  We report the detection in emission of various highly excited rotational
transitions of HNC (J = 6-5 through J =12-11) toward the carbon-star envelope
IRC +10 216 using the HIFI instrument on-board the Herschel Space Observatory.
Observations of the J = 1-0 and J = 3-2 lines of HNC with the IRAM 30-m
telescope are also presented. The lines observed with HIFI have upper level
energies corresponding to temperatures between 90 and 340 degrees Kelvin, and
trace a warm and smaller circumstellar region than that seen in the
interferometric maps of the J = 1-0 transition, whose emission extends up to a
radius of 20". After a detailed chemical and radiative transfer modeling, we
find that the presence of HNC in the circumstellar envelope of IRC +10 216 is
consistent with formation from the precursor ion HCNH+, which in turn is
produced through several proton transfer reactions which are triggered by the
cosmic-ray ionization. We also find that the radiative pumping through 21 um
photons to the first excited state of the bending mode v2 plays a crucial role
to populate the high-J HNC levels involved in the transitions observed with
HIFI. Emission in these high-J rotational transitions of HNC is expected to be
strong in regions which are warm and dense and/or have an intense infrared flux
at wavelengths around 21 um.
