Authors: Fonfria, J. P.; Montiel, E. J.; Cernicharo, J.; DeWitt, C. N.; Richter, M. J.; Lacy, J. H.; Greathouse, T. K.; Santander-Garcia, M.; Agundez, M.; Massalkhi, S.


Publication date: 2021/07/01

DOI: 10.1051/0004-6361/202040082

Abstract: 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 mu m 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 (HCN)-C-13 lines of bands nu(2), 2 nu(2), 2 nu(2) – nu(2), 3 nu(2) – 2 nu(2), 3 nu(2) – nu(2), and 4 nu(2) – 2 nu(2) were identified involving lower levels with energies up to similar or equal to 3900 K. These lines were complemented with the pure rotational lines J = 1-0 and 3-2 of the vibrational states up to 2 nu(2) 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 (similar or equal to 8 km s(-1)) from the photosphere to similar or equal to 3R(star), but there is evidence of higher velocities (greater than or similar to 9-10 km s(-1)) beyond this region. In the vicinity of the star, the line widths are as high as similar or equal to 10 km s(-1), which implies a maximum turbulent velocity of 6 km s(-1) 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 13-14 mu m. Our analysis finds a total HCN column density that ranges from similar or equal to 2.1 x 10(18) to 3.5 x 10(18) cm(-2), an abundance with respect to H-2 of 3.5 x 10(-5) to 1.3 x 10(-4), and a C-12/C-13 isotopic ratio of similar or equal to 2.5 throughout the whole envelope.