physical model adopted

• Information on rate coefficients is better than ever • Pure gas-phase chemistry is well understood • Not withstanding this, gas-phase chemistry has difficulties • Gas-grain interaction is critical to understanding molecules in star-forming regions • Sensitive to many uncertain physical parameters – grain composition, size and morphology; binding energies, mobilities, surface chemistry, cloud density, temperature, structure • But, increasingly sensitive observations of both gas and grain species at increasing resolution