New optical and near-infrared surface brightness fluctuation models. II. Young and intermediate-age stellar populations

We present theoretical surface brightness fluctuation (SBF) amplitudes for single-burst stellar populations of young and intermediate age (25 Myr <= t <= 5 Gyr) and metallicities Z = 0.0003, 0.001, 0.004, 0.008, 0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expected in the Johnson-Cousins (UBVRIJHK) photometric system are provided. We pay attention to the contribution of thermally pulsating asymptotic giant branch (TP-AGB) stars. The sensitivity of the predicted SBF to changes in the mass-loss scenario along the TP-AGB phase is examined. Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strong dependence on age and mass loss. We also evaluate SBF amplitudes using Monte Carlo techniques to reproduce the random variation in the number of stars experiencing bright and fast evolutionary phases (red giant branch, AGB, TP-AGB). On these grounds we provide constraints on the faintest integrated flux of real stellar populations required to derive reliable and meaningful SBF measurements. We analyze a technique for deriving SBF amplitudes of star clusters from the photometry of individual stars and estimate the uncertainty due to statistical effects, which may impinge on the procedure. The first optical SBF measurements for 11 Large Magellanic Cloud (LMC) star-rich clusters - with ages ranging from a few megayears to several gigayears - are derived using Hubble Space Telescope observations. The measurements are compared to our SBF predictions, providing a good agreement with models of metallicity Z = 0.0001-0.01. Our results suggest that, for TP-AGB stars, a mass loss as a power-law function of the star luminosity is required in order to properly reproduce the optical SBF data of the LMC clusters. Finally, near-IR models have been compared to available data, thus showing that the general trend is well fitted. We suggest how to overcome the general problem of SBF models in reproducing the details of the near-IR SBF measurements of the Magellanic Cloud star clusters.