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We present a new technique to quantify cluster-to-cluster variations in the observed present-day stellar mass functions of a large sample of star clusters. Our method quantifies these differences as a function of both the stellar mass and the total cluster mass, and offers the advantage that it is insensitive to the precise functional form of the mass function. We applied our technique to data taken from the ACS Survey for Globular Clusters, from which we obtained completeness-corrected stellar mass functions in the mass range 0.25-0.75 M$_{\odot}$ for a sample of 27 clusters. The results of our observational analysis were then compared to Monte Carlo simulations for globular cluster evolution spanning a range of initial mass functions, total numbers of stars, concentrations, and virial radii. We show that the present-day mass functions of the clusters in our sample can be reproduced by assuming an universal initial mass function for all clusters, and that the cluster-to-cluster differences are consistent with what is expected from two-body relaxation. A more complete exploration of the initial cluster conditions will be needed in future studies to better constrain the precise functional form of the initial mass function. This study is a first step toward using our technique to constrain the dynamical histories of a large sample of old Galactic star clusters and, by extension, star formation in the early Universe., Comment: 11 pages, 4 figures, 4 tables, accepted for publication in MNRAS, proof corrections made in updated version

A promising mechanism to form intermediate-mass black holes (IMBHs) is the runaway merger in dense star clusters, where main-sequence stars collide and form a very massive star (VMS), which then collapses to a black hole. In this paper we study the effects of primordial mass segregation and the importance of the stellar initial mass function (IMF) on the runaway growth of VMSs using a dynamical Monte Carlo code for N-body systems with N as high as 10^6 stars. Our code now includes an explicit treatment of all stellar collisions. We place special emphasis on the possibility of top-heavy IMFs, as observed in some very young massive clusters. We find that both primordial mass segregation and the shape of the IMF affect the rate of core collapse of star clusters and thus the time of the runaway. When we include primordial mass segregation we generally see a decrease in core collapse time (tcc). Moreover, primordial mass segregation increases the average mass in the core, thus reducing the central relaxation time, which also decreases tcc. The final mass of the VMS formed is always close to \sim 10^-3 of the total cluster mass, in agreement with the previous studies and is reminiscent of the observed correlation between the central black hole mass and the bulge mass of the galaxies. As the degree of primordial mass segregation is increased, the mass of the VMS increases at most by a factor of 3. Flatter IMFs generally increase the average mass in the whole cluster, which increases tcc. For the range of IMFs investigated in this paper, this increase in tcc is to some degree balanced by stellar collisions, which accelerate core collapse. Thus there is no significant change in tcc for the somewhat flatter global IMFs observed in very young massive clusters., Comment: Accepted for publication in ApJ

We study the dynamical evolution of the young star cluster Arches and its dependence on the assumed initial stellar mass function (IMF). We perform many direct $N$-body simulations with various initial conditions and two different choices of IMFs. One is a standard Kroupa IMF without any mass segregation. The other is a radially dependent IMF, as presently observed in the Arches. We find that it is unlikely for the Arches to have attained the observed degree of mass segregation at its current age starting from a standard non-segregated Kroupa IMF. We also study the possibility of a collisional runaway developing in the first $\sim 2-3 \rm{Myr}$ of dynamical evolution. We find that the evolution of this cluster is dramatically different depending on the choice of IMF: if a primordially mass segregated IMF is chosen, a collisional runaway should always occur between $2-3 \rm{Myr}$ for a broad range of initial concentrations. In contrast, for a standard Kroupa IMF no collisional runaway is predicted. We argue that if Arches was created with a mass segregated IMF similar to what is observed today then at the current cluster age a very unusual, high-mass star should be created. However, whether a collisional runaway leads to the formation of an intermediate-mass black hole (IMBH) depends strongly on the mass loss rate via winds from massive stars. Growth of stellar mass through collisions can be quenched by strong wind mass loss. In that case, the inter-cluster as well as intra-cluster medium are expected to have a significant Helium enrichment which may be observed via Helium recombination lines. The excess amount of gas lost in winds may also be observed via X-ray observations as diffused X-ray sources., Comment: 11 pages, 9 figures, 1 table. Submitted to ApJ

We investigate the dynamical decay of non-hierarchical accreting triple systems and its implications on the ejection model as Brown Dwarf formation scenario. A modified chain-regularization scheme is used to integrate the equations of motion, that also allows for mass changes over time as well as for momentum transfer from the accreted gas mass onto the bodies. We integrate an ensemble of triple systems within a certain volume with different accretion rates, assuming several prescriptions of how momentum is transferred onto the bodies. We follow their evolution until the systems have decayed. We find that the formation probability of Brown Dwarfs depends strongly on the assumed momentum transfer which is related to the motion of the gas. Due to ongoing accretion and consequent shrinkage of the systems, the median escape velocity is increased by a factor of 2 and the binary separations are decreased by a factor of 5 compared with non-accreting systems. Furthermore, the obtained semi-major axis distribution drops off sharply to either side of the median, which is also supported by observations. However, the disks around the ejected Brown Dwarfs seem to have too low accretion rates and masses to account for many of the observed disks in typical low-mass star-forming regions. We conclude that accretion of gas and the kinematic properties of the accreted gas during dynamical interactions strongly influence the abundance as well as the dynamical properties of Brown Dwarfs and, that the ejection scenario seems to be a promising scenario to produce both, very close Brown Dwarf binaries as well as single Brown Dwarfs, whereas it seems less likely to find very long-lived accretion disks around them.

Observations of brown dwarfs provide important feedback on theories of atmospheres and inner structure of substellar objects. Brown dwarf binary systems furthermore offer the unique opportunity to determine the mass of individual brown dwarfs, which is one of the fundamental astrophysical quantities., 4 pages, 6 figures, to appear in "Proc. 13th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun", Hamburg (Germany), 5-9 July 2004, F.Favata et al., eds. (ESA-SP, in press)