Imaging the water snowline in a protostellar envelope with (HCO+)-C-13

Journal article, 2018

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 (HCO+)-C-13 toward the envelope of the low-mass protostar NGC1333-IRAS2A, where the snowline is at a greater distance from the star than in disks. Comparison with previous observations of (H2O)-O-18 show whether (HCO+)-C-13 is indeed a good tracer of the water snwline. Methods. NGC1333-IRAS2A was observed using the NOrthern Extended Millimeter Array (NOEMA) at similar to 0:0.9 resolution, targeting the (HCO+)-C-13 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 (HCO+)-C-13. This profile was validated with a full chemical model. Results. The (HCO+)-C-13 emission peaks similar to 2" northeast of the continuum peak, whereas (H2O)-O-18 sh ows compact emission on source. Quantitative modeling shows that a decrease in (HCO+)-C-13 abundance by at least a factor of six is needed in the inner similar to 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 NGC1333-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 (HCO+)-C-13 and (H2O)-O-18 emission provide proof of concept that (HCO+)-C-13 can be used as a tracer of the water snowline.