Evolution of low mass stars: lithium problem and alpha enhanced tracks and isochrones

PARSEC (PAdova-TRieste Stellar Evolution Code) is the updated version of the the stellar evolution code used in Padova. It provides stellar tracks and isochrones for stellar mass from 0.1 M ⊙ to 350 M ⊙ and metallicity from Z=0.00001 to 0.06. The evolutionary phases are from pre-main sequence to the thermally pulsing asymptotic giant branch. The core of my Ph.D. project is building a new PARSEC database of α enhanced stellar evolutionary tracks and isochrones for Gaia. Precise studies on the Galactic bulge, globular cluster, Galactic halo and Galactic thick disk require stellar models with α enhancement and various helium contents. It is also important for extra-Galactic studies to have an α enhanced population synthesis. For this purpose we complement existing PARSEC models, which are based on the solar partition of heavy elements, with alpha-enhanced partitions. We collect detailed measurements on the metal mixture and helium abundance for the two populations of 47Tuc (NGC 104) from literature, and calculate stellar tracks and isochrones with these chemical compositions that are alpha-enhanced. By fitting precise color-magnitude diagram with HST ACS/WFC data from low main sequence till horizontal branch, we calibrate some free parameters that are important for the evolution of low mass stars like the mixing at the bottom of the convective envelope. This new calibration significantly improves the prediction of the RGB bump brightness. We also check that the He evolutionary lifetime is correctly predicted by the current version of PARSEC . As a further result of this calibration process, we derive an age of 12.00±0.2 Gyr, distance modulus(m-M) 0 =13.22^{+0.02}_{−0.01} , reddening E(V-I)=0.035^{-0.008} _{+0.005} , and red giant branch mass loss around 0.172 M ⊙ ∼ 0.177 M ⊙ for 47Tuc. We apply the new calibration and alpha-enhanced mixtures of the two 47Tuc populations ( [α/Fe] ∼0.4 and 0.2) to other metallicities. The new models reproduce the RGB bump observations much better than previous models, solving a long-lasting discrepancy concerning its predicted luminosity. This new PARSEC database, with the newly updated alpha-enhanced stellar evolutionary tracks and isochrones, will also be part of the new product for Gaia. Besides the α enhanced metal mixture in 47Tuc, we also calculate evolutionary tracks based on alpha-enhanced metal mixtures derived from ATLAS9 APOGEE atmosphere model. The full set of isochrones with chemical compositions suitable for globular clusters and Galactic bulge stars, will be soon made available online after the full calculation and calibration are performed. PARSEC is able to predict the evolution of stars for any chemical pattern of interest. Lithium is one of the most intriguing and complicated elements. The lithium abundance derived in metal-poor main sequence stars is about three times lower than the value of primordial Li predicted by the standard Big Bang nucleosynthesis when the baryon density is taken from the CMB or the deuterium measurements. This disagreement is generally referred as the “cosmological lithium problem”. We here reconsider the stellar Li evolution from the pre-main sequence to the end of the main sequence phase by introducing the effects of convective overshooting and residual mass accretion. We show that 7 Li could be significantly depleted by convective overshooting in the pre-main sequence phase and then partially restored in the stellar atmosphere by a tail of matter accretion which follows the Li depletion phase and that could be regulated by extreme ultraviolet (EUV) photo-evaporation. By considering the conventional nuclear burning and microscopic diffusion along the main sequence, we can reproduce the Spite plateau for stars with initial mass m 0 = 0.62 − 0.80 M ⊙ , and the Li declining branch for lower mass dwarfs, e.g, m 0 = 0.57 − 0.60 M ⊙ , with a wide range of metallicities (Z=0.00001 to Z=0.0005), starting from an initial Li abundance A(Li) = 2.72. This environmental Li evolution model also offers the possibility to interpret the decrease of Li abundance in extremely metal-poor stars, the Li disparities in spectroscopic binaries and the low Li abundance in planet hosting stars.