FAST search for circumstellar atomic hydrogen--I: the young planetary
  nebula IC 4997
  Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in
Guizhou, China, we detect the 21cm neutral atomic hydrogen absorption in the
young planetary nebula IC 4997. The absorption arises from a shell also
associated with Na I D lines. The H I shell has a mass of $1.46\times10^{-2}$
M$_\odot$ and a dynamic age of 990yr. The column density of H I is estimated to
be $7.1\times10^{20}$ cm$^{-2}$, which can be well explained in terms of a
photodissociation region around the ionized nebula, limited by self shielding
of H$_2$. We find that the atomic-to-ionized hydrogen ratio is 0.6, suggesting
that H I substantially contributes to overall nebular mass.
