A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: 19.34.1; 19.38.1; 19.41.1; 07.25.1; 19.69.1 ASTROPHYSICS On some properties of very metal-rich stars N. Mowlavi1, G. Meynet1, A. Maeder1, D. Schaerer2, and C. Charbonnel3 1 Geneva Observatory, CH-1290 Sauverny, Switzerland 2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 3 Laboratoire d’Astrophysique de Toulouse, CNRS - UMR 5572, 14 av. E. Belin, 31400 Toulouse, France Received 30 September 1997; accepted 6 April 1998 Abstract. An analysis of some properties of stellar mod- Schaller et al. 1992, hereafter Paper I, and Maeder & Conti els as a function of metallicity Z (and helium content Y ) 1994). In contrast, they are still poorly known at metal- is presented, with special attention to those stars with licities much higher than solar. Recent calculations have metallicities higher than twice or three times solar. been performed at metallicities five times solar (Fagotto It is shown that the stellar properties as a function of et al. 1994, Mowlavi et al. 1997). Mowlavi et al. (1997) Z are mainly determined by the effects of the opacities notice that those very metal-rich models present specific at sub-solar metallicities, and by the effects of the mean properties which are distinct from the ones at Z ≤ 0.04. In molecular weight and stellar mass loss at higher metal- particular, they are hotter and more luminous that those licities. As a result, very metal-rich stars (Z ∼> 0.05) ex- at solar metallicity, contrary to expectations. It is the pur- hibit properties that deviate from what is expected from pose of this paper to analyze in detail some of these prop- the known characteristics at lower metallicities. In par- erties of very metal-rich stars. These could have important ticular, they are more luminous and hotter than those at consequences on the observable properties of high metal- Z ∼< 0.05 due to the effect of the mean molecular weight. rich galaxies. They have main sequence lifetimes much shorter (60% The study is based on the extensive grids of stellar shorter at Z = 0.1 than at Z = 0.02) than those at so- models computed by the Geneva group with 0.001 ≤ Z ≤ lar metallicity due to their lower initial hydrogen content. 0.1. It covers the main phases of stellar evolution, exclud- Finally, the high mass loss rates at high metallicities af- ing the latest stages of evolution. The core helium burning fect significantly the population synthesis of massive stars phase of very metal-rich low-mass (M < 1.7 M⊙) stars, in very metal-rich regions. An analysis of expected Wolf- important for the study of elliptical galaxies, is not con- Rayet and supernovae populations in such conditions is sidered in this paper as it has been the subject of existing briefly presented. papers in the literature (see, for example, Dorman, Rood & O’Connell, 1993, Fagotto et al. 1994). arXiv:astro-ph/9804155v1 16 Apr 1998 Key words: Stars: evolution of; Stars: general; Stars: A general discussion on the dependence of stellar prop- Hertzsprung-Russell diagram; Galaxy (the): center of; erties on chemical composition is presented in Sect. 2. Stars: Wolf-Rayet Section 3 summarizes the model ingredients used in the computation of the Geneva grids. The stellar properties during the main sequence (MS) and the core helium burn- ing (CHeB) phase are analyzed in Sects. 4 and 5, respec- 1. Introduction tively. Wolf-Rayet stars in very metal-rich environments Observations as well as models of galactic chemical evo- are briefly discussed in Sect. 6. Conclusions are drawn in lution suggest that the metallicity at the center of our Sect. 7. galaxy, where the density of matter is high, can reach up to 3 to 5 times the solar metallicity (McWilliam & Rich 2. Chemical composition and stellar properties 1994, Simpson et al. 1995). Other objects such as elliptical galaxies, or maybe even quasars (Korista et al. 1996), also Chemical composition is traditionally described by X, Y show evidences of very metal-rich stellar populations. and Z, which are the mass fractions of hydrogen, helium The metallicity affects the evolution of stars mainly and heavier elements, respectively. In the course of galactic through its impact on the radiative opacities, the equa- evolution, the interstellar matter is progressively enriched tion of state, the nuclear reaction rates and the stellar in heavy elements, and Z increases through time. It is evi- mass loss rates. The impact of the metallicity on the stel- dent that helium is also enriched, the increase of its inter- lar properties has