Multi-frequency high spectral resolution observations of HCN toward the
  circumstellar envelope of Y CVn
  High spectral resolution observations toward the low mass-loss rate C-rich,
J-type AGB star Y CVn have been carried out at 7.5, 13.1 and 14.0 um with
SOFIA/EXES and IRTF/TEXES. Around 130 HCN and H13CN lines of bands v2, 2v2,
2v2-v2, 3v2-2v2, 3v2-v2, and 4v2-2v2 have been identified involving lower
levels with energies up to ~3900 K. These lines have been complemented with the
pure rotational lines J=1-0 and 3-2 of the vibrational states up to 2v2
acquired with the IRAM 30 m telescope, and with the continuum taken with ISO.
We have analyzed the data with a ro-vibrational diagram and a code which models
the absorption and emission of the circumstellar envelope of an AGB star. The
continuum is produced by the star with a small contribution from dust grains
comprising warm to hot SiC and cold amorphous carbon. The HCN abundance
distribution seems to be anisotropic. The ejected gas is accelerated up to the
terminal velocity (~8 km/s) from the photosphere to ~3R* but there is evidence
of higher velocities (>9-10 km/s) beyond this region. In the vicinity of Y CVn,
the line widths are as high as ~10 km/s, which implies a maximum turbulent
velocity of 6 km/s or the existence of other physical mechanisms probably
related to matter ejection that involve higher gas expansion velocities than
expected. HCN is rotationally and vibrationally out of LTE throughout the whole
envelope. A difference of about 1500 K in the rotational temperature at the
photosphere is needed to explain the observations at 7.5 and 13-14 um. Our
analysis finds a total HCN column density that ranges from ~2.1E+18 to 3.5E+18
cm^{-2}, an abundance with respect to H2 of 3.5E-05 to 1.3E-04, and a 12C/13C
isotopic ratio of ~2.5 throughout the whole envelope.
