Oxygen line formation in late-F through early-K disk/halo stars

Abstract

Abstract. In order to investigate the formation of O  7771-5 and [O ] 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. The formation of the triplet O  lines was found to be well described by the classical two-level-atom scattering model, and the non-LTE correction is practically determined by the parameters of the line-transition itself without any significant relevance to the details of the oxygen atomic model. This simplifies the problem in the sense that the non-LTE abundance correction is essentially determined only by the line-strength (W λ ), if the atmospheric parameters of T eff , log g, and ξ are given, without any explicit dependence of the metallicity; thus allowing a useful analytical formula with tabulated numerical coefficients. On the other hand, our calculations lead to the robust conclusion that LTE is totally valid for the forbidden An inspection of the parameter-dependence of this discordance indicates that the extent of the discrepancy tends to be comparatively lessened for higher T eff / log g stars, suggesting the preference of dwarf (or subgiant) stars for studying the oxygen abundances of metal-poor stars.

A holistic approach to carbon-enhanced metal-poor stars

“…This simple calculation shows that Ba stars are indeed the analogues of C stars at high metallicities. (bottom panel) [Ba/Fe] Takeda (2003). The corrections for O triplet measurements are of the order of 0.2 dex, in agreement with García Pérez et al (2006).…”

“…In this figure, we do not attempt to correct the O abundance for systematic effects because they come from different indicators ([OI], O triplet, and OH lines). Although NLTE and 3D effects are expected to reduce the O abundance, the corrections (≈0.2 dex ;Takeda 2003;García Pérez et al 2006) are smaller than the actual O enhancement in CEMP-rs compared to the normal galactic content represented by rI and rII stars. In this figure, CEMP-s stars also show a slight O enhancement but in this case, uncertainties from non-LTE and 3D effects are not negligible compared to the observed enhancement so that no firm conclusion can be drawn.…”

Oxygen from the λ7774 High‐Excitation Triplet in Open Cluster Dwarfs: Hyades

“…The two sets of features most often used for O abundance determinations are (1) two forbidden [O i] spectral lines at 6300.3 and 6363.8 8, and (2) the permitted high-excitation O i triplet in the 7771-7775 8 region. The formation of the [O i] lines is well described by current local thermodynamic equilibrium (LTE) calculations (Takeda 2003), but the lines are weak in the spectra of solar-type stars and suffer from blends (e.g., Johansson et al 2003). High-quality spectra and knowledge of the blending features are required if the forbidden lines are to be used for accurate abundance derivations.…”

Astrobiological Stoichiometry

“…The [O i] line at 6363 Å also has a small contribution from a CN molecular line in cool stars (Caffau et al, 2013) but is generally considered more reliable than the 6300 Å line. The O i triplet at 7774 Å is not affected by blends but is sensitive to non-local thermodynamic equilibrium effects (Takeda, 2003;Ramírez et al, 2007;Fabbian et al, 2009). From our intrinsic variation analysis, we expect reliable results only from the 6363 Å line and 7774 Å triplet.…”