Modelling realistic horizontal branch morphologies and their impact on
  spectroscopic ages of unresolved stellar systems
  The presence of an extended blue horizontal branch (HB) in a stellar
population is known to affect the age inferred from spectral fitting to stellar
population synthesis models. However, most population synthesis models still
rely on theoretical isochrones which do not include realistic modelling of
extended HBs. In this work, we create detailed models for a range of old simple
stellar populations (SSPs), to create a variety of realistic HB morphologies,
from extended red clumps, to extreme blue HBs. We achieve this by utilising
stellar tracks from the BaSTI database and implementing a different mass loss
prescription for each SSP created, resulting in different HB morphologies. We
find that, for each metallicity, there is some HB morphology which maximises
Hbeta, making an underlying 14Gyr population look ~5-6Gyr old for the low and
intermediate metallicity cases, and as young as 2Gyr for a solar metallicity
SSP. We explore whether there are any spectral indices capable of breaking the
degeneracy between an old SSP with extended blue HB and a truly young or
intermediate age SSP, and find that the CaII index of Rose(1984) and the
strength of the MgII doublet at 2800A are promising candidates, in combination
with Hbeta and other metallicity indicators such as Mgb and Fe5406. We also run
Monte Carlo simulations to investigate the level of statistical fluctuations in
the spectra of typical stellar clusters. We find that fluctuations in spectral
indices are significant even for average to large globular clusters, and that
various spectral indices are affected in different ways, which has implications
for full-spectrum fitting methods. Hence we urge caution if these types of
stellar clusters are to be used as empirical calibrating objects for various
aspects of SPS models. (Abridged)
