  How protostars accrete mass is one of the fundamental problems of star
formation. High column densities and complex kinematical structures make direct
observations challenging and they only provide a snapshot. Chemical tracers
provide an interesting alternative to characterise the infall histories of
protostars. Previous observations of H13CO+ towards the low-mass protostar
IRAS15398-3359 showed a depression in the abundance. 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. IRAS15398 was observed using ALMA at 0.5" resolution. Maps of
HDO(101-000) and H218O(414-321) 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. HDO is clearly detected and extended
over the scales of the H13CO+ depression, although it is displaced by ~500 AU
in the direction of the outflow. H218O is tentatively detected towards the
red-shifted outflow lobe, but otherwise it is absent from the mapped region,
which suggests that temperatures are low. 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.
