The photometric evolution of dissolving star clusters: II. Realistic
  models. Colours and M/L ratios
  Evolutionary synthesis models are the prime method to construct models of
stellar populations, and to derive physical parameters from observations. One
of the assumptions for such models so far has been the time-independence of the
stellar mass function. However, dynamical simulations of star clusters in tidal
fields have shown the mass function to change due to the preferential removal
of low-mass stars from clusters. Here we combine the results from dynamical
simulations of star clusters in tidal fields with our evolutionary synthesis
code GALEV to extend the models by a new dimension: the total cluster
disruption time. We reanalyse the mass function evolution found in N-body
simulations of star clusters in tidal fields, parametrise it as a function of
age and total cluster disruption time and use this parametrisation to compute
GALEV models as a function of age, metallicity and the total cluster disruption
time. We study the impact of cluster dissolution on the colour (generally, they
become redder) and magnitude (they become fainter) evolution of star clusters,
their mass-to-light ratios (off by a factor of ~2 -- 4 from standard
predictions), and quantify the effect on the cluster age determination from
integrated photometry (in most cases, clusters appear to be older than they
are, between 20 and 200%). By comparing our model results with observed M/L
ratios for old compact objects in the mass range 10^4.5 -- 10^8 Msun, we find a
strong discrepancy for objects more massive than 10^7 Msun (higher M/L). This
could be either caused by differences in the underlying stellar mass function
or be an indication for the presence of dark matter in these objects. Less
massive objects are well represented by the models. The models for a range of
total cluster disruption times are available online. (shortened)
