Analysis of the first overtone bands of isotopologues of CO and SiO in stellar spectra

Abstract

Context. This study is based on models of the first overtone (Δv = 2) bands of the monosubstituted isotopologues of CO at 2.3 μm in the spectrum of Arcturus (K2 III) and of the monosubstituted isotopologues of SiO at 4 μm in the spectrum of the red giant HD 196610 (M6 III). Aims. We aim to investigate problems involving the computation of the first overtone bands of isotopologues of CO and SiO in the spectra of late-type stars and to determine isotopic abundances. Methods. We used fits of theoretical synthetic spectra to the observed stellar molecular bands of CO and SiO for determining the abundances for isotopes of C, O, and Si. Results. Fits of synthetic spectra of the 12C16O first overtone bands at 2.3 μm computed with three available line lists to the observed spectrum of Arcturus provide the same carbon abundance [C] = - 0.6 and isotopic ratio of carbon 12C/13C = 10 ± 2. However, the quality of fits to the observed spectrum differ for three line lists used. Furthermore, the derived oxygen isotopic ratio 16O/18O = 2000 ± 500 is larger than that known in the solar system, where 16O/18O = 500. The silicon isotopic ratio in the atmosphere of the red giant HD 196610 has been revised. Using the ExoMol SiO line list with appropriate statistical weights for the SiO isotopologues, the "non-solar"ratio 28Si:29Si:30Si = 0.86 ± 0.03:0.12 ± 0.02:0.02 ± 0.01 is obtained. Conclusions. We find that: (a) the computed isotopic carbon and silicon ratios determined by the fits to the observed spectrum depend on the adopted abundance of C and Si, respectively; and (b) Correct treatment of the nuclear spin degeneracies parameter is of crucial importance for today's application of HITRAN and ExoMol line lists in the astrophysical computations.