2021A&A...651A...8F

High spectral resolution observations toward the low mass-loss rate C-rich, J-type asymptotic giant branch (AGB) star Y CVn were carried out at 7.5, 13.1, and 14.0 µm with the Echelon-cross-echelle Spectrograph mounted on the Stratospheric Observatory for Infrared Astronomy and the Texas Echelon-cross-echelle Spectrograph on the Infrared Telescope Facility. Around 130 HCN and H¹³CN lines of bands ν₂, 2ν₂, 2ν₂-ν₂, 3ν₂-2ν₂, 3ν₂-ν₂, and 4ν₂-2ν₂ were identified involving lower levels with energies up to ≃3900K. These lines were complemented with the pure rotational lines J=1-0 and 3-2 of the vibrational states up to 2ν₂ acquired with the Institut de Radioastronomie Millimetrique 30 m telescope, and with the continuum taken with Infrared Space Observatory. We analyzed the data in detail by means of a ro-vibrational diagram and with a code written to model the absorption and emission of the circumstellar envelope of an AGB star. The continuum is mostly 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 close to Y CVn and in the outer layers of its envelope. The ejected gas is accelerated up to the terminal velocity (≃8km/s) from the photosphere to ≃3R_*, but there is evidence of higher velocities (≥9-10km/s) beyond this region. In the vicinity of the star, the line widths are as high as ≃10km/s, which implies a maximum turbulent velocity of 6km/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 local thermodynamic equilibrium throughout the whole envelope. It is surprising that a difference of about 1500 K in the rotational temperature at the photosphere is needed to explain the observations at 7.5 and 13-14µm. Our analysis finds a total HCN column density that ranges from ≃2.1x10¹⁸ to 3.5x10¹⁸cm^–2, an abundance with respect to H₂ of 3.5x10^–5 to 1.3x10^–4, and a ¹²C/¹³C isotopic ratio of ≃2.5 throughout the whole envelope.