2022ApJ...932....6V

The water snowline in circumstellar disks is a crucial component in planet formation, but direct observational constraints on its location remain sparse owing to the difficulty of observing water in both young embedded and mature protoplanetary disks. Chemical imaging provides an alternative route to locate the snowline, and HCO⁺ isotopologues have been shown to be good tracers in protostellar envelopes and Herbig disks. Here we present ∼0.''5 resolution (∼35 au radius) Atacama Large Millimeter/submillimeter Array (ALMA) observations of HCO⁺J = 4 - 3 and H¹³CO⁺J = 3 - 2 toward the young (Class 0/I) disk L1527 IRS. Using a source-specific physical model with the midplane snowline at 3.4 au and a small chemical network, we are able to reproduce the HCO⁺ and H¹³CO⁺ emission, but for HCO⁺ only when the cosmic-ray ionization rate is lowered to 10^–18 s^–1. Even though the observations are not sensitive to the expected HCO⁺ abundance drop across the snowline, the reduction in HCO⁺ above the snow surface and the global temperature structure allow us to constrain a snowline location between 1.8 and 4.1 au. Deep observations are required to eliminate the envelope contribution to the emission and to derive more stringent constraints on the snowline location. Locating the snowline in young disks directly with observations of H₂O isotopologues may therefore still be an alternative option. With a direct snowline measurement, HCO⁺ will be able to provide constraints on the ionization rate.