FAST Search for Circumstellar Atomic Hydrogen. III. Planetary Nebulae at Galactic Latitudes ∣b∣ ≥ 10°

The detection of circumstellar atomic hydrogen (H i) via the 21 cm line remains a persistent challenge in planetary nebula (PN) studies, primarily due to contamination from ubiquitous interstellar H i 21 cm emission. In this paper, we report the…

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