Unveiling Central Ortho-H ₂ D ⁺ Depletion at Sub-kau Scales in Prestellar Core G205.46-14.56 M3: The First Interferometric Evidence and Implications for Deuterium Chemistry

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-H 2 D + depletion in the prestellar core G205.46−14.56 M3 using ALMA 820 μ m continuum and ortho-H 2 D + (1 10 –1 11 ) data at ∼300 au resolution. We confirm the previously reported two substructures, B1 and B2, and identify a central ortho-H 2 D + depletion zone toward B1 with ∼6 σ contrast and an inferred diameter ≲600 au, together with a peak x (N 2 D + )/ x (N 2 H + ) = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.0</mml:mn> <mml:msubsup> <mml:mn>3</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.56</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.07</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . The observationally inferred profiles of x (ortho-H 2 D + ) and x (N 2 D + )/ x (N 2 H + ) are reproduced by a deuteration-focused chemodynamical model; however, the central o-H 2 D + depletion is only marginally matched within the 2 σ upper limit, likely suggesting additional deuteration in the depletion zone. From these models we infer a core age of ∼0.42 Ma, comparable to the freefall time, suggesting that the substructures formed via rapid, turbulence-dominated fragmentation rather than slow, quasistatic contraction. Our observations also reveal that ortho-H 2 D + velocity dispersions are largely subsonic in the core and nearly thermal between B1 and B2, consistent with turbulence dissipating within a few freefall times. These results highlight the critical role of deuterated ions for both chemical evolution and dynamics in dense cores.