  The mass of stellar black holes (BHs) is currently thought to be in the 3-20
Msun range, but is highly uncertain: recent observations indicate the existence
of at least one BH with mass >20 Msun. The metallicity of the progenitor star
strongly influences the mass of the remnant, as only metal-poor stars can have
a final mass higher than ~40 Msun, and are expected to directly collapse into
BHs with mass >25 Msun. By means of N-body simulations, we investigate the
formation and evolution of massive stellar BHs (MSBHs, with mass >25 Msun) in
young dense star clusters. We study the effects of MSBHs on the population of
X-ray sources.
