Carbon, oxygen, and iron abundances in disk and halo stars

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 STAGGER-grid 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.

Light Elements in the Universe

“…For O, there is a step linear decrease in [O/Fe] that reflects the two main mechanisms of production of these elements: SNIa for iron and massive stars from oxygen. The results from GALAH are in general good agreement with previous high-spectral resolution observations (Bensby et al, 2014;Amarsi et al, 2019), but they are not completely consistent with the results of the APOGEE survey (Hayes et al, 2018) that indicates a plateau in the [O/Fe] and even a slight increase in [C/Fe] at super solar metallicities. This can be due to the determination of C and O abundances in APOGEE from molecular lines, which can be affected by important systematics (see also Weinberg et al, 2019, for oxygen).…”

“…The advent of large spectroscopic surveys made it possible the investigation, based on sizeable statistical samples, of the behavior of carbon in different populations of our Galaxy, and its origin, for which the relative importance of massive and LIMS is still a matter of debate (e.g., Gustafsson et al, 1999;Henry et al, 2000;Chiappini et al, 2003;Matteucci and Chiappini, 2003;Bensby and Feltzing, 2006;Gavilán et al, 2006;Mattsson, 2010;Amarsi et al, 2019;Romano et al, 2019). Recently, the determination of carbon abundance from stellar spectra has been extended to nearby galaxies (see Conroy et al, 2014).…”

“…Also, the C/O ratio can be used to investigate the scenarios of the formation of our Galaxy. Recently, Amarsi et al (2019) performed new calculations of C and O abundances in a sample of Galactic stars, considering NLTE and 3D effects, which improved the quality of the abundances and reduced the scatter. They found higher values of [C/ O] at a given [O/H] in thin-disk stars with respect to the thick disc ones.…”

Traits for chemical evolution in solar twins

“…This suggests that the C contribution to chemical evolution from s-process is lower than from α-capture, at least at solar metallicities. This reasoning might contradict the results of Amarsi et al (2019), who studied the [C/O] ratio for different stellar Fig. 1.…”

“…Carbon belongs to a special family that we call sa because carbon has multiple unique formation sites. It can be produced in AGB stars through s-process or from massive stars through α-capture (see Amarsi et al 2019, for extensive discussions). While other elements are also produced by a variety of channels, the carbon mass production is believed to be significant in all these channels.…”