2016MNRAS.462L..75C

In the context of the ASAI (Astrochemical Surveys At IRAM) project, we carried out an unbiased spectral survey in the millimetre window towards the well known low-mass Class I source SVS13-A. The high sensitivity reached (3-12 mK) allowed us to detect at least six HDO broad (full width at half-maximum ∼4-5 km s^–1) emission lines with upper level energies up to E_u = 837 K. A non-local thermodynamic equilibrium Large Velocity Gradient (LVG) analysis implies the presence of very hot (150-260 K) and dense (>=3 x 10⁷ cm^–3) gas inside a small radius (∼25 au) around the star, supporting, for the first time, the occurrence of a hot corino around a Class I protostar. The temperature is higher than expected for water molecules are sublimated from the icy dust mantles (∼100 K). Although we cannot exclude we are observing the effects of shocks and/or winds at such small scales, this could imply that the observed HDO emission is tracing the water abundance jump expected at temperatures ∼220-250 K, when the activation barrier of the gas phase reactions leading to the formation of water can be overcome. We derive X(HDO) ∼ 3 x 10^–6, and a H₂O deuteration >=1.5 x 10^–2, suggesting that water deuteration does not decrease as the protostar evolves from the Class 0 to the Class I stage.