Chronology of Episodic Accretion in Protostars -- an ALMA survey of the CO and H$_2$O snowlines

TL;DR

This study uses ALMA observations of N$_2$H$^+$ and HCO$^+$ to trace snowline shifts caused by episodic accretion bursts in protostars, revealing insights into burst history and star formation evolution.

Contribution

First survey to connect snowline locations with episodic accretion history in a large sample of protostars, providing new constraints on burst frequency and star formation processes.

Findings

Episodic accretion bursts cause snowline shifts outward.

Most Class 0 and I sources show evidence of past bursts.

Burst intervals increase from 2400 yr in Class 0 to 8000 yr in Class I.

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 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.