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Context. In recent years new observations of pre-main sequence stars (pre-MS) with Z ≤ Z have been made available. To take full advantage of the continuously growing amount of data of pre-MS stars in different environments, we need to develop updated pre-MS models for a wide range of metallicity to assign reliable ages and masses to the observed stars. Aims. We present updated evolutionary pre-MS models and isochrones for a fine grid of mass, age, metallicity, and helium values. Methods. We use a standard and well-tested stellar evolutionary code (i.e. FRANEC), that adopts outer boundary conditions from detailed and realistic atmosphere models. In this code, we incorporate additional improvements to the physical inputs related to the equation of state and the low temperature radiative opacities essential to computing low-mass stellar models. Results. We make available via internet a large database of pre-MS tracks and isochrones for a wide range of chemical compositions (Z = 0.0002−0.03), masses (M = 0.2−7.0 M ), and ages (1−100 Myr) for a solar-calibrated mixing length parameter α (i.e. 1.68). For each chemical composition, additional models were computed with two different mixing length values, namely α = 1.2 and 1.9. Moreover, for Z ≥ 0.008, we also provided models with two different initial deuterium abundances. The characteristics of the models have been discussed in detail and compared with other work in the literature. The main uncertainties affecting theoretical predictions have been critically discussed. Comparisons with selected data indicate that there is close agreement between theory and observation.

In this paper, we present an improved theoretical scenario concerning near‐infrared and visual magnitudes of RR Lyr variables, as based on up‐to‐date pulsating models. New relations connecting V and K absolute magnitudes with periods, mass, luminosity and metal content are discussed separately for fundamental and first‐overtone pulsators. We also show that the V−K colours are predicted to supply tight constraints on the pulsator intrinsic luminosity. On this basis, we revisit the case of the prototype variable RR Lyr, showing that the parallax inferred by this new pulsational approach appears in close agreement with Hubble Space Telescope absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyr variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity (MK–[Fe/H]–log P) as well as a new calibration of the MV–[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyr stars in the field and in galactic globular clusters (GGCs) should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of the current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade–Wesselink (BW) method suggest relations with a zero‐point which is fainter than is predicted by pulsation models, together with a milder metallicity dependence. However, preliminary results based on a new calibration of the BW method provided by Cacciari et al. (2000) for RR Cet and SW And appear in a much better agreement with the pulsational predictions.

Abstract. In the first part of this paper we revisit the history of theoretical predictions for HB luminosities in old Population II stellar clusters, starting from the results of "old" evolutionary computations to introduce in various steps all the available "new" physics. We discuss the influence of physical ingredients on selected evolutionary parameters, finally presenting models which incorporate all the most recent updating of the relevant physics. The evolutionary behavior of such models is extensively investigated for selected choices about the cluster metallicity, discussing theoretical predictions concerning both cluster isochrones and the calibration of the parameter R in terms of the original amount of He in stellar matter. One finds that the "new" physics has a relevant influence on both these parameters, moving cluster ages into a much better agreement with current cosmological evaluations. This scenario is implemented by a further set of stellar models where element diffusion is taken into account. The comparison between theoretical scenarios with or without diffusion is presented and discussed. A discussion of current observational constraints in the light of the updated theory closes the paper.Key words: stars: evolution; general; fundamental parameters; horizontal-branch Since galaxies were born in an already expanding Universe, the age of the Universe appears as a safe upper limit for the age of any star and any stellar cluster. The fact that several determinations of globular cluster ages yielded values larger than the age of the Universe as based on current evaluations of the Hubble constant (see, e.g., Van den Bergh 1994;Tanvir et al. 1995) has stimulated a renewed interest in the theory of globular cluster Send offprint requests to: V. Castellani, Dipartimento di Fisica Università di Pisa, piazza Torricelli 2, 56126 Pisa, Italy, vittorio@astrte.te.astro.it Pop. II stars. At the same time, significant improvements in the input physics needed for stellar evolution have been made, such that noticeable changes of the theoretical results could be expected. These improvements initially were motivated by the results of helioseismology, which opened a new window into the interior of the Sun, allowing an extremely accurate determination of the solar structure. The efforts undertaken resulted in a new generation of opacity data (Rogers & Iglesias 1992;Seaton et al. 1994; and equations of state (Mihalas et al. 1990;Rogers et al. 1996), which led to a much better prediction of solar oscillations and also resolved many long-standing problems in our understanding of pulsating stars. In addition, helioseismology required particle diffusion to be taken into account in solar models (see Bahcall et al. 1995 and references therein).The new opacities and equation of state, along with improvements in low-temperature opacities (e.g. Alexander & Ferguson 1994), nuclear cross-sections and neutrino emission rates, have now been applied to low-mass metalpoor stars in order to investigate the above-mentioned age pro...