been well studied for 0.001 ≤ Z ≤ 0.04 stellar abundance with respect to Z being conveniently de- on base of extensive stellar model calculations (see e.g. scribed by a ∆Y/∆Z law such that Y = Y0 +∆Y/∆Z ×Z 2 N. Mowlavi1 et al.: On some properties of very metal-rich stars −1 3 (where Y0 is the primordial He abundance). At low metal- κ T dT/dr for radiative transfer], leading to a decrease licities (Z ∼< 0.02), Y remains practically constant [an in the surface luminosity. As the radius is unaffected to absolute variation δZ of the metallicity induces an ab- first order, the surface temperature decreases too. In sum- solute variation of He of δY = ∆Y/∆Z × δZ, so that mary, an increase in κ tends to decrease both L and Teff . −2 −3 δY/Y ∼< 10 for Z ∼< 10 ], and the stellar properties In general, the opacities increase with increasing as a function of metallicity are thus expected to be de- metallicity. This is the case for the bound-free (see termined mainly by Z. At high metallicities (Z ∼> 0.02), Eq. 16.107 in Cox & Giuli 1969) and free-free (see on the other hand, Y increases (or, alternatively, X de- Eq. 16.95 in Cox & Giuli 1969) transitions. In contrast, creases) significantly with Z (Y passes from 0.30 to 0.48 electron scattering depends only on X [κe ≃ 0.20(1+ X)], when Z increases from 0.02 to 0.1, see Table 1). In those which is about constant at Z ∼< 0.01 and decreases with conditions, both Z and Y (and X =1 − Y − Z) determine Z at Z ∼> 0.01. As seen in Sect. 4, these considerations ex- the stellar properties as a function of metallicity. plain the stellar properties as a function of Z at sub-solar Chemical composition influences the stellar structure metallicities. basically through three contributions: the mean molecular weight µ, the radiative opacity κ, and the nuclear energy 2.3. The εnuc-effect production εnuc. The metallicity-dependent mass loss rate M˙ further affects stellar evolution as a function of Z. The nuclear energy production εnuc sustains the stellar luminosity. If εnuc is arbitrarily increased, the central re- gions of the star expand, leading to a decrease in the tem- 2.1. The µ-effect peratures and densities, and to an increase in the stellar The mean molecular weight acts in the hydrostatic equi- radius. Thus, an increase in εnuc tends to decrease both librium of the star. Let us imagine a thought experiment L and Teff , as well as the central temperature Tc and in which µ is arbitrarily increased over the whole star, all density ρc. other quantities such as κ or ε being unaltered (though The way εnuc depends on metallicity is closely related keeping, of course, their density and temperature depen- to the mode of nuclear burning. When the CNO cycles are dences). The reduction in the gas pressure leads to an the main mode of H burning, εnuc increases with Z. In overall contraction (and heating) until hydrostatic equi- contrast, when the main mode of burning is the pp chain, librium is re-established. As a result, the stellar radius de- εnuc is related to X. It is thus about independent of Z at creases and the surface temperature Teff increases. The Z ∼< 0.02, and decreases with increasing Z at Z ∼> 0.02. As surface luminosity L, on the other hand, increases with µ shown in Sect. 4, these properties determine the behavior 3 since L(r) ∝ T dT/dr [in the case of radiative transfer, of Tc and ρc in MS stars as a function of Z. with L(r) and T being the luminosity and temperature at radius r, respectively]. In summary, an increase in µ tends 2.4. The effect of M˙ to increase both L and Teff . This µ-effect is undetectable in low-metallicity stars, When mass loss is driven by radiation, Kudritzki et al. where µ is essentially constant as a function of metallic- (1989) showed that M˙ in O stars is proportional to Z0.5. ity. At metallicities higher than about solar, however, µ As a result, the effects of mass loss dominate in more increases as a result of increasing Y [Z remains unim- metal-rich stars. Mass loss reduces the stellar mass, mod- portant in this respect for all realistic values of Z, since ifying thereby the post-MS stellar properties (and even µ ≃ (2X +0.75Y +0.5Z)−1]. And indeed, it is shown in during the MS for the most massive ones). It also turns Sections 4 and 5 that very metal-rich models are more lu- the (massive) star into a Wolf-Rayet star at an earlier minous and hotter than solar-metallicity ones. It has to be stage than it would at lower metallicity. This is analysed noted that the ∆Y/∆Z law adopted in metal-rich models in more detail in Sect.