  The link between the shaping of bipolar planetary nebulae and the mass
ejection activity of their central stars is still poorly understood.
Appropriately characterizing the evolution of the shells ejected during the
late stages of evolution is vital to gain insight into the mechanism of nebular
shaping.
  Herschel/HIFI provides an invaluable tool by opening a new window from which
to probe warm molecular gas (~50-1000 K). We present a radiative-transfer,
spatio-kinematic modeling of the molecular envelope of the young planetary
nebula NGC 7027 in high- and low-J 12CO and 13CO transitions observed by
Herschel/HIFI and IRAM 30-m, and discuss the structure and dynamics of the
molecular envelope.
  We have developed a code which, used along with the existing SHAPE software,
implements spatio-kinematic modeling with accurate non-LTE calculations of line
excitation and radiative transfer in molecular species. We have used this code
to build a relatively simple "russian doll" model of the molecular envelope of
NGC 7027.
  The model nebula consists of four nested, mildly bipolar shells plus a pair
of high-velocity blobs. The innermost shell is the thinnest and shows a
significant jump in physical conditions (temperature, density, abundance and
velocity) with respect to the adjacent shell. This is a clear indication of a
shock front in the system. Each of the high-velocity blobs is divided into two
sections with different physical conditions. The presence of H2O in NGC 7027, a
C-rich nebula, is likely due to photo-induced chemistry from the hot central
star. The computed molecular mass of the nebula is 1.3 Msun, compatible with
estimates from previous works.
