Oral Presentation
First Interferometric Observation of o-H2D+ Depletion in a Prestellar Core
Presenter: Sheng-Jun Lin (ASIAA)
Prestellar cores represent the initial conditions of star formation; however, heavy molecules, such as CO, are highly depleted in cold and dense regions, making them ineffective tracers of core conditions. Recently, NH₃ has also been found to deplete in core centers. Deuterated molecules, particularly o-H₂D⁺, become key tracers due to enhanced deuterium fractionation at low temperatures. G205.46M3, a low-mass prestellar core in Orion B, uniquely harbors two substructures, B1 and B2, on the kilo-au scale, believed to be dense seeds of a future binary system. We present high-resolution o-H₂D⁺ (111-110), N₂H⁺ (1-0, 4-3), and archival N₂D⁺ (3-2) data from ALMA observations of this core. We discover significant o-H₂D⁺ depletion toward both substructures, which is contrary to expectations. Our 1D non-LTE radiative transfer modeling of B1 reveals an abundance drop toward the core center, with a volumetric depletion factor exceeding 100 within the central ~600 au region. Nevertheless, o-H₂D⁺ remains an effective tracer within the inner ~2000 au, being more abundant than the more depleted N₂H⁺ and N₂D⁺. Additionally, the N₂D⁺/N₂H⁺ ratio peaks at $1.39_{-0.91}^{+0.34}$ in the center of B1. This enhanced deuteration indicates that o-H₂D⁺ depletion is driven by further deuteration, forming D₂H⁺ and D₃⁺, indicating advanced chemical evolution.
Our chemo-dynamical model suggests a core age of 0.42 Ma by reproducing both the observed N₂H⁺ deuteration profile and density structure. This is comparable to the core's free-fall time, indicating that the two substructures likely formed via turbulent-dominated core fragmentation.
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