FAST Search for Circumstellar Atomic Hydrogen. I. the Young Planetary Nebula IC 4997

Using the Five-hundred-meter Aperture Spherical radio Telescope in Guizhou, China, we detect the 21 cm neutral atomic hydrogen absorption in the young planetary nebula IC 4997. The absorption arises from a shell that is also associated with Na i D lines. The H i shell has a mass of 1.46 × 10−2 M ⊙ and a dynamic age of 990 yr. The column density of H i is estimated to be 7.1 × 1020 cm−2, which can be well explained in terms of a photodissociation region around the ionized nebula, limited by the self-shielding of H2. We find that the atomic-to-ionized hydrogen ratio is 0.6, suggesting that H i substantially contributes to the overall nebular mass.