The Fraction of Stars That Form in Clusters in Different Galaxies
  We estimate the fraction of stars that form in compact clusters (bound and
unbound), Gamma_F, in a diverse sample of eight star-forming galaxies,
including two irregulars, two dwarf starbursts, two spirals, and two mergers.
The average value for our sample is Gamma_F ~ 24 +/- 9%. We also calculate the
fraction of stars in clusters that survive to ages between t1 and t2, denoted
by Gamma_S(t1,t2), and find Gamma_S(10,100)=4.6 +/- 2.5% and
Gamma_S(100,400)=2.4 +/- 1.1 %, significantly lower than Gamma_F for the same
galaxies. We do not find any systematic trends in Gamma_F or Gamma_S with the
star formation rate (SFR), the SFR per unit area (Sigma_SFR), or the surface
density of molecular gas (Sigma_H2) within the host galaxy. Our results are
consistent with those found previously from the CMF/SFR statistic (where CMF is
the cluster mass function), and with the quasi-universal model in which
clusters in different galaxies form and disrupt in similar ways. Our results,
however, contradict many previous claims that the fraction of stars in bound
clusters increases strongly with Sigma_SFR and Sigma_H_2. We find that the
previously reported trends are largely driven by comparisons that mixed Gamma_F
~ Gamma_S(0,10) and Gamma_S(10,100), where Gamma_S(0,10) was systematically
used for galaxies with higher Sigma_SFR and Sigma_H2, and Gamma_S(10,100) for
galaxies with lower Sigma_SFR and Sigma_H_2.
