New Optical and Near-Infrared Surface Brightness Fluctuations Models. II. Young and Intermediate Age Stellar Populations

We present theoretical surface-brightness fluctuations (SBF) amplitudes for single-burst stellar populations of young and intermediate age ($25 Myr \leq t \leq 5 Gyr$), and $Z=0.0003$, 0.001, 0.004, 0.008, 0.01, 0.02, and 0.04. We pay attention to the contribution of thermal-pulses 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 NIR SBF models exhibit a strong dependence on age and mass loss. We also evaluate SBF amplitudes by 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). We provide constraints upon 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 indetermination 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 age ranging from few $Myr$ to several $Gyr$ - are derived by using Hubble Space Telescope observations. The measurements are successfully compared to our SBF predictions, thus providing a good agreement with models of metallicity $Z = 0.0001$-0.01. A mass loss as a power-law function of the TP-AGB star luminosity is required in order to properly reproduce the optical SBF data of the LMC clusters. We suggest how to overcome the general problem of SBF models in reproducing the details of the near-infrared SBF measurements of the Magellanic Cloud star clusters.