Pilot HI Survey of Planck Galactic Cold Clumps with FAST
  We present a pilot HI survey of 17 Planck Galactic Cold Clumps (PGCCs) with
the Five-hundred-meter Aperture Spherical radio Telescope (FAST). HI Narrow
Self-Absorption (HINSA) is an effective method to detect cold HI being mixed
with molecular hydrogen H$_2$ and improves our understanding of the atomic to
molecular transition in the interstellar medium. HINSA was found in 58\% PGCCs
that we observed. The column density of HINSA was found to have an intermediate
correlation with that of $^{13}$CO, following $\rm log( N(HINSA)) = (0.52\pm
0.26) log(N_{^{13}CO}) + (10 \pm 4.1) $. HI abundance relative to total
hydrogen [HI]/[H] has an average value of $4.4\times 10^{-3}$, which is about
2.8 times of the average value of previous HINSA surveys toward molecular
clouds. For clouds with total column density N$\rm_H >5 \times 10^{20}$
cm$^{-2}$, an inverse correlation between HINSA abundance and total hydrogen
column density is found, confirming the depletion of cold HI gas during
molecular gas formation in more massive clouds. Nonthermal line width of
$^{13}$CO is about 0-0.5 km s$^{-1}$ larger than that of HINSA. One possible
explanation of narrower nonthermal width of HINSA is that HINSA region is
smaller than that of $^{13}$CO. Based on an analytic model of H$_2$ formation
and H$_2$ dissociation by cosmic ray, we found the cloud ages to be within
10$^{6.7}$-10$^{7.0}$ yr for five sources.
