Imaging the water snowline in a protostellar envelope with H13CO+

Abstract

Context. 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 because of the close proximity of this snowline to the central star. Aims. 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 isotopolog H13CO+ toward the envelope of the low-mass protostar NGC 1333-IRAS2A, where the snowline is at a greater distance from the star than in disks. Comparison with previous observations of H218O show whether H13CO+ is indeed a good tracer of the water snowline. Methods. NGC 1333-IRAS2A was observed using the NOrthern Extended Millimeter Array (NOEMA) at ~0.′′9 resolution, targeting the H13CO+ J = 3 − 2 transition at 260.255 GHz. The integrated emission profile was analyzed using 1D radiative transfer modeling of a spherical envelope with a parametrized abundance profile for H13CO+. This profile was validated with a full chemical model. Results. The H13CO+ emission peaks ~ 2′′ northeast of the continuum peak, whereas H218O 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 ~360 AU to reproduce the observed emission profile. Chemical modeling indeed predicts a steep increase in HCO+ just outside the water snowline; the 50% decrease in gaseous H2O 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 NGC 1333-IRAS2A. In contrast, DCO+ observations show that the CO snowline is at the expected location, making an outburst scenario unlikely. Conclusions. The spatial anticorrelation of H13CO+ and H218O emission provide proof of concept that H13CO+ can be used as a tracer of the water snowline.