Water around IRAS 15398-3359 observed with ALMA

Context. Understanding how protostars accrete their mass is one of the fundamental problems of star formation. High dust column densities and complex kinematical structures make direct observations challenging. Moreover, direct observations only provide a snapshot. Chemical tracers provide an interesting alternative to characterise the infall histories of protostars. Aims. We aim to map the distribution and kinematics of gaseous water towards the low-mass embedded protostar IRAS 15398-3359. Previous observations of (HCO+)-C-13 showed a depression in the abundance towards IRAS 15398-3359. This is a sign of destruction of HCO+ by an enhanced presence of gaseous water in an extended region, possibly related to a recent burst in the accretion. Direct observations of water vapour can determine the exact extent of the emission and confirm the hypothesis that HCO+ is indeed a good tracer of the water snow-line. Methods. IRAS 15398-3359 was observed using the Atacama Large Millimeter/submillimeter Array (ALMA) at 0.5 '' resolution in two setups at 390 and 460 GHz. Maps of HDO(1(01)-0(00)) and (H2O)-O-18(4(14)-3(21)) were taken simultaneously with observations of the CS (8-7) and N2H+ (5-4) lines and continuum at 0.65 and 0.75 mm. The maps were interpreted using dust radiative transfer calculations of the protostellar infalling envelope with an outflow cavity. Results. HDO is clearly detected and extended over the scales of the (HCO+)-C-13 depression, although it is displaced by similar to 500 AU in the direction of the outflow. (H2O)-O-18 is tentatively detected towards the red-shifted outflow lobe, but otherwise it is absent from the mapped region, which suggests that temperatures are low. Although we cannot entirely exclude a shock origin, this indicates that another process is responsible for the water emission. Conclusions. Based on the temperature structure obtained from dust radiative transfer models, we conclude that the water was most likely released from the grains in an extended hour-glass configuration during a recent accretion burst. HDO is only detected in the region closest to the protostar, at distances of up to 500 AU. These signatures can only be explained if the luminosity has recently been increased by orders of magnitudes. Additionally, the densities in the outflow cones must be sufficiently low.