Hot and dense water in the inner 25 au of SVS13-A

Abstract

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 Eu = 837 K. A non-local thermodynamic equilibrium Large Velocity Gradient (LVG) analysis implies the presence of very hot (150-260 K) and dense (≥3 × 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 × 10-6, and a H2O deuteration ≥1.5 × 10-2, suggesting that water deuteration does not decrease as the protostar evolves from the Class 0 to the Class I stage.