r. t. garrod & e. herbst the ohio state university r. t. garrod & e. herbst grain surface...

R. T. Garrod & E. Herbst

The Ohio State University


Grain Surface Formation of Methyl Formate

Grain Surface Formation of Methyl Formate in the Warm-up Phase of Hot Molecular Cores

(submitted: A&A)

Hot cores



• nH ~ 107 cm-3 T > 100 K

• Associated with protostellar sources (geometry uncertain)• Rich, complex chemistry, large molecules present

- H2O, H2CO, CH3OH, HCOOCH3, CH3OCH3,

HCOOH, CH3CHO ...

• Chemistry influenced by desorption from dust grains

n(HCOOCH3) / n(H2) ~ 10-8

n(CH3OCH3) / n(H2) ~ 10-8

Gas Phase Methyl Formate production



CH3OH2+ + H2CO → [HC(OH)OCH3]

+ + H2 X- Large potential barrier (128 kJ mol-1 ≈ 15,000 K)

- Horn et al., 2004, ApJ, 611, 605- Other isomers/ionic pre-cursors may provide a route

Inefficient- HCOOCH3 branching fraction probably ~ 5 % (cf. 50 %)

- See e.g. Geppert et al., 2006, Faraday Discussion133, paper 13

- Inefficiency may well apply to many such molecules

[HC(OH)OCH3]+ + e- → HCOOCH3 + H

A Grain Surface Solution?



HCO + CH3O → HCOOCH3

andCH3O + CH3 → CH3OCH3

HCO + OH → HCOOH

(Allen & Robinson, 1977, ApJ, 212, 396)

CH3OH2+ + H2CO → CH3OH2OCH2

+ + hν1%H2COH+ + H2CO → H2COHOCH2

+ + hν1%CH3

+ + HCOOH → HC(OH)OCH3+ + hν5%

Also include G-P mechanisms from Horn et al. (2004):Recombination efficiencyfor HCOOCH3 formation



OSU gas-grain chemical/dynamical model

Rate equations are used to model:

Usual gas phase chemistry

AccretionThermal desorption

Cosmic ray heating desorptionSurface photodissociation

Grain surface reactions(Langmuir-Hinshelwood)

Can now handle T = T(t), nH = nH(t)

Typical chemical models: Step-change in temperature



10 K(Collapse)

Radicals are immobileToo cold!

100 K(Hot core)

Mantles have evaporatedToo hot!

Warm-up phaseJust right!

Typically ignored(except e.g. Viti et al. 2001, 2004)

Time

Temp

Physical model



2) Warm-up:T = 10 → 200 K

nH statictimescale = 2 x 105 yr

Surface H quickly evaporates.Heavy radicals become mobile.

Based on observationally determined protostellar luminosity functions and warm-up timescales, see: - Viti et al., 2001, 2004 - Molinari et al., 2000, A&A, 355, 617 - Bernasconi & Maeder, 1996, A&A, 307, 829

1) Isothermal collapse:T = 10 K

nH = 3 x 103 → 107 cm-3

timescale ~106 yrSurface chemistry H-dominated.

Ices build up.

Results – HCOOCH3 is formed



Gas Grain surface

T ~ 100 K:all ices

evaporateH2CO evaporates

HCOOCH3

10-8 10-8

Warm-up phase

How HCOOCH3 is formed:



H2Ohν

OH

H H3COH2CO

HCOHCOOCH3

CO

CO2

ICE Evaporates (~25 K)

X

Evaporates (~ 40 K)

H2CO H2COH+

GAS PHASE

H2COHOCH2+

e-



Gas phase

Grain surface

H2CO evaporates

HCOOCH3

10-8 10-8

Results – HCOOCH3 is formed T ~ 100 K:all ices

evaporate

Warm-up phase



• HCOOCH3 (and CH3OCH3, HCOOH) reach observed

levels.

• Surface reactions are sufficient; gas phase routes may be

plausible if recombination can provide ~1%.

• HCOOCH3 : Surface formation (~75%) from 25 → 40 K

Gas phase formation (~25%) from 40 (→ 60) K

• Longer timescales improve agreement (i.e. larger abundances).

• Radicals originate from C.R.-induced photodissociation of ices.

• Chemistry is non-trivial; relative diffusion/desorption energies

are important.

Conclusions



• Effects of warm-up phase on sulphur chemistry:

with S. Viti, E. Herbst• Extension to larger network of large organic molecules:

with S. Widicus Weaver, E. Herbst

Current/future work

Thank you for listening

r. t. garrod & e. herbst the ohio state university r. t. garrod & e. herbst grain surface...

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