Chronology of Episodic Accretion in Protostars -- an ALMA survey of the
  CO and H$_2$O 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 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 which 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 $\sim10-100~L_\odot$. 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 \ce{H2O} and 10,000 yr for CO, we
found that the intervals between bursts increases from 2400 yr in the Class 0
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.
