lpsc 2016-presentation

New Experimental Evidence of Silicate Formation with Meteorite Like Oxygen Isotopes on a Dust Surface Analog

Subrata ChakrabortyHaiyang Kehoe and M. H. Thiemens

University of California, San DiegoDepartment of Chemistry and Biochemistry, 9500 Gilman Drive, La Jolla, CA 92093-0356

•Dust particles are obiquitous throughout the universe•Dying stars are the ‘dust factories’ •Constitutes ~1 mass % in the ISM•As high as 10 % in planetary nebula•Dust particles are mainly of two types: carbonaceous and silicates•Long lifetime journey– stellar region through diffuse, dense or molecular clouds to the new born star or its planetary system •Significantly processed during this journey

Background

Rational

Condensation of oxides in gas phase were theoretically calculated

Ferrarotti and Gail, AA, 2002Mg + H2O → MgO(s) + H2Fe + H2O → FeO(s) + H22Fe + 3H2O → Fe2O3(s) + 3H23Fe + 4H2O → Fe3O4(s) + 4H2

Rational

Oxidation Reaction on the surface of an existing oxide: SiO, FeO, Al2O3 etc….

With absorbed water molecule Specifically in colder environments

Experimental

Experimental

Sample: Si, SiO, Al2O3…..

Experimental

Experimental

Four different sets of experiments:1.SiO vapor deposit + H2O2 vapor deposit2.SiO vapor deposit + H2O vapor deposit3.(Al2O3 + Si) vapor deposit + H2O vapor

deposit4.(Al2O3 + SiO) vapor deposit + H2O vapor

deposit

1 mm

1.SiO Vapor Deposit + H2O2@ 150 K

1.SiO Vapor Deposit + H2O2

Collection Plate

Side Wall- Foil

Collection Plate- Foil

Element

Atomic (%)

Si 44.2O 55.8

SiOx (<2)

Element

Atomic (%)

Si 38.9O 61.1

Element

Atomic (%)

Si 34.9O 65.1

SiO2

SiO2

@ 150 K

SiO2

1.SiO Vapor Deposit + H2O2@ 150 K

2. SiO Vapor Deposit + H2O @ 150 K

2 mm

Element

Atomic (%)

Si 45.5O 54.5

Stoichiometry: SiO1.2


Element

Atomic (%)

Si 41.8O 58.2

Stoichiometry: SiO1.4



Oxygen Isotopes

2. SiO Vapor Deposit + H2OTime Progression

27 Hrs

19 Hrs

5 Hrs

2.5 Hrs

1 Hr

@ 150 K

@ 300 K

3. (Al2O3 + Si) Vapor Deposit + H2O @150 K

2 mm


Element

Atomic (%)

Si 63.8Al 9.4O 26.8

Stoichiometry: AlSi6.7O2.9

4. (Al2O3 + SiO) Vapor Deposit + H2O @150 K

2 mm


Element

Atomic (%)

Si 38.6Al 16.9O 44.5


4. (Al2O3 + SiO) Vapor deposit + H2O vapor deposit

Element

Atomic (%)

Si 42.3Al 13.3O 44.4

~ 10 micron particle


Mechanism

Duley and Millar, 1978….. Model (theoretical) for the reaction of +ve ions

with –ve charged species on oxide grain surfaces

Recombination of gas phase ions with surface O- and OH-

Explains selective depletion of Al, Ca, Cr, Ti, Mn etc…..

MechanismDuley and Millar, 1978….. Model (theoretical) for the reaction of +ve ions

with –ve charged species on oxide grain surfaces

Recombination of gas phase ions with surface O- and OH-

Explains selective depletion of Al, Ca, Cr, Ti, Mn etc…..Other way round….

Reaction of adsorbed species (H2O) with the host species (Si, Al etc…)

Bikondoa et al., Nature 2006

MechanismSteps: Water adsorbs into O vacancies Dissociates into OH and H

fragments. The OH fragment is pinned at the

position of the O vacancy The H fragment attaches to

another oxygen from the bridging-O rows forming a second OH groupHypothesis:XO + OH → (XOOH)* → XOO + H

Mechanism

Based on Marcus, 2004

ComparisonGas Phase Reactions

Chakraborty et al., Science, 2013

Group-2H2 + O → OH + HSiO + OH → SiO2 + HOH + OH → H2O + HOH + O2 → HO2 + O SiO + HO2 → SiO2 + OH

Group 1SiO (g) + O2 → SiO2 + OSiO (g) + O → SiO2

ComparisonGas Phase Reactions

Chakraborty et al., Science, 2013

Group 1SiO (g) + O2 → SiO2 + OSiO (g) + O → SiO2

Group-2H2 + O → OH + HSiO + OH → SiO2 + HOH + OH → H2O + HOH + O2 → HO2 + O SiO + HO2 → SiO2 + OH

Surface Reactions

ConclusionsSurface assisted oxidation reation is a

possible mechanismEffective in the dark and cold molecular

clouds and colder parts of the solar nebula

Observed: Growth of new particles through reactions

Observed: Mass-independent oxygen isotopic compositions in the newly formed SiOx particles

Multicle element oxide grains do form in this process

Need more studies in this direction

lpsc 2016-presentation

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