Forming deuterated methanol (and other COMs) in pre-stellar core conditions
Abstract
The extremely cold (=10K) environment of pre-stellar cores are ideal physical conditions for the isotopic fractionation of hydrogen. It was previously believed that these conditions would restrict the degree of chemical complexity to simpler molecular species. However, there is recent observational evidence (e.g: Jiménez-Serra+2016) that even complex organic molecules (COMs) form in these dark and cold environments. I updated a gas-grain chemical code capable of deuterium chemistry (Sipilä+2015a, Sipilä+2019b) by various mechanisms essential to study the formation of methanol and larger COMs on the surface of icy dust grains. This includes multiple mechanisms for nondiffusive chemistry. Using the improved code, I performed several 1D simulations of the pre-stellar core L1544 and derived column density profiles for methanol and its deuterated isotopologues. I will show that on one hand, when applying a single collision reaction probability (Hasegawa+1992), an increase in the reaction rate is needed to attain observed methanol levels. On the other hand, when applying reaction-diffusion competition (Chang+2007), reactions proceeding by thermal diffusion is enough to reach observed levels. I find that, in contrast to other COMs, the introduced nondiffusive mechanisms play only a secondary role in the formation and deuteration of methanol. In more recent efforts, I am focusing on models for the heavier COMs with the aim to make first predictions about their D/H ratios.
Full text
Forming deuterated methanol (and other COMs) in pre-stellar core conditions Wiebke Riedel¹, Olli Sipilä¹, Elena Redaelli2,1, Miwha Jin3,4, Paola Caselli1, Anton Vasyunin5, Rob Garrod6 1 Max-Planck-Institut für extraterrestrische Physik 2 European Southern Observatory 3 Astrochemistry Laboratory, NASA Goddard Space Flight Center 4 Department of Physics, Catholic University of America 5 Ural Federal University 6 Departments of Astronomy&Chemistry, University of Virginia credit: Herschel! 1
Jiménez-Serra+2016 others at the time: ! •prestellar core L1689B Bacmann+2012! •dense core B1-b Cernicharo+2012 How do complex organic molecules form? gradual warm-up phase: COMs form at 30K-100K Garrod+2008 credit: talk by Rob Garrod 2010 saturated molecules produced by hydrogenation! photodissociation produces radicals! gradual warm-up of molecular cloud! diffusion of radicals! reaction & desorption of products! observations in dark molecular clouds show COMs already form to some extent at 10K! but L1544! others today:! •Quijote project! •Gotham project! models & observations 2 2
How do complex organic molecules form? 1. Fedoseev+2015 - CO+H: formaldehyde ( ), methanol ( ), glycoaldehyde ( ), ethylene glycol ( ) H2CO CH3OH HC(O)CH2OH H2C(OH)CH2OH 2. Chuang+2016 - CO+H+H2CO & CO+H+CH3OH: formaldehyde ( ), methanol ( ), glycoaldehyde ( ), ethylene glycol ( ) + methyl formate ( ) H2CO CH3OH HC(O)CH2OH H2C(OH)CH2OH CH3OCHO 3. Fedoseev+2017 - CO+H+HOCH2CHO: glycerol ( ), glyceraldehyde ( )! HOCH2CH(OH)CH2OH HOCH2CH(OH)CHO 4. Qasim+2019 - C2H2+CO+H: propanal ( ), 1-propanol ( )! H3CCH2CHO CH3CH2CH2OH 5. Ioppolo+2021 - CO+NH2CH3+O2+H: glycine ( )! NH2CH2COOH 6. Fedoseev+2022 - CO+H+C+H2O: ketene ( ), possibly acetaldehyde ( ) and ethanol ( )! CH2CO CH3CHO CH3CH2OH 7. Santos+2022 - H+H2CO: methanol ( ) by ! CH3OH H2CO + CH3O→CH3OH + HCO 13K 15K 13K 10K 13K 10K 10K-16K experiments 3 3
