Chemically tracing the water snowline in protoplanetary disks with HCO

Context. The formation of planets is expected to be enhanced around snowlines in protoplanetary disks, in particular around the water snowline. Moreover, freeze-out of abundant volatile species in disks alters the chemical composition of the planet-forming material. However, the close proximity of the water snowline to the host star combined with the difficulty of observing water from Earth makes a direct detection of the water snowline in protoplanetary disks challenging. HCO + is a promising alternative tracer of the water snowline. The destruction of HCO + is dominated by gas-phase water, leading to an enhancement in the HCO + abundance once water is frozen out. Aims. Following earlier observed correlations between water and H 13 CO + emission in a protostellar envelope, the aim of this research is to investigate the validity of HCO + and the optically thin isotopologue H 13 CO + as tracers of the water snowline in protoplanetary disks and the required sensitivity and resolution to observationally confirm this. Methods. A typical Herbig Ae disk structure is assumed, and its temperature structure is modelled with the thermochemical code DALI . Two small chemical networks are then used and compared to predict the HCO + abundance in the disk: one without water and one including water. Subsequently, the corresponding emission profiles are modelled for the J = 2−1 transition of H 13 CO + and HCO + , which provides the best balance between brightness and the optical depth effects of the continuum emission and is less affected by blending with complex molecules. Models are then compared with archival ALMA data. Results. The HCO + abundance jumps by two orders of magnitude over a radial range of 2 AU outside the water snowline, which in our model is located at 4.5 AU. We find that the emission of H 13 CO + and HCO + is ring-shaped due to three effects: destruction of HCO + by gas-phase water, continuum optical depth, and molecular excitation effects. Comparing the radial emission profiles for J = 2−1 convolved with a 0′′.05 beam reveals that the presence of gas-phase water causes an additional drop of only ~13 and 24% in the centre of the disk for H 13 CO + and HCO + , respectively. For the much more luminous outbursting source V883 Ori, our models predict that the effects of dust and molecular excitation do not limit HCO + as a snowline tracer if the snowline is located at radii larger than ~40 AU. Our analysis of recent archival ALMA band 6 observations of th