  The young, compact, very high surface brightness but low excitation planetary
nebula (PN) BD+303639 is one of the very few PNe that have been reported to
exhibit the 21cm HI emission line. As part of a long-term programme to search
for circumstellar atomic hydrogen, we observed the 21cm feature toward
BD+303639 with the Five-hundred-meter Aperture Spherical radio Telescope
(FAST). Assuming a direct association between the PN and the detected HI
emission, these new observations show that this surrounding emission is
significantly more spatially extended than indicated by previous
interferometric observations, and can be resolved into two velocity components.
The estimated HI mass is larger than 100M_sun, invalidating an origin from the
host star itself or its ejecta for the emitting material. We discuss the
possibility that the extended HI emission stems from the interstellar medium
(ISM) swept out over time by the stellar wind. Moreover, we report tentative
detections of HI absorption features lying near and blueward of the systemic
velocity of this PN, which are probably from a stalled asterosphere at the
outer boundary of the expanding ionized region. The mass of the gas producing
the HI absorption is insufficient to solve the so-called `PN missing mass
problem'. We demonstrate the capability of FAST to investigate the interaction
process between a PN and the surrounding ISM.
