Academia Sinica Logo

Abstract:

Prestellar cores set the initial conditions for star formation, yet heavy molecules like CO are heavily depleted in these cold, dense environments, limiting our ability to probe core centers. In contrast, deuterated molecular ions, particularly ortho-H₂D⁺, emerge as key tracers owing to enhanced deuterium fractionation at low temperatures. We present the first interferometric map of ortho-H₂D⁺ depletion in the prestellar core G205.46M3 based on high-sensitivity ALMA 820 µm continuum and molecular line observations. Our data reveal two distinct substructures, B1 and B2, separated by ~1200 au, with a significant ortho-H₂D⁺ depletion zone of ~600 au in diameter toward B1. Chemo-dynamical modeling reproduces the observed deuteration profiles and indicates a core age comparable to the free-fall time, implying that the substructures likely formed via turbulent-dominated fragmentation. Furthermore, the enhanced N₂D⁺/N₂H⁺ ratio, peaking at ~1.4, suggests that further deuteration converts H₂D⁺ into D₂H⁺ and D₃⁺, underscoring the advanced chemical evolution of G205.46M3. Our results demonstrate the critical role of deuterated ions in probing both the chemical evolution and dynamical state of dense cores.