Modeling Galactic Oxygen Enrichment via the NOFNe Cycle in Halo and Disk Stars

We present a theoretical analysis of oxygen abundances in a sample of 67 metal-poor field stars belonging to the Galactic halo and disk populations. The primary objective of this study is to investigate whether proton-capture nucleosynthesis pathways can reproduce the observed oxygen abundance trends in metal-poor stellar environments. Oxygen abundances,[O/H]LTE, are derived under the assumption of local thermodynamic equilibrium. The NOFNe nuclear reaction cycle is modelled under advanced stellar burning conditions, spanning temperatures of 1 × 108–3.5 × 108 K and a characteristic density of 102 g cm−3. Enhanced stellar atmosphere models and refined abundance calculations yield excellent agreement with reported values, with mean absolute deviations in the range 0.00–0.02 dex. The calculations further reveal subtle but systematic variations in oxygen abundance associated with changes in hydrogen (XH) and oxygen (XO) mass fractions. For the full sample, we obtain a mean oxygen abundance of <[O/H]> = −0.49 dex with a mean absolute deviation of 0.19 dex, consistent with expectations for metal-poor stellar populations. Distinct abundance signatures are identified among thick-disk, high-α, and low-α halo stars, reflecting differences in their nucleosynthetic histories and formation pathways. These results provide important constraints on oxygen synthesis in massive stars and supernovae and support a role for proton-capture reactions within the NOFNe cycle, together with rotationally induced mass loss, in shaping the observed surface oxygen abundances. Overall, this study offers new insights into Galactic chemical evolution and the assembly history of the stellar halo.