Chronology of Episodic Accretion in Protostars—An ALMA Survey of the CO and H 2 O Snowlines

Abstract

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 N 2 H + (1−0) and HCO + (3−2) toward 39 Class 0 and Class I sources in the Perseus molecular cloud. N 2 H + and HCO + are destroyed via gas-phase reactions with CO and H 2 O, respectively, thus tracing the CO and H 2 O 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 N 2 H + 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 ∼10–100 L ⊙ . 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 H 2 O 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.