Chronology of Episodic Accretion in Protostars-An ALMA Survey of the CO and H2O Snowlines

Episodic accretion has been used to explain the wide range of protostellar luminosities, but its origin and influence on the star-forming process are not yet fully understood. We present an ALMA survey of N2H+ (1-0) and HCO+ (3-2) toward 39 Class 0 and Class I sources in the Perseus molecular cloud. N2H+ and HCO+ are destroyed via gas-phase reactions with CO and H2O, respectively, thus tracing the CO and H2O snowline locations. A snowline location at a much larger radius than that expected from the current luminosity suggests that an accretion burst has occurred in the past that has shifted the snowline outward. We identified 18/18 Class 0 and 9/10 Class I post-burst sources from N2H+ and 7/17 Class 0 and 1/8 Class I post-burst sources from HCO+. The accretion luminosities during the past bursts are found to be similar to 10-100 L-circle dot. This result can be interpreted as either evolution of burst frequency or disk evolution. In the former case, assuming that refreeze-out timescales are 1000 yr for H2O and 10,000 yr for CO, we found that the intervals between bursts increase from 2400 yr in the Class 0 stage to 8000 yr in the Class I stage. This decrease in the burst frequency may reflect that fragmentation is more likely to occur at an earlier evolutionary stage when the young stellar object is more prone to instability.