Probing Episodic Accretion in Very Low Luminosity Objects

TL;DR

This study uses ALMA observations of N2H+ and CO isotopologues in VeLLOs to identify recent accretion bursts, revealing that such bursts are more frequent in these very low luminosity objects than in more evolved protostars.

Contribution

It provides observational evidence linking CO snow line positions to recent accretion bursts in VeLLOs, suggesting shorter intervals between bursts compared to Class 0/I sources.

Findings

Approximately 70% of studied VeLLOs show signs of recent accretion bursts.

CO snow lines are located at larger radii than predicted by current luminosities.

Most VeLLOs have experienced recent accretion activity.

Abstract

Episodic accretion has been proposed as a solution to the long-standing luminosity problem in star formation; however, the process remains poorly understood. We present observations of line emission from N2H+ and CO isotopologues using the Atacama Large Millimeter/submillimeter Array (ALMA) in the envelopes of eight Very Low Luminosity Objects (VeLLOs). In five of the sources the spatial distribution of emission from N2H+ and CO isotopologues shows a clear anti-correlation. It is proposed that this is tracing the CO snow line in the envelopes: N2H+ emission is depleted toward the center of these sources in contrast to the CO isotopologue emission which exhibits a peak. The positions of the CO snow lines traced by the N2H+ emission are located at much larger radii than those calculated using the current luminosities of the central sources. This implies that these five sources have experienced a recent accretion burst because the CO snow line would have been pushed outwards during the burst due to the increased luminosity of the central star. The N2H+ and CO isotopologue emission from DCE161, one of the other three sources, is most likely tracing a transition disk at a later evolutionary stage. Excluding DCE161, five out of seven sources (i.e., ~70%) show signatures of a recent accretion burst. This fraction is larger than that of the Class 0/I sources studied by J{\o}rgensen et al. (2015) and Frimann et al. (2016) suggesting that the interval between accretion episodes in VeLLOs is shorter than that in Class 0/I sources.