FAST Search for Circumstellar Atomic Hydrogen. II. Is BD+30°3639 an Interacting Planetary Nebula?

The young, compact, very high surface brightness but low excitation planetary nebula (PN) BD+30 & DEG;3639 is one of the very few PNe that have been reported to exhibit the 21 cm H i emission line. As part of a long-term program to search for circumstellar atomic hydrogen, we observed the 21 cm feature toward BD+30 & DEG;3639 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Assuming a direct association between the PN and the detected H i 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 H i mass is larger than 100 M (& ODOT;), invalidating an origin from the host star itself or its ejecta for the emitting material. We discuss the possibility that the extended H i emission stems from the interstellar medium (ISM) swept out over time by the stellar wind. Moreover, we report tentative detections of H i 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 H i 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.