The Stellar "Snake" I: Whole Structure and Properties
  To complement our previous discovery of the young snake-like structure in the
solar neighborhood and reveal the structure's full extent, we build two samples
of stars within the Snake and its surrounding territory from {\tt Gaia EDR3}.
With the friends-of-friends algorithm, we identify 2694 and 9615 Snake member
candidates from the two samples. Thirteen open clusters are embedded in these
member candidates. By combining the spectroscopic data from multiple surveys,
we investigate the comprehensive properties of the candidates and find that
they \thj{are very likely to} belong to one sizable structure, since most of
the components are well bridged in their spatial distributions, and follow a
single stellar population with an age of $30-40$\,Myr and solar metallicity.
This sizable structure is best explained as hierarchically primordial, and
probably formed from a filamentary giant molecular cloud with unique formation
history in localized regions. To analyze the dynamics of the Snake, we divide
the structure into five groups according to their tangential velocities; we
find that the groups are expanding at a coherent rate
($\kappa_X\sim3.0\,\times10^{-2}\,\rm km\,s^{-1}\,pc^{-1}$) along the length of
the structure ($X$-direction). \thj{The corresponding expansion age
($\tau\sim33$\,Myr) is highly consistent with the age of the Snake}. With over
ten thousand member stars, the Snake is an ideal laboratory to study nearby
coeval stellar formation, stellar physics, and environmental evolution over a
large spatial extent.
