The young embedded disk L1527 IRS: constraints on the water snowline and
  cosmic ray ionization rate from HCO+ observations
  The water snowline in circumstellar disks is a crucial component in planet
formation, but direct observational constraints on its location remain sparse
due 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
$\sim$0.5$^{\prime\prime}$ resolution ($\sim$35 au radius) Atacama Large
Millimeter/submillimeter Array (ALMA) observations of HCO$^+$ $J=4-3$ and
H$^{13}$CO$^+$ $J=3-2$ 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 H$^{13}$CO$^+$
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 H$_2$O isotopologues may therefore
still be an alternative option. With a direct snowline measurement, HCO$^+$
will be able to provide constraints on the ionization rate.
