Carbon, oxygen, and iron abundances in disk and halo stars. Implications of 3D non-LTE spectral line formation

Abstract

The abundances of carbon, oxygen, and iron in late-type stars are important parameters in exoplanetary and stellar physics, as well as key tracers of stellar populations and Galactic chemical evolution. However, standard spectroscopic abundance analyses can be prone to severe systematic errors, based on the assumption that the stellar atmosphere is one-dimensional (1D) and hydrostatic, and by ignoring departures from local thermodynamic equilibrium (LTE). In order to address this, we carried out three-dimensional (3D) non-LTE radiative transfer calculations for C i and O i, and 3D LTE radiative transfer calculations for Fe ii, across the staggergrid of 3D hydrodynamic model atmospheres. The absolute 3D non-LTE versus 1D LTE abundance corrections can be as severe as −0.3 dex for C i lines in low-metallicity F dwarfs, and −0.6 dex for O i lines in high-metallicity F dwarfs. The 3D LTE versus 1D LTE abundance corrections for Fe ii lines are less severe, typically less than +0.15 dex. We used the corrections in a re-analysis of carbon, oxygen, and iron in 187 F and G dwarfs in the Galactic disk and halo. Applying the differential 3D non-LTE corrections to 1D LTE abundances visibly reduces the scatter in the abundance plots. The thick disk and high-α halo population rise in carbon and oxygen with decreasing metallicity, and reach a maximum of [C/Fe] ≈ 0.2 and a plateau of [O/Fe] ≈ 0.6 at [Fe/H] ≈ −1.0. The low-α halo population is qualitatively similar, albeit offset towards lower metallicities and with larger scatter. Nevertheless, these populations overlap in the [C/O] versus [O/H] plane, decreasing to a plateau of [C/O] ≈ −0.6 below [O/H] ≈ −1.0. In the thin-disk, stars having confirmed planet detections tend to have higher values of C/O at given [O/H]; this potential signature of planet formation is only apparent after applying the abundance corrections to the 1D LTE results. Our grids of line-by-line abundance corrections are publicly available and can be readily used to improve the accuracy of spectroscopic analyses of late-type stars.

The Gaia–ESO Survey: Carbon Abundance in the Galactic Thin and Thick Disks*

“…The left panels show for both the thin and thick disk stars a large scatter in the [C/H] values. Such a scatter was also found by Nissen & Gustafsson (2018); Amarsi et al (2019c) and they suggested that it can be explained by variations in the dust to gas ratio in different star-forming gas clouds and/or by the need of applying differential 3D non-LTE corrections to 1D LTE abundances (see discussion at the end of Section 2.2.2).…”

“…Amarsi et al 2019a). The paper by Amarsi et al (2019c) shows that in the metallicity regime of our stars ([Fe/H]> −1) the 3D-NLTE corrections do not change significantly the [C/Fe] ratios (see their Figure 11). Their paper provides, in any case, a tool to correct 1D-LTE carbon abundances to take into account the 3D-NLTE effects.…”

“…We used a code kindly provided us by A. M. Amarsi (private communication) to compute for all our stars the required 3D-NLTE corrections. It is worth noticing that our C/H estimates were not derived from equivalent width measurements and that (Amarsi et al 2019c) provides corrections only for 2 CI lines given in Table 1 thus compelling us to consider the corrections only as a first approximation. The corrections we computed are all between ±0.05 dex i.e.…”

The Explosion of Helium Stars Evolved with Mass Loss

“…Using Ni+Tr/2 gives 19%, perhaps on the low side. Observationally the fraction of iron from core-collapse supernovae, based on measurements of O/Fe or α/Fe (where α may be Mg, Si, S, Ca, or Ti), is usually thought to be about 1/4-1/3 (e.g., Fulbright et al 2007;Barbuy et al 2018;Amarsi et al 2019), though some recent observations suggest it may be as large as 50% (Griffith et al 2019). Taking 0.073 for 0.75×(Ni+Tr+a) increases the maximum average 56 Ni production for W18 and raises the fraction to 33% which is also acceptable.…”

The evolution of CNO elements in galaxies

“…6 we display different datasets in the [C/Fe]-[Fe/H] and [C/Mg]-[Mg/H] planes (top and bottom panels, respectively). The red and blue symbols refer to abundance measurements from highresolution, high signal-to-noise ratio (SNR) spectra of single thin-and thick-disc stars, respectively, presented in Bensby and Feltzing (2006), Bensby et al (2014), Nissen and Schuster (2010), Nissen et al (2014), Amarsi et al (2019). The solid red and blue lines represent the average trends derived by Franchini et al (2020) from, respectively, about 1300 thin-disc and nearly 100 thick-disc dwarf stars from the fifth internal data release of GES (GES iDR5).…”