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Dust temperature and time-dependent effects in the chemistry of photodissociation regions

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Esplugues,  G.
Center for Astrochemical Studies at MPE, MPI for Extraterrestrial Physics, Max Planck Society;

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Citation

Esplugues, G., Cazaux, S., Caselli, P., Hocuk, S., & Spaans, M. (2019). Dust temperature and time-dependent effects in the chemistry of photodissociation regions. Monthly Notices of the Royal Astronomical Society, 486(2), 1853-1874. doi:10.1093/mnras/stz1009.


Cite as: https://hdl.handle.net/21.11116/0000-0004-E57D-E
Abstract
When studying chemistry of photodissociation regions (PDRs), time dependence becomes important as visual extinction increases, since certain chemical time-scales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of the Meijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of high G0 PDRs, where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O2), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long time-scales promote the destruction of oxygen-bearing molecules in the inner parts of low G0 PDRs, while favouring their formation and that of carbon-bearing molecules in high G0 PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO2 is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H2O in lower layers than CO2. Regarding steady state, the PDR edge reaches chemical equilibrium at early times (≲10 5 yr). This time is even shorter (<10 4 yr) for high G0 PDRs. By contrast, inner regions reach equilibrium much later, especially low G0 PDRs, where steady state is reached at ∼106–107 yr.