Zusammenfassung
Precise studies on the Galactic bulge, halo, thick disk, and globular
clusters require stellar models with alpha enhancement and various He contents.
For this purpose we complement existing PARSEC (PAdova-TRieste Stellar
Evolution Code) models, which are based on the solar partition, with
alpha-enhanced partitions. We collect detailed measurements on the metal
mixture and He abundance for the two populations of 47Tuc from literature, and
calculate stellar tracks and isochrones with these chemical compositions
(alpha/Fe~0.4 and 0.2). By fitting precise CMD with HST data from low main
sequence till horizontal branch, we calibrate free parameters that are
important for the evolution of low mass stars. The new calibration and
alpha-enhanced mixtures are applied to other metallicities. The new models
reproduce the RGB bump observations much better than previous models.
PARSEC is able to predict any chemical pattern in stellar evolution. Li is
one of the most intriguing ones. Li abundance derived in POP II dwarfs is about
three times lower than the standard Big Bang Nucleosynthesis predicts. This
disagreement is referred as the cosmological lithium problem. We reconsider the
stellar Li evolution from pre-main sequence to the end of main sequence by
introducing the effects of convective overshooting and residual accretion.
$^7$Li could be significantly depleted by convective overshooting in the
pre-main sequence and then partially restored in the stellar atmosphere by a
tail of matter accretion which could be regulated by EUV photo-evaporation. By
considering the conventional nuclear burning and microscopic diffusion along
the main sequence, we reproduce the Spite plateau for $m_0=0.62-0.80M_ødot$
stars, and the Li declining branch for lower mass dwarfs, with a wide range of
metallicities (Z=0.00001 to Z=0.0005), starting from the primordial abundance
A(Li)=2.72.
Nutzer