Xudong Gao

Xudon Gao  (China)

gao @ mpia.de

3D non-equilibriummodeling of the first stars

The astrophysical interest in lithium is disproportional to its relatively low cosmic abundance. Lithium is a fragile element and therefore a sensitive tracer of convection and other types of mixing in the stellar interior. The abundances of metal-poor stars in the Galactic halo can also help us explore primordial nucleosynthesis in the earliest phases of the universe. But the fact that confounds astronomers is that abundances of the lithium isotopes in these old stars are not compatible with the predictions from the standard model of Big Bang nucleosynthesis. To avoid that large systematic uncertainties in the stellar abundance analysis influence this important discrepancy, it is important to lift the the common simplifying assumptions of stellar atmospheres in 1D hydrostatic and thermodynamic equilibrium. I will therefore perform 3D NLTE modelling to investigate if the observed stellar abundances of light lithium indeed exceed the expected, as often claimed. 3D NLTE calculations are computationally very demanding and are not yet feasible for analysis of large spectroscopic data set. For this purpose, 1D NLTE or <3D> NLTE calculations must be employed. I have begun to study the behaviour of non-LTE effects by performing 1D NLTE calculations of lithium and other elements for a wide range of late-type stellar parameters. These will be implemented in the chemical abundance analysis of the GALAH survey of a million bright stars, to study the formation and evolution of our Galaxy. The next step is to use 3D NLTE models to analyse lithium isotopes from the very high resolution spectra of a handful of metal poor stars and eventually even extend to other interesting isotopes to give better constraints on stellar nucleosynthesis.

Supervisor: Karin Lind  (MPIA)

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