2025ApJ...992...55T

Observing and characterizing pre- and protostellar cores in the earliest and densest stages of star formation is challenging due to their short timescales and high densities, limiting the suitable tracers and targets. We conducted Atacama Large Millimeter/submillimeter Array-Atacama Compact Array standalone observations of ortho-H₂D⁺ (1_1,0-1_1,1) emission, which is believed to trace cold high-density regions, toward three dense cores in the Taurus molecular cloud: (1) L1544, likely in the densest prestellar phase; (2) MC 35 mm, a candidate for the first hydrostatic core; and (3) MC 27/L1521F, which hosts a Class 0 very-low-luminosity object. These observations provide high angular resolution data for the line across a set of cores selected to represent consecutive stages around the onset of star formation, offering a unique opportunity to trace the time evolution of ∼10⁴ yr. With the single-dish total-power array, we detected ortho-H₂D⁺ emission in all three cores, revealing its presence over scales of ∼10,000 au. In the interferometric 7 m array data with a beam size of 3$\mathop{.}\limits^{^{\prime\prime} }$5 (∼500 au), emission was detected only toward the central continuum source of MC 35 mm, with a significance of ∼3σ. No significant detections were found in the other targets, placing an upper limit on the H₂D⁺ abundance of ∼10^–11 in the dense components traced by the interferometric continuum emission. These results suggest that ortho-H₂D⁺ predominantly exhibits an extended distribution over several thousand au in the early stages of star formation. Detection in compact, dense central structures may only be achieved within a few ×10⁴ yr immediately before or after protostar formation.