Imaging the water snowline around protostars with water and HCO$^+$
  isotopologues
  The water snowline location in protostellar envelopes provides crucial
information about the thermal structure and the mass accretion process as it
can inform about the occurrence of recent ($\lesssim$1,000 yr) accretion
bursts. In addition, the ability to image water emission makes these sources
excellent laboratories to test indirect snowline tracers such as
H$^{13}$CO$^+$. We study the water snowline in five protostellar envelopes in
Perseus using a suite of molecular line observations taken with the Atacama
Large Millimeter/submillimeter Array (ALMA) at
$\sim$0.2$^{\prime\prime}-$0.7$^{\prime\prime}$ (60--210 au) resolution. B1-c
provides a textbook example of compact H$_2^{18}$O ($3_{1,3}-2_{2,0}$) and HDO
($3_{1,2}-2_{2,1}$) emission surrounded by a ring of H$^{13}$CO$^+$ ($J=2-1$)
and HC$^{18}$O$^+$ ($J=3-2$). Compact HDO surrounded by H$^{13}$CO$^+$ is also
detected toward B1-bS. The optically thick main isotopologue HCO$^+$ is not
suited to trace the snowline and HC$^{18}$O$^+$ is a better tracer than
H$^{13}$CO$^+$ due to a lower contribution from the outer envelope. However,
since a detailed analysis is needed to derive a snowline location from
H$^{13}$CO$^+$ or HC$^{18}$O$^+$ emission, their true value as snowline tracer
will lie in the application in sources where water cannot be readily detected.
For protostellar envelopes, the most straightforward way to locate the water
snowline is through observations of H$_2^{18}$O or HDO. Including all
sub-arcsecond resolution water observations from the literature, we derive an
average burst interval of $\sim$10,000 yr, but high-resolution water
observations of a larger number of protostars is required to better constrain
the burst frequency.
