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Description
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 aims to map the optically thin isotopologue H^13^CO^+^ toward the envelope of the low-mass protostar NGC1333-IRAS2A, where the snowline is at larger distance from the star than in disks. Comparison with previous observations of H_2_^18^O will show whether H^13^CO^+^ is indeed a good tracer of the water snowline. NGC1333-IRAS2A was observed using NOEMA at ~0.9 arcsec resolution, targeting the H^13^CO^+^ J=3-2 transition at 260.255GHz. The integrated emission profile was analyzed using 1D radiative transfer modeling of a spherical envelope with a parametrized abundance profile for H^13^CO^+^. This profile was validated with a full chemical model. The H^13^CO^+^ emission peaks ~2-arcsec northeast of the continuum peak, whereas H_2_^18^O shows compact emission on source. Quantitative modeling shows that a decrease in H13CO+ abundance by at least a factor of six is needed in the inner ~360AU 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 225AU, 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.
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