2022ApJ...924....5V

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 (<=1000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H¹³CO⁺. 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 ∼0.''2-0.''7 (60-210 au) resolution. B1-c provides a textbook example of compact H₂¹⁸O (3_1,3-2_2,0) and HDO (3_1,2-2_2,1) emission surrounded by a ring of H¹³CO⁺ (J = 2-1) and HC¹⁸O⁺ (J = 3-2). Compact HDO surrounded by H¹³CO⁺ is also detected toward B1-bS. The optically thick main isotopologue HCO⁺ is not suited to trace the snowline, and HC¹⁸O⁺ is a better tracer than H¹³CO⁺ due to a lower contribution from the outer envelope. However, because a detailed analysis is needed to derive a snowline location from H¹³CO⁺ or HC¹⁸O⁺ emission, their true value as a 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 H₂¹⁸O or HDO. Including all subarcsecond-resolution water observations from the literature, we derive an average burst interval of ∼10,000 yr, but high-resolution water observations of a larger number of protostars are required to better constrain the burst frequency.