On the initial binary population for star cluster simulations
  Colour-magnitude diagrams (CMDs) are powerful tools that might be used to
infer stellar properties in globular clusters (GCs), for example, the binary
fraction and their mass ratio ($q$) distribution. In the past few years,
observations have revealed that q distributions of GC main-sequence (MS)
binaries are generally flat, and a distribution characterized by a strong
increase towards q $\approx$ 1 is not typical in GCs. In numerical simulations
of GC evolution with the initial binary population (IBP) described by Kroupa,
synthetic CMD colour distributions exhibit a peak associated with binaries that
have q $\approx$ 1. While the Kroupa IBP reproduces binary properties in
star-forming regions, clusters and the Galactic field, the peak in the q
distribution towards q $\approx$ 1 observed for GC simulations is not
consistent with distributions derived from observations. The objective of this
paper is to refine and further improve the physical formulation of
pre-main-sequence eigenevolution proposed by Kroupa in order to achieve CMD
colour distributions of simulated GC models similar to those observed in real
GCs, and to get a similarly good agreement with binary properties for late-type
binaries in the Galactic field. We present in this paper a modified Kroupa IBP,
in which early-type stars follow observational distributions, and late-type
stars are generated according to slightly modified pre-main-sequence
eigenevolution prescriptions. Our modifications not only lead to a
qualitatively good agreement with respect to long-term observations of
late-type binaries in the Galactic field, but also resolve the above-mentioned
problem related to binary distributions in GC models.
