Non-LTE analysis of K I in late-type stars

Abstract

Context. Older models of Galactic chemical evolution (GCE) predict [K/Fe] ratios as much as 1 dex lower than those inferred from stellar observations. Abundances of potassium are mainly based on analyses of the 7698 Å resonance line, and the discrepancy between GCE models and observations is in part caused by the assumption of local thermodynamic equilibrium (LTE) in spectroscopic analyses. Aims. We study the statistical equilibrium of K i, focusing on the non-LTE effects on the 7698 Å line. We aim to determine how non-LTE abundances of potassium can improve the analysis of its chemical evolution, and help to constrain the yields of GCE models. Methods. We construct a new model K i atom that employs the most up-to-date atomic data. In particular, we calculate and present inelastic e+K collisional excitation cross-sections from the convergent close-coupling (CCC) and the B-Spline R-matrix (BSR) methods, and H+K collisions from the two-electron model (LCAO). We constructed a fine, extended grid of non-LTE abundance corrections based on 1D MARCS models that span 4000 < T eff /K < 8000, 0.50 < log g < 5.00, −5.00 < [Fe/H] < +0.50, and applied the corrections to potassium abundances extracted from the literature. Results. In concordance with previous studies, we find severe non-LTE effects in the 7698 Å line. The line is stronger in non-LTE and the abundance corrections can reach ∼ −0.7 dex for solar-metallicity stars such as Procyon. We determine potassium abundances in six benchmark stars, and obtain consistent results from different optical lines. We explore the effects of atmospheric inhomogeneity by computing for the first time a full 3D non-LTE stellar spectrum of K i lines for a test star. We find that 3D modeling is necessary to predict a correct shape of the resonance 7698Å line, but the line strength is similar to that found in 1D non-LTE. Conclusions. Our non-LTE abundance corrections reduce the scatter and change the cosmic trends of literature potassium abundances. In the regime [Fe/H] −1.0 the non-LTE abundances show a good agreement with the GCE model with yields from rotating massive stars. The reduced scatter of the non-LTE corrected abundances of a sample of solar twins shows that line-by-line differential analysis techniques cannot fully compensate for systematic LTE modelling errors; the scatter introduced by such errors introduces a spurious dispersion to K evolution.

Detailed Abundances in the Ultra-faint Magellanic Satellites Carina II and III

“…The 7665Å line was also clear of telluric lines for a few stars, and when measurable is always consistent with the 7699Å line. K has negative NLTE corrections that could be as large as −0.9 dex (Ivanova & Shimanskiȋ 2000), although Reggiani et al (2019) have recently calculated grids of corrections that are more typically −0.0 to −0.4 dex in our stellar parameter range.…”

The GALAH+ survey: Third data release

“…The non-LTE grids of departure coefficients that we use for the on-the-fly synthesis of 1D non-LTE spectra are described in Amarsi et al (2020). In brief, new grids of departure coefficients were constructed by adopting the non-LTE model atoms presented for H (Amarsi et al 2018a), Li (Lind et al 2009;Wang et al 2020), C (Amarsi et al 2019), O (Amarsi et al 2018b), Na (Lind et al 2011), Mg (Osorio et al 2015), Al (Nordlander & Lind 2017), Si (Amarsi & Asplund 2017), K (Reggiani et al 2019), Ca (Osorio et al 2019), Mn (Bergemann et al 2019), and Ba (Gallagher et al 2020), and running on the model atmosphere grid using the non-LTE radiative transfer code (Amarsi et al 2018a), a modified version of 3 (Leenaarts & Carlsson 2009). For Fe, the same non-LTE grids of departure coefficients that were used in GALAH DR2 were adopted here (Amarsi et al 2016;Lind et al 2017).…”

APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South

“…Not many optical measurements of K exist to assess the accuracy of the APOGEE abundances, and the one optical comparison sample used is systematically different to APOGEE by 0.27 dex (see 12). However, the optical K lines have been shown to suffer large NLTE-effects (Reggiani et al 2019).…”