  We investigate the role of binaries and chromospheric emission on HST
photometry of globular clusters' stars. We quantify their respective effects on
the position of stars in the chromosome map, especially among the first
population. We computed atmosphere models and synthetic spectra for stars of
different chemical compositions, based on isochrones produced by stellar
evolution calculations with abundance variations representative of first and
second populations in GCs. From this we built synthetic chromosome maps for a
mixture of stars of different chemical compositions. We subsequently replaced a
fraction of stars with binaries, or stars with chromospheric emission, using
synthetic spectroscopy. We studied how the position of stars is affected in the
chromosome map. Binaries can, in principle, explain the extension of the first
population in the chromosome map. However, we find that given the binary
fraction reported for GCs, the density of stars in the extended part is too
small. Another difficulty of the binary explanation is that the shape of the
distribution of the first population in the chromosome map is different in
clusters with similar binary fractions. Additionally, we find that the
contribution of chromospheric emission lines to the HST photometry is too small
to have an observable impact on the shape of the chromosome map. Continuum
chromospheric emission has an effect qualitatively similar to binaries. We
conclude that binaries do have an impact on the morphology of the chromosome
map of GCs, but they are unlikely to explain entirely the shape of the extended
distribution of the first population stars. Uncertainties in the properties of
continuum chromospheric emission of stars in GCs prevent any quantitative
conclusion. Therefore, the origin of the extended first population remains
unexplained.
