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Paper: The Effect of Surface Inhomogeneities on the Determination of the Lithium Abundance in Cool Stellar Atmospheres
Volume: 154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop
Page: 979
Authors: Uitenbroek, H.
Abstract: I investigate the formation of the 670.6 nm LiI resonance doublet in the presence of stellar surface inhomogeneities. This doublet is widely used for lithium abundance determination in stars. To explore the possibility that the presence of hot and cool elements in a stellar atmosphere due to convective heat transport might lead to an under- or overestimate of lithium abundance if the equivalent width of the doublet is analyzed in terms of a one-dimensional plane-parallel model atmosphere, I solved the two-dimensional non-LTE radiative transfer equations for lithium in a hydrodynamic simulation snapshot of the solar granulation. For different lithium abundances the effects of the inhomogeneities in the atmosphere on lithium line-strength is small, never amounting to more than 0.1 dex in the derived abundance. This is mainly for three reasons: - Due to the exponential decrease of density with height in the gravitationally stratified stellar atmosphere, radiation escapes mostly vertically with little horizontal exchange. - The sharp drop in temperature over hot upwelling material, in contrast to the much shallower gradient over the dark intergranular lanes, causes the 670.6 nm doublet to be deeper and narrower in the first and broader in the latter. Consequently, the contrast of equivalent line width between profiles emerging over hot upflows and cold downflows is small. - Because of its small abundance the opacity scale in lithium ionizing continua is mostly set by H^- bound-free processes. Optical depth unity at the photoionization edges, therefore, follows the contours of electron temperature, moderating contrast in the ionizing radiation field.
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