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The water snowline in circumstellar disks is a crucial component in planetformation, but direct observational constraints on its location remain sparsedue to the difficulty of observing water in both young embedded and matureprotoplanetary disks. Chemical imaging provides an alternative route to locatethe snowline, and HCO$^+$ isotopologues have been shown to be good tracers inprotostellar envelopes and Herbig disks. Here we present$\sim$0.5$^{\prime\prime}$ resolution ($\sim$35 au radius) Atacama LargeMillimeter/submillimeter Array (ALMA) observations of HCO$^+$ $J=4-3$ andH$^{13}$CO$^+$ $J=3-2$ toward the young (Class 0/I) disk L1527 IRS. Using asource-specific physical model with the midplane snowline at 3.4 au and a smallchemical network, we are able to reproduce the HCO$^+$ and H$^{13}$CO$^+$emission, but for HCO$^+$ only when the cosmic ray ionization rate is loweredto $10^{-18}$ s$^{-1}$. Even though the observations are not sensitive to theexpected HCO$^+$ abundance drop across the snowline, the reduction in HCO$^+$above the snow surface and the global temperature structure allow us toconstrain a snowline location between 1.8 and 4.1 au. Deep observations arerequired to eliminate the envelope contribution to the emission and to derivemore stringent constraints on the snowline location. Locating the snowline inyoung disks directly with observations of H$_2$O isotopologues may thereforestill be an alternative option. With a direct snowline measurement, HCO$^+$will be able to provide constraints on the ionization rate.

The young embedded disk L1527 IRS: constraints on the water snowline and cosmic ray ionization rate from HCO+ observations