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A promising mechanism to form intermediate-mass black holes (IMBHs) is therunaway merger in dense star clusters, where main-sequence stars collide andform a very massive star (VMS), which then collapses to a black hole. In thispaper we study the effects of primordial mass segregation and the importance ofthe stellar initial mass function (IMF) on the runaway growth of VMSs using adynamical Monte Carlo code for N-body systems with N as high as 10^6 stars. Ourcode now includes an explicit treatment of all stellar collisions. We placespecial emphasis on the possibility of top-heavy IMFs, as observed in some veryyoung massive clusters. We find that both primordial mass segregation and theshape of the IMF affect the rate of core collapse of star clusters and thus thetime of the runaway. When we include primordial mass segregation we generallysee a decrease in core collapse time (tcc). Moreover, primordial masssegregation increases the average mass in the core, thus reducing the centralrelaxation time, which also decreases tcc. The final mass of the VMS formed isalways close to \sim 10^-3 of the total cluster mass, in agreement with theprevious studies and is reminiscent of the observed correlation between thecentral black hole mass and the bulge mass of the galaxies. As the degree ofprimordial mass segregation is increased, the mass of the VMS increases at mostby a factor of 3. Flatter IMFs generally increase the average mass in the wholecluster, which increases tcc. For the range of IMFs investigated in this paper,this increase in tcc is to some degree balanced by stellar collisions, whichaccelerate core collapse. Thus there is no significant change in tcc for thesomewhat flatter global IMFs observed in very young massive clusters.

Formation of Massive Black Holes in Dense Star Clusters. II. IMF and Primordial Mass Segregation