FAST Search for Circumstellar Atomic Hydrogen. I. The Young Planetary Nebula IC 4997
Zhang, Yong; Zhang, Chuan-Peng; Parker, Quentin A.; Nakashima, Jun-ichi; Zijlstra, Albert; Ouyang, Xu-Jia
China, United Kingdom, Hong Kong SAR
Abstract
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.