  We present realistic models for massive Population III stars and stellar
populations based on non-LTE model atmospheres, recent stellar evolution tracks
and up-to-date evolutionary synthesis models, to study their spectral
properties, including their dependence on age, star formation history, and IMF.
(..) The main results regarding integrated stellar populations are: * For young
bursts and the case of a constant SFR, nebular continuous emission - neglected
in previous studies - dominates the spectrum redward of Lyman-alpha (...).
Therefore predicted emission line equivalent widths are considerably smaller
than found in earlier studies, whereas the detection of the continuum is eased.
Nebular line and continuous emission strongly affect the broad band photometric
properties of Pop III objects. * Due to stellar evolution, the hardness of the
ionising spectrum decreases rapidly, leading to the disappearance of the
characteristic HeII 1640 recombination lines after ~ 3 Myr in instantaneous
bursts. * The relative efficiency of ionising photon energy to heavy element
rest mass production, eta, of metal-poor and metal-free populations is
increased by factors of ~ 4 to 18 with respect to solar metallicity and for
``standard'' IMFs. * The lowest values of eta ~ 1.6 - 2.2 % are obtained for
IMFs exclusively populated with high mass stars (M_low >~ 50 Msun). If correct,
the yields dominated by pair creation SNae then predict large overabundances of
O/C and Si/C compared to solar abundance ratios. Detailed results are given in
tabular form and as fit formulae for implementation in other calculations.
(abridged abstract)
