O/Fe in metal-poor main sequence and subgiant stars

The Galactic chemical evolution of oxygen inferred from 3D non-LTE spectral-line-formation calculations

“…The 3D corrections for the [OI] 630 nm line are close to zero in the solar metallicity case, but become increasingly more negative at lower [Fe/H], in line with previous findings (Nissen et al 2002;Collet et al 2007). Differences in the predicted mean temperature stratification between the hydrodynamical 3D models and the hydrostatic 1D models are more severe in this regime; 1D models typically overestimate the atmospheric temperature in the upper layers where this line forms (Asplund et al 1999).…”

“…Abundances inferred from the low-excitation forbidden [OI] 630 nm line tend to show a plateau at [O/Fe] ≈ 0.5 below [Fe/H] ≈ −1 (Nissen et al 2002;Cayrel et al 2004;García Pérez et al 2006), as do those inferred from OH infrared (IR) vibration rotation lines (Balachandran et al 2001;Meléndez & Barbuy 2002). Analyses of the high-excitation permitted OI 777 nm lines (Carretta et al 2000;Nissen et al 2002;García Pérez et al 2006) and of the OH ultraviolet (UV) excitation lines (Israelian et al 1998(Israelian et al , 2001Boesgaard et al 1999;González Hernández et al 2010) Israelian et al 2001) that these different trends could reflect in part different rates of circumstellar mixing in dwarfs, subgiants and giants, because at low metallicities the different diagnostics are typically used in different types of stars. Spite et al (2005) compared abundances in giants which have and have not undergone internal mixing.…”

“…These errors arise from the use of one dimensional (1D) hydrostatic model atmospheres and from the assumption of local thermodynamic equilibrium (LTE). Although the [OI] 630 nm line forms in LTE (Kiselman & Nordlund 1995), the effects of inhomogeneities and threedimensional structures in late-type stellar atmospheres on the [OI] 630 nm line in the metal-poor regime are significant (Nissen et al 2002;Collet et al 2007). The OI 777 nm lines are a classic case study of non-LTE line formation (Sedlmayr 1974;Kiselman 1993;Takeda 2003; Asplund et al 2004;Pereira et al 2009;Prakapavičius et al 2013;Steffen et al 2015).…”

Electron-impact excitation of neutral oxygen

“…The main observational evidence regarding the evolution of oxygen in the Galaxy comes from the measurement of oxygen abundances in the atmospheres of cool stars. The magnitude and trend of measured oxygen abundance with age or metallicity is not yet well constrained; in particular, the results at low metallicity differ between different studies (e.g., Israelian et al 1998;Boesgaard et al 1999;Nissen et al 2002;Garcia-Perez et al 2006). The use of different spectral diagnostics is most likely the reason for these discrepancies.…”

Tracing the evolution of the Galactic bulge with chemodynamical modelling of alpha-elements

“…Hydrodynamical three-dimensional (3D) model atmospheres show that this line is instead affected by granulation, increasing with decreasing metallicity and increasing temperatures, as shown by Nissen (2002), for example. Oxygen abundances in the Galactic bulge derived from these forbidden lines were presented by Zoccali et al (2006), Lecureur et al (2007), Fulbright et al (2006Fulbright et al ( , 2007, Alves-Brito et al (2010) and Johnson et al (2014).…”