2024A&A...691A.281K

Context. Very Low Luminosity Objects (VeLLOs) are deeply embedded, and extremely faint objects (L_int < 0.1 L_⊙), and are thought to be in the quiescent phase of the episodic accretion process. They fill an important gap in our understanding of star formation.
Aims. The VeLLO in the isolated DC3272+18 cloud has undergone an outburst in the past ∼10⁴ yr, and is thus an ideal target for investigating the chemical inventory in the gas phase of an object of its type. The aim of this study is to investigate the direct impact of the outburst on the chemical processes in the object and identify molecules that can act as tracers of past heating events.
Methods. Observations with the Atacama Pathfinder EXperiment (APEX) in four spectral windows in the frequency range of 213.6-272.4 GHz have been carried out to identify molecules that can be directly linked to the past outburst; to utilize the line fluxes, column densities, and the abundance ratios of the detected species to characterize the different physical components of the VeLLO; and to probe for the presence of complex organic molecules.
Results. Nitric oxide (NO) is detected for the first time in a source of this type, and its formation could be induced by the sublimation of grain-surface species during the outburst. In addition, the observations securely detect CH₃OH, H₂CO, D₂CO, SO, SO₂, CO, ¹³CO, C¹⁸O, N₂D⁺, HCO⁺, DCO⁺, HCN, DCN, HNC, c-C₃H₂, and C₂D. The upper state energies of the securely detected lines and their derived line intensity ratios indicate that most of the probed material stems from regions of cold gas in the envelope enshrouding the VeLLO in the DC3272+18 cloud with a temperature of ∼10 K. In addition, c-C₃H₂ traces a second, warmer gas reservoir with a temperature of ∼35 K. The high D/H ratio derived from D₂CO points toward its origin from the prestellar stage, while deuteration of the gas-phase species DCO⁺, DCN, and C₂D could still be ongoing in the gas in the envelope.
Conclusions. The gas probed by the observations already cooled down after the past heating event caused by the outburst, but it still has lasting effects on the chemistry in the envelope of the VeLLO. CH₃OH, H₂CO, SO, SO₂, and CO sublimated from grains during the outburst and have not fully frozen out yet, which indicates that the outburst took place < 10⁴ yr ago. A pathway to form NO directly in the gas phase is from the photodissociation products created after the sublimation of H₂O and NH₃ from the ices. While the present time water snowline has likely retreated to a pre-outburst small radius, the volatile NO species is still extensively present in the gas phase, as is evident by its high column density relative to methanol in the observations. This suggests that NO could be potentially used to trace the water snowline in outbursting sources. In order to rule out nonthermal desorption processes that could also have led to the formation of NO, this proposition has to be verified with future observations at a higher spatial resolution, and by searching for NO in additional targets.