Zero-metallicity stars - I. Evolution at constant mass

We present extensive evolutionary models of stars with initial zero-metalli city, covering a large range of initial masses (i.e. 0.7 M-circle dot less than or equal to M less than or equal to 100 M-circle dot). Calculations ar e carried out at constant mass, with updated input physics, and applying an overshooting scheme to convective boundaries. The nuclear network includes all the important reactions of the p-p chain, CNO-cycle and alpha -capture s, and is solved by means of a suitable semi-implicit method. The evolution is followed up to the thermally pulsing AGB in the case of low- and interm ediate-mass stars, or to the onset of carbon burning in massive stars. The main evolutionary features of these models are discussed, also in compariso n with models of non-zero metallicity. Among several interesting aspects, p articular attention has been paid to describe: i) the first synthesis of C- 12 inside the stars, that may suddenly trigger the CNO-cycle causing partic ular evolutionary features; ii) the pollution of the stellar surface by the dredge-up events, that are effective only within particular mass ranges; i ii) the mass limits which conventionally define the classes of low-, interm ediate-, and high-mass stars on the basis of common evolutionary properties , including the upper mass limit for the achievement of super-Eddington lum inosities before C-ignition in the high-mass regime; and iv) the expected p ulsational properties of zero-metallicity stars. All relevant information r eferring to the evolutionary tracks and isochrones is made available in com puter-readable format.