SIMBAD references

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 x 10^–2M_☉ and a dynamic age of 990 yr. The column density of H I is estimated to be 7.1 x 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.