Browsing by Author "Dib, Sami"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Massive stars reveal variations of the stellar initial mass function in the Milky Way stellar clusters
    (Oxford : Oxford Univ. Press, 2016) Dib, Sami; Schmeja, Stefan; Hony, Sacha
    We investigate whether the stellar initial mass function (IMF) is universal, or whether it varies significantly among young stellar clusters in the MilkyWay.We propose a method to uncover the range of variation of the parameters that describe the shape of the IMF for the population of young Galactic clusters.These parameters are the slopes in the low and high stellar mass regimes, γ and Γ, respectively, and the characteristic mass, Mch. The method relies exclusively on the high-mass content of the clusters, but is able to yield information on the distributions of parameters that describe the IMF over the entire stellar mass range. This is achieved by comparing the fractions of single and lonely massive O stars in a recent catalogue of the Milky Way clusters with a library of simulated clusters built with various distribution functions of the IMF parameters. The synthetic clusters are corrected for the effects of the binary population, stellar evolution, sample incompleteness, and ejected O stars. Our findings indicate that broad distributions of the IMF parameters are required in order to reproduce the fractions of single and lonely O stars in Galactic clusters. They also do not lend support to the existence of a cluster mass-maximum stellar mass relation. We propose a probabilistic formulation of the IMF whereby the parameters of the IMF are described by Gaussian distribution functions centred around γ = 0.91, Γ = 1.37, and Mch = 0.41 M⊙, and with dispersions of σγ = 0.25, σΓ = 0.60, and σMch = 0.27 M⊙ around these values.
  • Thumbnail Image
    Item
    Structure and mass segregation in Galactic stellar clusters
    (Oxford : Oxford Univ. Press, 2017) Dib, Sami; Schmeja, Stefan; Parker, Richard J.
    We quantify the structure of a very large number of Galactic open clusters and look for evidence of mass segregation for themostmassive stars in the clusters.We characterize the structure and mass segregation ratios of 1276 clusters in the Milky Way Stellar Cluster (MWSC) catalogue containing each at least 40 stars and that are located at a distance of up to ≈2 kpc from the Sun. We use an approach based on the calculation of the minimum spanning tree of the clusters, and for each one of them, we calculate the structure parameter Q and the mass segregation ratio ΓMSR. Our findings indicate that most clusters possess a Q parameter that falls in the range 0.7-0.8 and are thus neither strongly concentrated nor do they show significant substructure. Only 27 per cent can be considered centrally concentrated with Q values > 0.8. Of the 1276 clusters, only 14 per cent show indication of significant mass segregation (ΓMSR > 1.5). Furthermore, no correlation is found between the structure of the clusters or the degree of mass segregation with their position in the Galaxy. A comparison of the measured Q values for the young open clusters in the MWSC to N-body numerical simulations that follow the evolution of the Q parameter over the first 10 Myr of the clusters life suggests that the young clusters found in the MWSC catalogue initially possessed local mean volume densities of ρ* ≈ 10-100 M⊙ pc-3.