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 results of a H i survey of 12 high surface brightness PNe located at Galactic latitudes ∣ b ∣ ≥ 10°, performed with the Five-hundred-meter Aperture Spherical radio Telescope, which is currently the most sensitive telescope in the L band. Although the contamination from interstellar emission is still severe, we detect or tentatively detect circumstellar H i 21 cm absorption associated with two PNe: NGC 6905 and NGC 7662. H i exhibits a comparatively high detection frequency in bipolar PNe. We estimate the H i masses of the two PNe to range from 0.01 to 0.14 M ⊙ , resulting in atomic-to-ionized hydrogen ratios of 0.02–0.3. The H i shells have dynamical ages of 2100–2400 yr. Our measurements confirm previous findings that the optical depth of H i decreases with increasing linear radius of the nebula. The mass loss rates traced by the H i absorption are larger than 10 −5 M ⊙ yr −1 , indicating that they originate from the superwind phase at the tip of the asymptotic giant branch.