Probing Episodic Accretion in Very Low Luminosity Objects
  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.
