2026ApJ...996...92D

Protostellar jets provide valuable insight into the evolutionary stage and formation history of star-forming cores in their earliest phases. We investigated the inner envelope structures of three extremely young protostars, selected for having the shortest dynamical timescales in their outflows and jets. Our analysis is based on Atacama Large Millimeter/submillimeter Array observations of the N₂D⁺, DCO⁺, DCN, C¹⁸O, CH₃OH, and H₂CO lines, along with 1.3 mm continuum data, obtained at two spatial resolutions of ∼500 and 150 au. By examining molecular depletion and sublimation patterns and emission extents at core-scale and outflow rotational temperatures, we assessed the relative evolutionary stages of the three sources. In G208.68-19.20N1, the absence of N₂D⁺ toward the core-despite a semi-ringlike distribution-and the presence of bright DCN and DCO⁺ emission cospatial with C¹⁸O indicate a warmer envelope, possibly suggesting a more advanced evolutionary state. In contrast, G208.68-19.20N3 shows no dense central structures in C¹⁸O, DCN, DCO⁺, or N₂D⁺, with emission instead appearing scattered around the continuum and along large-scale filaments, consistent with a likely younger stage than G208.68-19.20N1. The third source, G215M, exhibits compact C¹⁸O emission at the continuum peak but spatially extended N₂D⁺, DCN, and DCO⁺ along the continuum, pointing to a cooler envelope and likely the youngest stage among the three. This comparative analysis across three protostars demonstrates the effectiveness of molecular tracers for evolutionary staging, though variations in luminosity or accretion may also shape chemical morphologies. These results highlight the promise of broader surveys for advancing our understanding of early protostellar evolution.