Non-Thermal Desorption and Binding Energy Computations of Interstellar Ices

Background

Stars and their surrounding planetary systems form within dense molecular clouds. During the gravitational collapse of these clouds, dust and gas flows through a collapsing envelope, feeding a so-called protoplanetary disk, providing the material required to form a new planetary system.[1]

The cold midplane of protoplanetary disks is seeded with ices that originally formed on dust grains at the low densities (10³ – 10⁴ cm⁻³) and temperatures (10 K) in the parent molecular cloud.[2] Infrared  ices  subsequently react with atomic hydrogen to form larger and more complex species such as CH₂O and CH₃OH, the processing of which via heat and/or radiation can further enhance the chemical complexity of interstellar ices.[3]

The presence of complex organic molecules (in the form of gas-phase methanol) in protoplanetary disks have finally been revealed around young stars in observations with ALMA (the Atacama Large Millimeter/submillimeter Array);[4] methanol is found to be rotationally cold and likely arising from non-thermal desorption from the cold icy reservoir in the disk midplane. The confirmation of methanol in protoplanetary disks has revealed the presence of a complex organic ice reservoir in disk midplanes for the first time. This reservoir may be processed to form molecules of higher complexity, the rates of which depend on the binding energies and binding environments of key radicals under the conditions in disk midplanes.[5]

I currently model amorphous solid water (ASW) and methanol ices, as well as mixed water-methanol ices at the following temperatures; 10 K, 20 K, 50 K, and 70 K (protoplanetary disk temperatures), yielding a range of interstellar ices. I conduct binding and non-thermal desorption investigations of these complex molecular systems using a range of QM (quantum mechanical), MM (molecular mechanical) and semi-empirical methods.

[1] E. F. van Dishoeck et al. arXiv Astro-ph.EP (2020).

[2] J. K. Jørgensen, et al. Ann Rev of Astron. Astrophys. 58 (2020): 727-778.

[3] K. I. Öberg, Chem Rev 116 (2016): 9631−9663.

[4] C. Walsh, et al. ApJ Letters 823 (2016): L10

[5] A. S. Booth, et al., Nature Astron 5 684-690 (2021)