Spectroscopic analysis of RGB stars in nine open clusters

Abstract

Stellar clusters are crucial tools for studying the age, spatial distribution, dynamics, kinematics, and chemical composition of different Galactic stellar populations. In this work, we used red giant stars from open clusters to better understand the extra-mixing process through the CNO abundances and $^{12}$C/$^{13}$C, $^{16}$O/$^{17}$O and $^{16}$O/$^{18}$O isotopic ratios determined using high-quality spectra in the visible and near-infrared regions. We analysed the radial velocities and chemical composition of 22 K-type giant stars from nine open clusters (NGC188, NGC2682, NGC3680, NGC5822, IC4756, NGC6633, NGC3532, NGC6281, and NGC5460). High-resolution and high signal-to-noise spectra of stars in the NGC188 cluster were obtained with the ESPaDOnS spectrograph at the CFHT in the visible region. The stars in the other clusters were observed with the CRIRES spectrograph at the VLT. We used IRAF to compute radial velocities and Turbospectrum and MOOG for the chemical analysis. The values obtained for the radial velocities and abundances of the sample are similar to those found in the literature. The results in the visible and infrared support the occurrence and predicted mass dependence of thermohaline mixing on the red giant branch and of rotation-induced mixing on the main sequence. Variations of the initial abundances of $^{17}$O and $^{18}$O may be needed to explain the dispersion of the oxygen isotopic ratios in red giant stars.