  Snowlines are key ingredients for planet formation. Providing observational
constraints on the locations of the major snowlines is therefore crucial for
fully connecting planet compositions to their formation mechanism.
Unfortunately, the most important snowline, that of water, is very difficult to
observe directly in protoplanetary disks due to its close proximity to the
central star. Based on chemical considerations, HCO$^+$ is predicted to be a
good chemical tracer of the water snowline, because it is particularly abundant
in dense clouds when water is frozen out. This work maps the optically thin
isotopologue H$^{13}$CO$^+$ ($J=3-2$) toward the envelope of the low-mass
protostar NGC1333-IRAS2A (observed with NOEMA at ~0.9" resolution), where the
snowline is at larger distance from the star than in disks. The H$^{13}$CO$^+$
emission peaks ~2" northeast of the continuum peak, whereas the previously
observed H$_2^{18}$O shows compact emission on source. Quantitative modeling
shows that a decrease in H$^{13}$CO$^+$ abundance by at least a factor of six
is needed in the inner ~360 AU to reproduce the observed emission profile.
Chemical modeling predicts indeed a steep increase in HCO$^+$ just outside the
water snowline; the 50% decrease in gaseous H$_2$O at the snowline is not
enough to allow HCO$^+$ to be abundant. This places the water snowline at 225
AU, further away from the star than expected based on the 1D envelope
temperature structure for NGC1333-IRAS2A. In contrast, DCO$^+$ observations
show that the CO snowline is at the expected location, making an outburst
scenario unlikely. The spatial anticorrelation of the H$^{13}$CO$^+$ and
H$_2^{18}$O emission provide a proof of concept that H$^{13}$CO$^+$ can be used
as a tracer of the water snowline.
