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

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.