  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.