How do complex organic molecules form? 1. Fedoseev+2015 - CO+H: formaldehyde ( ), methanol ( ), glycoaldehyde ( ), ethylene glycol ( ) H2CO CH3OH HC(O)CH2OH H2C(OH)CH2OH 2. Chuang+2016 - CO+H+H2CO & CO+H+CH3OH: formaldehyde ( ), methanol ( ), glycoaldehyde ( ), ethylene glycol ( ) + methyl formate ( ) H2CO CH3OH HC(O)CH2OH H2C(OH)CH2OH CH3OCHO 3. Fedoseev+2017 - CO+H+HOCH2CHO: glycerol ( ), glyceraldehyde ( )! HOCH2CH(OH)CH2OH HOCH2CH(OH)CHO 4. Qasim+2019 - C2H2+CO+H: propanal ( ), 1-propanol ( )! H3CCH2CHO CH3CH2CH2OH 5. Ioppolo+2021 - CO+NH2CH3+O2+H: glycine ( )! NH2CH2COOH 6. Fedoseev+2022 - CO+H+C+H2O: ketene ( ), possibly acetaldehyde ( ) and ethanol ( )! CH2CO CH3CHO CH3CH2OH 13K 15K 13K 10K 13K 10K 10K-16K experiments proposed reaction pathways include (at least) one reaction step involving two heavy immobile reactants How are they possible? 4 4
How do complex organic molecules form? 1. Fedoseev+2015 - CO+H: formaldehyde ( ), methanol ( ), glycoaldehyde ( ), ethylene glycol ( ) H2CO CH3OH HC(O)CH2OH H2C(OH)CH2OH 2. Chuang+2016 - CO+H+H2CO & CO+H+CH3OH: formaldehyde ( ), methanol ( ), glycoaldehyde ( ), ethylene glycol ( ) + methyl formate ( ) H2CO CH3OH HC(O)CH2OH H2C(OH)CH2OH CH3OCHO 3. Fedoseev+2017 - CO+H+HOCH2CHO: glycerol ( ), glyceraldehyde ( )! HOCH2CH(OH)CH2OH HOCH2CH(OH)CHO 4. Qasim+2019 - C2H2+CO+H: propanal ( ), 1-propanol ( )! H3CCH2CHO CH3CH2CH2OH 5. Ioppolo+2021 - CO+NH2CH3+O2+H: glycine ( )! NH2CH2COOH 6. Fedoseev+2022 - CO+H+C+H2O: ketene ( ), possibly acetaldehyde ( ) and ethanol ( )! CH2CO CH3CHO CH3CH2OH 13K 15K 13K 10K 13K 10K 10K-16K experiments proposed reaction pathways include (at least) one reaction step involving two heavy immobile reactants 5 5
Introducing nondiffusive chemistry following Jin&Garrod(2020) & Garrod+2022 6 similar processes tested in:! •single three-body reaction to explain formation (Garrod&Pauly 2011)! •Eley-Rideal mechanism (e.g.: Ruaud+2015)! •chain reaction mechanism (Chang&Herbst 2016)! •single three-body reaction to explain formation (Dulieu+2019)! CO2 NH2CHO 6
Introducing nondiffusive chemistry following Jin&Garrod(2020) & Garrod+2022 7 similar processes tested in:! •single three-body reaction to explain formation (Garrod&Pauly 2011)! •Eley-Rideal mechanism (e.g.: Ruaud+2015)! •chain reaction mechanism (Chang&Herbst 2016)! •single three-body reaction to explain formation (Dulieu+2019)! CO2 NH2CHO 7
Introducing nondiffusive chemistry following Jin&Garrod(2020) & Garrod+2022 8 similar processes tested in:! •single three-body reaction to explain formation (Garrod&Pauly 2011)! •Eley-Rideal mechanism (e.g.: Ruaud+2015)! •chain reaction mechanism (Chang&Herbst 2016)! •single three-body reaction to explain formation (Dulieu+2019)! CO2 NH2CHO 8
How are COMs deuterated? H+ 3+ HD →H2D++ H2+ 230K isotopic exchange reaction: H2D++ X →DX++ H2 DX++ e−→X+D production of H and D atoms by electron recombination of protonated ions Hidaka+2009 deuteration mechanism in the gas deuteration mechanism on grains cosmic D/H ratio: molecular cloud D/H ratio: 0.1-1.0 10−5 9 9
Results I. reactive desorption rate of methanol increases due to cyclic H-abstraction events 16 16
Results I. reactive desorption rate of methanol increases due to cyclic H-abstraction events I. increases due to preferential abstraction of to and N(CH2DOH)/N(CH3OH) CH2DOH CHDOH CH2DO 17 17
Take home messag es observations: COM formation already in pre-stellar core phase! experiments: formation pathways involve 2 heavier reaction partners 1. addition of nondiffusive reaction mechanisms provides only minor contributions to methanol formation! 2. 3. 4. does not play a major role ! H2CO + CH3O→CH3OH + HCO 18 addition of H-abstraction reactions increases & ! CH3OH N(CH2DOH)/N(CH3OH) 18
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