Cumulative physical uncertainty in modern stellar models - II. The dependence on the chemical composition

摘要

Aims. We extend our previous work on the effects of the uncertainties on the main input physics for the evolution of low-mass stars. We analyse the dependence of the cumulative physical uncertainty affecting stellar tracks on the chemical composition. Methods. We calculated more than 6000 stellar tracks and isochrones, with metallicity ranging from Z?=?0.0001 to 0.02, by changing the following physical inputs within their current range of uncertainty: 1H(p,νe+)2H, 14N(p,γ)15O and triple-α reaction rates, radiative and conductive opacities, neutrino energy losses, and microscopic diffusion velocities. The analysis was performed using a latin hypercube sampling design. We examine in a statistical way – for different metallicities – the dependence on the variation of the physical inputs of the turn-off (TO) luminosity, the central hydrogen exhaustion time (tH), the luminosity and the helium core mass at the red-giant branch (RGB) tip, and the zero age horizontal branch (ZAHB) luminosity in the RR Lyrae region. Results. For the stellar tracks, an increase in the metallicity from Z?=?0.0001 to Z?=?0.02 produces a cumulative physical uncertainty error variation in TO luminosity from 0.028 dex to 0.017 dex, while the global uncertainty on tH increases from 0.42 Gyr to 1.08 Gyr. For the RGB tip, the cumulative uncertainty on the luminosity is almost constant at 0.03 dex, whereas the one on the helium core mass decreases from 0.0055 M⊙ to 0.0035 M⊙. The dependence of the ZAHB luminosity error is not monotonic with Z, and it varies from a minimum of 0.036 dex at Z?=?0.0005 to a maximum of 0.047 dex at Z?=?0.0001. Regarding stellar isochrones of 12 Gyr, the cumulative physical uncertainty on the predicted TO luminosity and mass increases respectively from 0.012 dex to 0.014 dex and from 0.0136 M⊙ to 0.0186 M⊙. Consequently, from Z?=?0.0001 to Z?=?0.02 for ages typical of galactic globular clusters, the uncertainty on the age inferred from the TO luminosity increases from 325 Myr to 415 Myr.