NLTE Effects Of Li In Late-type Stellar Atmosphere

Lithium is one of the few elements to have been produced in the Big Bang. It offers a unique probe into the evolution of the Galaxy, nucleosynthesis and stellar structure and evolution. Observational constraints on the primordial lithium abundance are important for the evaluation of the Big Bang Theory, baryonic density of the universe and even the neutrino family number. So, it is very important to investigate the stellar lithium abundances. As Li I has a low ionizaion potential, non-local thermodynamic equilibrium (NLTE) effects can be expected to be important. Especially for lithium rich stars the NLTE effects should be taken into account.This thesis includes four parts. In the first part, the importance of lithium abundances in astrophysics and the main mechanisms of lithium formation and depletion are introduced. Besides the Standard Big Bang nucleosynsis (SBBN), the main sources of ⁷Li production are the cosmic rays, type II SNe, Novae, AGB stars and T Tau stars. While the available depletion for lithium are the mass loss, convective overshooting, micro-diffusion, rotation and so on. We review the recent observational results of lithium abundance in disk stars, metal-poor stars and stars in clusters.In the second part, the reasons for studying NLTE effects for lithium are introduced and the importance of studying the NLTE effects for lithium are discussed. The possible mechanisms that influence the NLTE effects for lithium are presented, such as the resonance line scattering and photon loss, photo suction, ultraviolet overionization, (infra)red overrecombination, bound-bound pumping and inelastic collision with H atom and H^-. And the development history of lithium atomic models and the lithium NLTE corrections for the Sun, metal-poor stars and disk stars are summarized.In the third part, we introduce the main work in detail and make conclusion. Based on high-resolution and high signal-to-noise spectra the stellar parameters, such as T_eff, log g, [Fe/H] andξ_t, are determined for nine lithium rich stars. Here, the T_eff is derived by fitting the wings of E_αline, the advantage is that the H_α-based T_eff is not affected by possible errors in the interstellar extinction and color indexes. The surface gravities (log g) are based on the Hipparcos parallaxes. The iron and lithium abundances are derived by the spectra synthesis method. And we analysis the lithium NLTE effects with and without the charge reactions included for these lithium rich stars. Our main conclusions are:1. The NLTE effects are all less than 0.1 dex for our program stars whether the charge reactions are included or not.2. The NLTE corrections decrease with the decreasing lithium abundances.3. For metal-rich stars the same lithium abundaces are obtained with and without the charge reactions included. However, they may be inportant for metal-poor stars, and it will decrease the lithium abundaces when the charge reactions are included.4. It is obviously that the lithium abundances depend on their stellar masses, and the lithium abundaces decrease when M <1.2 M☉.In the fourth Part, we summary and look ahead.