Grouped star formation: converting sink particles to stars in
  hydrodynamical simulations
  Modelling star formation and resolving individual stars in numerical
simulations of molecular clouds and galaxies is highly challenging. Simulations
on very small scales can be sufficiently well resolved to consistently follow
the formation of individual stars, whilst on larger scales sinks that have
masses sufficient to fully sample the IMF can be converted into realistic
stellar populations. However, as yet, these methods do not work for
intermediate scale resolutions whereby sinks are more massive compared to
individual stars but do not fully sample the IMF. In this paper, we introduce
the grouped star formation prescription, whereby sinks are first grouped
according to their positions, velocities, and ages, then stars are formed by
sampling the IMF using the mass of the groups. We test our grouped star
formation method in simulations of various physical scales, from sub-parsec to
kilo-parsec, and from static isolated clouds to colliding clouds. With suitable
grouping parameters, this star formation prescription can form stars that
follow the IMF and approximately mimic the original stellar distribution and
velocity dispersion. Each group has properties that are consistent with a
star-forming region. We show that our grouped star formation prescription is
robust and can be adapted in simulations with varying physical scales and
resolution. Such methods are likely to become more important as galactic or
even cosmological scale simulations begin to probe sub-parsec scales.
