Unveiling Central ortho-H2D+ Depletion at Sub-kau Scales in Prestellar Core G205.46-14.56M3: The First Interferometric Evidence and Implications for Deuterium Chemistry

TL;DR

This work tackles the problem of heavy-molecule depletion masking dense core centers and demonstrates the diagnostic power of deuterated ions, focusing on ortho-H2D+ in the prestellar core G205.46-14.56M3. It combines high-resolution ALMA 820 μm continuum and ortho-H2D+ observations with a deuteration-focused chemodynamical model to interpret the data. The observations show a central o-H2D+ depletion zone near substructure B1 with a diameter up to 600 au, challenging simple depletion models and hinting at ongoing deuteration in the core center. A chemical-dynamical model yields a core age around 0.42 Myr, consistent with the free-fall time, and indicates rapid, turbulence-dominated fragmentation with subsonic to near-thermal kinematics between B1 and B2, underscoring the role of deuterated ions in core chemistry and dynamics.

Abstract

Prestellar cores represent the initial conditions of star formation, but heavy molecules such as CO are strongly depleted in their cold, dense interiors, limiting the ability to probe core centers. Deuterated molecular ions therefore emerge as key tracers because deuterium fractionation is enhanced at low temperatures. We present the first direct observation of ortho-H2D+ depletion in the prestellar core G205.46-14.56M3 using ALMA 820um continuum and ortho-H2D+(110-111) data at ~300-au resolution. We confirm the previously reported two substructures, B1 and B2, and identify a central ortho-H2D+ depletion zone toward B1 with ~6$σ$ contrast and an inferred diameter $\lesssim$600au, together with a peak $x$(N2D+)/$x$(N2H+)=$1.03^{+0.07}_{-0.56}$. The observationally inferred profiles of $x$(ortho-H2D+) and $x$(N2D+)/$x$(N2H+) are reproduced by a deuteration-focused chemo-dynamical model; however, the central ortho-H2D+ depletion is only marginally matched within the $2σ$ upper limit, likely suggesting additional deuteration in the depletion zone. From these models we infer a core age of ~0.42Ma, comparable to the free-fall time, suggesting that the substructures formed via rapid, turbulence-dominated fragmentation rather than slow, quasi-static contraction. Our observations also reveal that ortho-H2D+ velocity dispersions are largely subsonic in the core and nearly thermal between B1 and B2, consistent with turbulence dissipating within a few free-fall times. These results highlight the critical role of deuterated ions for both chemical evolution and dynamics in dense cores.