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

¹ Main Astronomical Observatory of NAS Ukraine, 27 Zabolotnoho, Kyiv, 01137, Ukraine
e-mail: yp@mao.kiev.ua
² Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK

Received: 30 September 2019
Accepted: 26 November 2019

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.