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Snowlines are key ingredients for planet formation. Providing observationalconstraints on the locations of the major snowlines is therefore crucial forfully connecting planet compositions to their formation mechanism.Unfortunately, the most important snowline, that of water, is very difficult toobserve directly in protoplanetary disks due to its close proximity to thecentral star. Based on chemical considerations, HCO$^+$ is predicted to be agood chemical tracer of the water snowline, because it is particularly abundantin dense clouds when water is frozen out. This work maps the optically thinisotopologue H$^{13}$CO$^+$ ($J=3-2$) toward the envelope of the low-massprotostar NGC1333-IRAS2A (observed with NOEMA at ~0.9" resolution), where thesnowline is at larger distance from the star than in disks. The H$^{13}$CO$^+$emission peaks ~2" northeast of the continuum peak, whereas the previouslyobserved H$_2^{18}$O shows compact emission on source. Quantitative modelingshows that a decrease in H$^{13}$CO$^+$ abundance by at least a factor of sixis needed in the inner ~360 AU to reproduce the observed emission profile.Chemical modeling predicts indeed a steep increase in HCO$^+$ just outside thewater snowline; the 50% decrease in gaseous H$_2$O at the snowline is notenough to allow HCO$^+$ to be abundant. This places the water snowline at 225AU, further away from the star than expected based on the 1D envelopetemperature structure for NGC1333-IRAS2A. In contrast, DCO$^+$ observationsshow that the CO snowline is at the expected location, making an outburstscenario unlikely. The spatial anticorrelation of the H$^{13}$CO$^+$ andH$_2^{18}$O emission provide a proof of concept that H$^{13}$CO$^+$ can be usedas a tracer of the water snowline.

Imaging the water snowline in a protostellar envelope with H$^{13}$CO$^+$