The bound fraction of young star clusters
  The residual gas within newly formed star clusters is expelled through
stellar feedback on timescales ~ 1 Myr. The subsequent expansion of the cluster
results in an unbinding of a fraction of stars before the remaining cluster
members can re-virialize and form a surviving cluster. We investigate the bound
fraction after gas expulsion as a function of initial cluster mass in stars and
gauge the influence of primordial mass segregation, stellar evolution and the
tidal field at the solar distance. We also assess the impact of the
star-formation efficiency and gas expulsion velocity. We perform N-body
simulations using Sverre Aarseth's NBODY7 code, starting with compact clusters
in their embedded phase and approximate the gas expulsion by means of an
exponentially depleting external gravitational field. We follow the process of
re-virialization through detailed monitoring of different Lagrange radii over
several Myr, examining initial half-mass radii of 0.1 pc, 0.3 pc and 0.5 pc and
initial masses usually ranging from $5\times10^3 M_\odot$ to $5\times10^4
M_\odot$. The adopted star-formation efficiency of 0.33 in the cluster volume
results in a distinct sensitivity to the gas expulsion velocity over a wide
mass range, while a variation of the star-formation efficiency can make the
cluster robust to the rapidly decreasing external potential. We confirm that
primordial mass segregation leads to a smaller bound fraction, its influence
possibly decreasing with mass. Stellar evolution has a higher impact on lower
mass clusters, but heating through dynamical friction could expand the cluster
to a similar extent. The examined clusters expand well within their tidal radii
and would survive gas expulsion even in a strong tidal field.
