Chemical Composition of the interstellar grain mantle ...idmc2011/presentation/CT10.pdf ·...
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Chemical Composition of the interstellar grain mantle
under the effect of physical parameters
Dr. Ankan Das
Indian Centre for Space Physics
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1.Introduction 2.Basic physical Process 3.Results 4.Summary
Plan of my presentation
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Interstellar Medium
Composition: 99% gas (89% hydrogen 9% helium and 2% elements heavier than H or He) and 1% dust.
ISM is the matter that exists in the space between the star systems in a galaxy .
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Number Density (cm-3)
T(K) Visual
Extinction Chemistry
Diffuse atomic cloud
10-100 30-100
< 0.1 fnH2 < 0.1
Diffuse molecular
cloud 100-500
30-100
~ 1 fn
H2 > 1
CO, CH, CN, C2, C3, H3+, HCO+, OH, C2H
Translucent cloud
500-1000 15-50
>1
Chemistry differs due to absence of c+. conversion of atomic carbon occurs at substantial rate
Dense Molecular
cloud >104 10 >5
ISRF attenuated so much that rich chemistry occurs, complex molecules might be formed
Interstellar cloud
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Formation of proto-stars
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Type of grains
Olivine: (Mg,Fe)2SiO4
Amorphous Carbon
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Basic Physical Process on Grain Surfaces
Accretion
Fx= v Nx Nd
Diffusion tdiff=Nstn
tn= thop= n0-1 exp(Eb/KTd)
tn=ttun=n0-1[(4pa/h)(2mEb)1/2]
Ri,j = ki,j (Rdiff,i + Rdiff,j) ni nj Nd
Evaporation tevap =n0
-1exp(Ed/KTd)
Reaction
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Different approaches to handle the Grain Chemistry
A. Deterministic Approach B. Stochastic Approach
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Surface Reactions considered in our H,O & CO network
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Initial gas phase abundance
Species High density
(105 cm-3)
Intermediate density
(104 cm-3)
Low density
(103 cm-3) H 1.10 1.15 1.15
O 7.0 0.75 0.09
CO 7.5 0.75 0.075
Stantcheva et al., 2002
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Methanol formation in Mono-layer regime
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Das et al., 2008, A&A
Methanol formation in mono-layer regime
Observation: 2%<CH3OH<30% w.r.t. solid water
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Das et al., 2010, MNRAS
Typical structure of the grain mantle
Dominated by, H2O, CH3OH and CO2
Observation
No. of layers ~ 100 Surface coverage of H2O > 60% (~10-4) 2%<CH3OH<30% w.r.t. solid water 2%<CO2<20% w.r.t solid water
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Das & Chakrabarti, 2011, MNRAS
Different routes to the formation of Water
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Time evolution of some selected gas-grain species
Das et al., 2010, MNRAS
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Time dependent study
Das et al., 2010, MNRAS
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Variation of final abundance as a function of initial gas phase O
Das et al., 2010, MNRAS
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Variation of final abundance as a function of initial gas phase CO
Das et al., 2010, MNRAS
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Favourable zone for T=10K, AV=10, and nH=104 cm-3
Das et al., 2010, MNRAS
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Effect of Photo-dissociation
Cosmic Ray induced Photoreaction rates:
Direct cosmic ray rates:
Woodall et al., 2007 & Cazaux et al., 2010
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Das & Chakrabarti 2011, MNRAS
Percentage of Photo-dissociation as a function of extinction parameter
For AV<6 photo-dissociation effects are prominent
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Effect of Photo-dissociation
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Effect of Photo-dissociation
Das & Chakrabarti, 2011, MNRAS
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Following Lee et al., 1996 the relationship between hydrogen number density and visual extinction is given by,
Das & Chakrabarti, 2011, MNRAS
Variation of Column density with the extinction parameter
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Summary
For the lower value of Av(<6) photo-dissociation dominates
Stable species like H2O, CH3OH, and CO2 photo-dissociating more strongly in the Rate equation method
Formation of H2O via H2O2 and O3 also contributes significantly around the high density region
In a narrow region of parameter space the observed abundances of H2O, CH3OH, and CO2 could be reproduced
Monte Carlo method should be used to have an exact information about the interstellar grain mantle
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Thank You