Interstellar Glycolaldehyde, Methyl Formate, and Acetic Acid. II. Chemical Modeling of the Bimodal Abundance Pattern in NGC 6334I
Name
Shope_2024_ApJ_972_146.pdf
Description
Published version
Size
1.16 MB
Format
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Checksum (MD5)
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Author(s)
Shope, Brielle M
El-Abd, Samer J
Brogan, Crystal L
Hunter, Todd R
Willis, Eric R
McGuire, Brett A
Garrod, Robin T
Date Issued
September 3, 2024
Journal
The Astrophysical Journal
Publisher
American Astronomical Society
Citation
Brielle M. Shope et al 2024 ApJ 972 146
Version
Final published version
Abstract
Gas-phase abundance ratios between C2H4O2 isomers methyl formate (MF), glycolaldehyde (GA), and acetic acid (AA) are typically on the order of 100:10:1 in star-forming regions. However, an unexplained divergence from this neat relationship was recently observed toward a collection of sources in the massive protocluster NGC 6334I; some sources exhibited extreme MF:GA ratios, producing a bimodal behavior between different sources, while the MF:AA ratio remained stable. Here, we use a three-phase gas-grain hot-core chemical model to study the effects of a large parameter space on the simulated C2H4O2 abundances. A combination of high gas densities and long timescales during ice-mantle desorption (∼125–160 K) appears to be the physical cause of the high MF:GA ratios. The main chemical mechanism for GA destruction occurring under these conditions is the rapid adsorption and reaction of atomic H with GA on the ice surfaces before it has time to desorb. The different binding energies of MF and GA on water ice are crucial to the selectivity of the surface destruction mechanism; individual MF molecules rapidly escape the surface when exposed by water loss, while GA lingers and is destroyed by H. Moderately elevated cosmic-ray ionization rates can increase absolute levels of “complex organic molecule” (COM) production in the ices and increase the MF:GA ratio, but extreme values are destructive for gas-phase COMs. We speculate that the high densities required for extreme MF:GA ratios could be evidence of COM emission dominated by COMs desorbing within a circumstellar disk.
MIT Department
Massachusetts Institute of Technology. Department of Chemistry
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DOI of Published Version
https://doi.org/10.3847/1538-4357/ad5d5f
