NLTE analysis of the methylidyne radical (CH) molecular lines in metal-poor stellar atmospheres

Abstract

An analysis of the CH molecule in non-local thermodynamic equilibrium (NLTE) is performed for the physical conditions of cool stellar atmospheres typical of red giants (log g = 2.0, Teff = 4500 K) and the Sun. The aim of the present work is to explore whether the G-band of the CH molecule, which is commonly used in abundance diagnostics of Carbon-Enhanced Metal-Poor (CEMP) stars, is sensitive to NLTE effects. LTE and NLTE theoretical spectra are computed with the MULTI code. We use one-dimensional (1D) LTE hydrostatic MARCS model atmospheres with parameters representing eleven red giant stars with metallicities ranging from [Fe/H] = -4.0 to [Fe/H] = 0.0 and carbon-to-iron ratios [C/Fe] = 0.0, +0.7, +1.5, and +3.0. The CH molecule model is represented by 1981 energy levels, 18377 radiative bound-bound transitions, and 932 photo-dissociation reactions. The rates due to transitions caused by collisions with free electrons and hydrogen atoms are computed using classical recipes. Our calculations suggest that NLTE effects in the statistical equilibrium of the CH molecule are significant and cannot be neglected for precision spectroscopic analysis of C abundances. The NLTE effects in the G-band increase with decreasing metallicity. We show that the C abundances are always under-estimated if LTE is assumed. The NLTE corrections to C abundance inferred from the CH feature range from +0.04 dex for the Sun to +0.21 dex for a red giant with metallicity [Fe/H] = -4.0. Departures from the LTE assumption in the CH molecule are non-negligible and NLTE effects have to be taken into account in the diagnostic spectroscopy based on the CH lines. We show here that the NLTE effects in the optical CH lines are non-negligible for the Sun and red giant stars, but further calculations are warranted to investigate the effects in other regimes of stellar parameters.

Accepted for publication in Astronomy & Astrophysics

References in corpus (22)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Two Stellar Components in the Halo of the Milky Way
• Carbon Enhanced Metal-Poor Stars. I. Chemical Compositions of 26 Stars
• CH in stellar atmospheres: an extensive linelist
• Observational constraints on the origin of the elements. IV: The standard composition of the Sun
• Observational Constraints on First-Star Nucleosynthesis. I. Evidence for Multiple Progenitors of CEMP-no Stars
• 3D Hydrodynamical Simulations of Surface Convection in Red Giant Stars. Impact on spectral line formation and abundance analysis
• Binarity in Carbon-Enhanced Metal-Poor stars
• Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
• Solar oxygen abundance
• Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods
• Red Supergiant Stars as Cosmic Abundance Probes. III. NLTE effects in J-band Magnesium lines
• Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo*; A new classification scheme based on Sr and Ba
• Non-LTE Radiative Transfer with Turbospectrum
• Are some CEMP-s stars the daughters of spinstars?
• G64-12 and G64-37 are Carbon-Enhanced Metal-Poor Stars
• Spectroscopic study of CEMP-(s & r/s) stars- Revisiting classification criteria and formation scenarios, highlighting i-process nucleosynthesis
• Stellar winds and metal enrichment from fast-rotating Population III stars
• Nucleosynthetic yields of Z=$10^{-5}$ intermediate-mass stars
• Metal-Poor Stars Observed with the Southern African Large Telescope II. An Extended Sample
• Probing Massive Star Nucleosynthesis with Data on Metal-Poor Stars and the Solar System
• Atomic Radiative Data for Oxygen and Nitrogen for Solar Photospheric Studies

Cited by in corpus (9)

• Chemo-Dynamically Tagged Groups of CEMP Stars in the Halo of the Milky Way. I. Untangling the Origins of CEMP-$s$ and CEMP-no Stars
• Data structures for photoabsorption within the ExoMol project
• Discovery of an $[Fe/H]\sim -4.8$ Star in $Gaia$ XP Spectra
• SDSS J102915.14+172927.9: Revisiting the chemical pattern
• Observational constraints on the origin of the elements. IX. 3D NLTE abundances of metals in the context of Galactic Chemical Evolution Models and 4MOST
• M3DIS -- A grid of 3D radiation-hydrodynamics stellar atmosphere models for stellar surveys. II. Carbon-enhanced metal-poor stars
• Implications of time-dependent molecular chemistry in metal-poor dwarf stars
• Changing CO Bands in Near-IR Spectra of CP Cephei
• On the Chemical Composition of the Evolved Very Bright Metal-Poor Star HD 1936