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An analysis of some properties of stellar models as a function of metallicity Z (and helium content Y) is presented, with special attention to those stars with metallicities higher than twice or three times solar. It is shown that the stellar properties as a function of Z are mainly determined by the effects of the opacities at sub-solar metallicities, and by the effects of the mean molecular weight and stellar mass loss at higher metallicities. As a result, very metal-rich stars (Z >~0.05) exhibit properties that deviate from what is expected from the known characteristics at lower metallicities. In particular, they are more luminous and hotter than those at Z <~ 0.05 due to the effect of the mean molecular weight. They have main sequence lifetimes much shorter (60 % shorter at Z=0.1 than at Z=0.02) than those at solar metallicity due to their lower initial hydrogen content. Finally, the high mass loss rates at high metallicities affect significantly the population synthesis of massive stars in very metal-rich regions. An analysis of expected Wolf-Rayet and supernovae populations in such conditions is briefly presented., Comment: Accepted for publication in A&A. 11 pages, including 11 encapsulated figures. Stellar evolution models available at ftp://obsftp.unige.ch/pub/evol/modelz100/

Stellar nucleosynthesis is the corner-stone of many astrophysical problems. Its understanding, which can be tested by countless observations, leads to insights into the stellar structure and evolution, and provides crucial clues to the physics of the galaxies and of the universe. Precise answers can be given to the questions ``When, where and how the chemical elements are synthesized in stars?", and will be summarized in this paper for what concerns stars of low and intermediate mass. However, in spite of the observational confirmation of many predictions, important and complementary data reveal some weaknesses in the theory of stellar physics. In particular, the mixing processes of chemicals inside the stars, as well as the mass loss and ejection mechanisms, are poorly known. Their rudimentary treatment imposes the use of parameters which strongly influence the predicted chemical yields. In our discussion, we will underline the future developments that may lead to quantitative changes in the predictions of chemicals production by low and intermediate mass stars., Comment: 9 pages, 1 figure, to be published in the proceedings of the workshop "Abundance Profiles : Diagnostic Tools for Galaxy History", Quebec (1997)

We have computed new solar models using the same stellar evolution code as described in Charbonnel, Vauclair and Zahn (1992). This code, originating from Geneva, now includes the computation of element segregation for helium and 12 heavier isotopes. It may also include any type of mixing of the stellar gas, provided this mixing may be parametrized with an effective diffusion coefficient as a function of radius. Here we introduced rotation-induced mixing as prescribed by Zahn (1992). We present five solar models: the standard model; two models including pure element segregation; two models with both element segregation and rotation-induced mixing. The $u = {P \over \rho }$ function computed as a function of radius in these new solar models are compared to the helioseismological results obtained for the same function by Dziembowski et al (1994). Improving the physics of the models leads to a better consistency with helioseismology. In our best model, which includes both segregation and mixing, the relative difference in the $ u $ function between the model and the helioseismological results is smaller than 0.5 per cent at all radii except at the center and the surface. Meanwhile lithium is depleted by a factor 155 and beryllium by a factor 2.9, which is consistent with the observations. The bottom of the convective zone lies at a fractional radius of 0.716, consistent with helioseismology. The neutrino fluxes are not decreased in any of these models. The models including the computations of element segregation lead to a present surface helium abundance of: $ Y_{surf} $ between 0.248 and 0.258, which is in satisfactory agreement with the value derived from helioseismology., Comment: 14 pages, 5 postscript figures (included), for LaTeX, uses epsf.sty and A&A style file l-aa.sty version 3.0 (not included), gzip'ed tar file. Accepted for publication in A&A

Recent observations of 3He/H in different environments tend to prove that 3He is not produced in low mass stars, contrary to the standard predictions of stellar evolution theory. We show how a simple consistent mechanism, namely rotation-induced mixing, can lead to the destruction of 3He in low mass stars and simultaneously account for the low 12C/13C ratios and low lithium abundances observed in giant stars of different populations. This process should both naturally account for the recent measurements of 3He/H in galactic HII regions and allow for high values of 3He observed in some planetary nebulae., Comment: Proceedings of the meeting "From Stars to Galaxies" Crete, 9-13 oct.1995, C.Leitherer, U.Fritze-von Alvensleben, L.Hucra eds, PASP in press, 5 pages, uuencoded, compressed Postscript file including all figures