The GASTAG evolutionary tracks and isochrones based on coupled 1D GARSTEC interior and 3D Stagger stellar atmosphere models

by Dr Yixiao Zhou (周一啸) (ITA/RoCS, UiO)

Wednesday 18 Feb 2026, 13:30 → 14:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

Models of stellar structure and evolution are among the most important instruments in astrophysics. They are a major method for determining the age of stars. Nevertheless, a significant weakness in stellar structural and evolution modeling is the simplified treatment of convection, which leads to erroneous near-surface stratification and substantial uncertainties in predicted fundamental properties of low-mass stars. We developed a novel method that couples stellar structural models with 3D surface convection simulations during stellar evolution, across a wide range of metallicities. This 1D-3D coupling method makes evolutionary tracks effectively independent of the mixing-length parameter, meanwhile providing oscillation frequencies in much better agreement with asteroseismic observations. This methodology was systematically validated against 18 main-sequence stars with high-quality asteroseismic data. The inferred stellar radii, masses, and ages are fully consistent with results from other pipelines.

Building on these successes, we constructed a new set of evolutionary tracks and isochrones, named GASTAG, based on the novel method. Comparing surface temperatures from the APOKASC3 catalog with GASTAG predictions, we find the theoretical temperatures are cooler by about 70 K near solar metallicity. The temperature discrepancy shows a weak correlation with iron abundance [Fe/H], qualitatively agrees with previous studies employing standard evolution models. Our isochrones are compared with observed color-magnitude diagrams of star clusters spanning from [Fe/H] = 0.3 to -1.9. In all cases, the synthesized and observed diagrams agree excellently in the main-sequence, turn-off and subgiant region, while isochrones underestimate temperatures for M67 and 47 Tuc along the red giant branch. Because GASTAG is constructed using a method that greatly reduces uncertainties associated with surface boundary conditions and mixing-length parameters, the difference between modeling and observation can be more confidently attributed to other factors, such as helium or α-element abundances or uncertainties in opacities.