Research

Shallow IMFs and primordial mass segregation in starburst clusters -- In this paper, we explore the role of core coalescence as a viable mechanism to produce shallow IMFs and a primordial mass segregation in young massive clusters such as the Arches clusters and NGC 3603.  We investigate the time-evolution of the mass distribution of pre-stellar cores (PSCs) and their transition to the initial stellar mass function (IMF) in the central parts of a molecular cloud (MC) under the assumption that the coalescence of cores is important. Our aim is to explain the observed shallow IMF in dense stellar clusters such as the Arches cluster. The initial distributions of PSCs at various distances from the MC centre are those of gravitationally unstable cores resulting from the gravo-turbulent fragmentation of the MC. As time evolves, there is a competition between the rates of coalescence and collapse of the PSCs. Whenever the local rate of collapse is larger than the rate of coalescence in a given mass bin, cores are collapsed into stars. With appropriate parameters, we find that the coalescence-collapse model reproduces very well all the observed characteristics of the Arches stellar cluster IMF: namely, the slopes at high- and low-mass ends and the peculiar bump observed at ~ 5-6Msolar. Our results suggest that today's IMF of the Arches cluster is very similar to the primordial one and is little affected by the mass segregation due to dynamical effects. Ref: Dib, Kim & Shadmehri (2007).

The co-evolution of the Core Mass Function (CMF) and of the IMF in a protocluster clump -- We developed a model that follows the co-evolution of the mass function of dense gravitationally bound cores and of the stellar mass function in a protocluster clump. In the model, dense cores are injected, at a uniform rate, at different locations in the clump and evolve under the effect of gas accretion. Gas accretion on to the cores follows a time-dependent accretion rate that describes accretion in a turbulent medium. Once the accretion time-scales of cores of a given age, of a given mass and located at a given distance from the centre of the protocluster clumps exceed their contraction time-scales, they are turned into stars. The stellar initial mass function (IMF) is thus built up from successive generations of cores that undergo this accretion-collapse process. We also include the effect of feedback by the newly formed massive stars through their stellar winds. A fraction of the wind's energy is assumed to counter gravity and disperse the gas from the protocluster and as a consequence quench further star formation. We used this model to explore variations of the IMF as a function of a number of parameters in the models (e.g., contraction timescales and concentration of the collapsing and acreting cores; density profiles of the clumps). Ref  Dib et al. (2010).

The Supernova-Velocity Dispersion Relation in the interstellar medium -- We investigate the relationship between the velocity dispersion of the HI gas and the supernova (SN) rate and feedback efficiency with 3D numerical simulations of SN-driven turbulence in the interstellar medium. We aim to explore the constancy of the velocity dispersion profiles in the outer parts of galactic disks at ~6-8 km s-1 and the transition to the starburst regime. With our fiducial value of the SN feedback efficiency (i.e., = 0.25, corresponding to an injected energy per SN of 0.25 × 1051 ergs), our results show that (1) SN driving leads to constant velocity dispersions of σ ~ 6 km s-1 for the total gas and σ ~ 3 km s-1 for the H I gas, independent of the SN rate, for values of the rate between 0.01 and 0.5 the Galactic value (ηG); (2) the position of the transition to the starburst regime at around SFR/area 5 × 10-3 to 10-2 M☉ yr-1 kpc-2 observed in the simulations is in good agreement with the observations (e.g., NGC 628 and NGC 6949). Ref: Dib, Bell, & Burkert (2006)