Imaging the water snowline in protostellar envelopes

TL;DR

This study demonstrates that H$^{13}$CO$^+$ emission can be used as an indirect tracer to locate the water snowline in protostellar envelopes, aiding understanding of planet formation processes.

Contribution

The paper provides observational evidence that H$^{13}$CO$^+$ emission is spatially anticorrelated with H$_2$^{18}$O, confirming its potential as an indirect tracer of the water snowline.

Findings

H$^{13}$CO$^+$ is anticorrelated with H$_2$^{18}$O emission.

H$^{13}$CO$^+$ can trace the water snowline in protostellar envelopes.

Proof of concept for using chemical tracers to locate snowlines.

Abstract

Determining the locations of the major snowlines in protostellar environments is crucial to fully understand the planet formation process and its outcome. Despite being located far enough from the central star to be spatially resolved with ALMA, the CO snowline remains difficult to detect directly in protoplanetary disks. Instead, its location can be derived from N$_2$H$^+$ emission, when chemical effects like photodissociation of CO and N$_2$ are taken into account. The water snowline is even harder to observe than that for CO, because in disks it is located only a few AU from the protostar, and from the ground only the less abundant isotopologue H$_2^{18}$O can be observed. Therefore, using an indirect chemical tracer, as done for CO, may be the best way to locate the water snowline. A good candidate tracer is HCO$^+$, which is expected to be particularly abundant when its main destructor, H$_2$O, is frozen out. Comparison of H$_2^{18}$O and H$^{13}$CO$^+$ emission toward the envelope of the Class 0 protostar IRAS2A shows that the emission from both molecules is spatially anticorrelated, providing a proof of concept that H$^{13}$CO$^+$ can indeed be used to trace the water snowline in systems where it cannot be imaged directly.