FAST search for circumstellar atomic hydrogen. III. Planetary nebulae at Galactic latitudes |b|>10degree

The detection of circumstellar atomic hydrogen (\ion{H}{1}) via the 21,cm line remains a persistent challenge in planetary nebula (PN) studies, primarily due to contamination from ubiquitous interstellar \ion{H}{1} 21,cm emission. In this paper, we report the results of a \ion{H}{1} survey of 12 high surface brightness PNe located at Galactic latitudes $\mathrm{\mid }b\mathrm{\mid }\ge 10\text{arcdeg}$, performed with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), 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 \ion{H}{1} 21,cm absorption associated with two PNe: NGC,6905 and NGC,7662. \ion{H}{1} exhibits a comparatively high detection frequency in bipolar PNe. We estimate the \ion{H}{1} masses of the two PNe to range from 0.01 to 0.14 $M_{\text{sun}}$, resulting in atomic-to-ionized hydrogen ratios of 0.02–0.3. The \ion{H}{1} shells have dynamical ages of 2100–2400 years. Our measurements confirm previous findings that the optical depth of \ion{H}{1} decreases with increasing linear radius of the nebula. The mass loss rates traced by the \ion{H}{1} absorption are larger than 10${}^{-5}$,M${}_{\text{sun}}$ yr${}^{-1}$, indicating that they originate from the superwind phase at the tip of the asymptotic giant branch.