  A snow-line is the region of a protoplanetary disk at which a major volatile,
such as water or carbon monoxide, reaches its condensation temperature.
Snow-lines play a crucial role in disk evolution by promoting the rapid growth
of ice-covered grains. Signatures of the carbon monoxide snow-line (at
temperatures of around 20 kelvin) have recently been imaged in the disks
surrounding the pre-main-sequence stars TW Hydra and HD163296, at distances of
about 30 astronomical units (au) from the star. But the water snow-line of a
protoplanetary disk (at temperatures of more than 100 kelvin) has not hitherto
been seen, as it generally lies very close to the star (less than 5 au away for
solar-type stars). Water-ice is important because it regulates the efficiency
of dust and planetesimal coagulation, and the formation of comets, ice giants
and the cores of gas giants. Here we report ALMA images at 0.03-arcsec
resolution (12 au) of the protoplanetary disk around V883 Ori, a protostar of
1.3 solar masses that is undergoing an outburst in luminosity arising from a
temporary increase in the accretion rate. We find an intensity break
corresponding to an abrupt change in the optical depth at about 42 au, where
the elevated disk temperature approaches the condensation point of water, from
which we conclude that the outburst has moved the water snow-line. The spectral
behaviour across the snow-line confirms recent model predictions: dust
fragmentation and the inhibition of grain growth at higher temperatures results
in soaring grain number densities and optical depths. As most planetary systems
are expected to experience outbursts caused by accretion during their formation
our results imply that highly dynamical water snow-lines must be considered
when developing models of disk evolution and planet formation.
