R. T. Garrod & E. Herbst The Ohio State University R. T. Garrod & E. Herbst 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 R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate n H ~ 10 7 cm -3 T > 100 K Associated with protostellar sources (geometry uncertain) Rich, complex chemistry, large molecules present - H 2 O, H 2 CO, CH 3 OH, HCOOCH 3, CH 3 OCH 3, HCOOH, CH 3 CHO... Chemistry influenced by desorption from dust grains n(HCOOCH 3 ) / n(H 2 ) ~ n(CH 3 OCH 3 ) / n(H 2 ) ~ 10 -8

Gas Phase Methyl Formate production R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate CH 3 OH H 2 CO → [HC(OH)OCH 3 ] + + H 2 X - Large potential barrier (128 kJ mol -1 ≈ 15,000 K) - Horn et al., 2004, ApJ, 611, Other isomers/ionic pre-cursors may provide a route Inefficient - HCOOCH 3 branching fraction probably ~ 5 % (cf. 50 %) - See e.g. Geppert et al., 2006, Faraday Discussion 133, paper 13 - Inefficiency may well apply to many such molecules [HC(OH)OCH 3 ] + + e - → HCOOCH 3 + H

A Grain Surface Solution? R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate HCO + CH 3 O → HCOOCH 3 and CH 3 O + CH 3 → CH 3 OCH 3 HCO + OH → HCOOH (Allen & Robinson, 1977, ApJ, 212, 396) CH 3 OH H 2 CO → CH 3 OH 2 OCH hν1% H 2 COH + + H 2 CO → H 2 COHOCH hν1% CH HCOOH → HC(OH)OCH hν 5% Also include G-P mechanisms from Horn et al. (2004): Recombination efficiency for HCOOCH 3 formation

R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate OSU gas-grain chemical/dynamical model Rate equations are used to model: Usual gas phase chemistry Accretion Thermal desorption Cosmic ray heating desorption Surface photodissociation Grain surface reactions (Langmuir-Hinshelwood) Can now handle T = T(t), n H = n H (t)

Typical chemical models: Step-change in temperature R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate 10 K (Collapse) Radicals are immobile Too cold! 100 K (Hot core) Mantles have evaporated Too hot! Warm-up phase Just right! Typically ignored (except e.g. Viti et al. 2001, 2004) Time Temp

Physical model R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate 2)Warm-up: T = 10 → 200 K n H static timescale = 2 x 10 5 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, Molinari et al., 2000, A&A, 355, Bernasconi & Maeder, 1996, A&A, 307, 829 1)Isothermal collapse: T = 10 K n H = 3 x 10 3 → 10 7 cm -3 timescale ~10 6 yr Surface chemistry H-dominated. Ices build up.

Results – HCOOCH 3 is formed R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate Gas Grain surface T ~ 100 K: all ices evaporate H 2 CO evaporates HCOOCH Warm-up phase

How HCOOCH 3 is formed: R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate H2OH2O hνhν OH H H 3 CO H 2 CO HCO HCOOCH 3 CO CO 2 ICE Evaporates (~25 K) X Evaporates (~ 40 K) H 2 COH 2 COH + GAS PHASE H 2 COHOCH 2 + e-e-

R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate Gas phase Grain surface H 2 CO evaporates HCOOCH Results – HCOOCH 3 is formed T ~ 100 K: all ices evaporate Warm-up phase

R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate HCOOCH 3 (and CH 3 OCH 3, HCOOH) reach observed levels. Surface reactions are sufficient; gas phase routes may be plausible if recombination can provide ~1%. HCOOCH 3 :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

R. T. Garrod & E. Herbst Grain Surface Formation of Methyl Formate 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