Abstract
Carbon abundances in late-type stars are important in a variety of astrophysical contexts. However C I lines, one of the main abundance diagnostics, are sensitive to departures from local thermodynamic equilibrium (LTE). We present a model atom for non-LTE analyses of C I lines, that uses a new, physically-motivated recipe for the rates of neutral hydrogen impact excitation. We analyse C I lines in the solar spectrum, employing a three-dimensional (3D) hydrodynamic model solar atmosphere and 3D non-LTE radiative transfer. We find negative non-LTE abundance corrections for C I lines in the solar photosphere, in accordance with previous studies, reaching up to around 0.1 dex in the disk-integrated flux. We also present the first fully consistent 3D non-LTE solar carbon abundance determination: we infer log ɛC = 8.44 ± 0.02, in good agreement with the current standard value. Our models reproduce the observed solar centre-to-limb variations of various C I lines, without any adjustments to the rates of neutral hydrogen impact excitation, suggesting that the proposed recipe may be a solution to the long-standing problem of how to reliably model inelastic collisions with neutral hydrogen in late-type stellar atmospheres.
Highlights
As the second most abundant metal at the present epoch, carbon is interesting in a variety of astrophysical contexts
In late-type stellar atmospheres, C I is sensitive to departures from local thermodynamic equilibrium (LTE; e.g. Vernazza et al 1981; Stürenburg & Holweger 1990; Takeda 1992; Asplund et al 2005; Takeda & Honda 2005; Fabbian et al 2006; Alexeeva & Mashonkina 2015)
The 1D non-LTE versus 3D non-LTE abundance differences are caused by differences in the atmospheric stratification in 1D and in 3D; in support of this, we found that 3D non-LTE versus 3D non-LTE abundance corrections are much less severe
Summary
As the second most abundant metal at the present epoch, carbon is interesting in a variety of astrophysical contexts. Via the C/O ratio, carbon has an oversized influence on the structure of the atmospheres Madhusudhan 2012) and interiors Carbon is influential on the structure of the atmospheres Gallagher et al 2017) and interiors Accurate carbon abundances measured across the Galaxy are of great interest to the community. In late-type stars, C I lines are among the most commonly used carbon abundance diagnostics Fabbian et al 2009). In late-type stellar atmospheres, C I is sensitive to departures from local thermodynamic equilibrium
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