Warm gas phase chemistry as possible origin of high HDO/H2O ratios in hot and dense gases: application to inner protoplanetary discs

TL;DR

This study explores how high HDO/H2O ratios can form in warm, dense gas via gas-phase chemistry, challenging the idea that such ratios solely indicate cometary water origin, with implications for understanding Earth's water source.

Contribution

It demonstrates that high HDO/H2O ratios can originate from gas-phase reactions in warm, dense environments, without grain surface chemistry, supported by analytical and numerical models.

Findings

High HDO/H2O ratios predicted in inner protoplanetary disks.

Gas-phase chemistry can produce deuterium enrichment at high temperatures.

HDO/H2O ratio alone cannot distinguish water's origin on Earth.

Abstract

The origin of Earth oceans is controversial. Earth could have acquired its water either from hydrated silicates (wet Earth scenario) or from comets (dry Earth scenario). [HDO]/[H2O] ratios are used to discriminate between the scenarios. High [HDO]/[H2O] ratios are found in Earth oceans. These high ratios are often attributed to the release of deuterium enriched cometary water ice, which was formed at low gas and dust temperatures. Observations do not show high [HDO]/[H2O] in interstellar ices. We investigate the possible formation of high [HDO]/[H2O] ratios in dense (nH> 1E6 cm^{-3}) and warm gas (T=100-1000 K) by gas-phase photochemistry in the absence of grain surface chemistry. We derive analytical solutions, taking into account the major neutral-neutral reactions for gases at T>100 K. The chemical network is dominated by photodissociation and neutral-neutral reactions. Despite the high gas temperature, deuterium fractionation occurs because of the difference in activation energy between deuteration enrichment and the back reactions. The analytical solutions were confirmed by the time-dependent chemical results in a 1E-3 MSun disc around a typical TTauri star using the photochemical code ProDiMo. The ProDiMo code includes frequency-dependent 2D dust-continuum radiative transfer, detailed non-LTE gas heating and cooling, and hydrostatic calculation of the disc structure. Both analytical and time-dependent models predict high [HDO]/[H2O] ratios in the terrestrial planet forming region (< 3 AU) of circumstellar discs. Therefore the [HDO]/[H2O] ratio may not be an unique criterion to discriminate between the different origins of water on Earth.