Research

My research

my research is mostly dedicated to understanding how stars form, and how the outcome of the star formation process in galaxies depends on the local dynamical, and chemical conditions. I am particularly interested in the mass distribution of stars at their birth (a function called the stellar initial mass function, or in short the IMF).  I also work on the dynamics of gas and the origin of turbulent motions in the interstellar medium of galaxies. Some specific problems I have worked on include:

 The stellar initial mass function

Testing the universality of the IMF with Monte Carlo simulations -- 

We compare the fraction of single O stars with masses  > 15 Msol (i.e., O stars that are unique in  their respective clusters) and lonely O stars (a lonely O star is unique in its cluster and the cluster has no high mass B stars in the range 10-15 Msol) in Galactic stellar clusters measure using the Milky Way Stellar Cluster (MWSC) catalogue with that found in populations of synthetic clusters constructed with various distributions functions on the IMF parameters. We show that broad distributions of the IMF parameters are required to explain the observational values. To find out more: Dib, Schmeja, Hony (2017)

Testing the Universality of the IMF in young stellar clusters -- In this work, the universality of the stellar initial mass function (IMF) is tested using  Bayesian statistics with a sample of eight young Galactic stellar clusters (IC 348, ONC, NGC 2024, NGC 6611, NGC 2264, rho Ophiuchi, Chameleon I, and Taurus). The posterior probability distribution function (pPDF) of the IMF parameters is inferred when the likelihood function is described by a tapered power-law function, a lognormal distribution at low masses coupled to a power law at higher masses, and a multi-component power-law function. The intercluster comparison of the pPDFs of the IMF parameters for each likelihood function shows that these distributions do not overlap within the 1sigma uncertainty level. Furthermore, the most probable values of the IMF parameters for most of the clusters deviate substantially from their values for the Galactic field stellar IMF.  Given the current data, these results suggest that the IMF is not universal. Ref: Dib (2014).

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)

Can the orientation of molecular clouds in the Milky way be used to infer the physical scale of the turbulence driver ? -- In this work, we assess whether the driving of turbulence by supernovae is also important in the outer Galactic disc, where the star formation rates are lower, we study the spatial distribution of molecular cloud (MC) inclinations with respect to the Galactic plane. The latter contains important information on the nature of the mechanism of energy injection into the ISM. We analyse the spatial correlations between the position angles (PAs) of a selected sample of MCs (the largest clouds in the catalogue of the outer Galaxy published by Heyer et al). Our results show that when the PAs of the clouds are all mapped to values into the [0°, 90°] interval, there is a significant degree of spatial correlation between the PAs on spatial scales in the range of 100–800 pc. These scales are of the order of the sizes of individual SN shells in low-density environments such as those prevailing in the outer Galaxy and where the metallicity of the ambient gas is of the order of the solar value or smaller. These findings suggest that individual SN explosions, occurring in the outer regions of the Galaxy and in likewise spiral galaxies, albeit at lower rates, continue to play an important role in shaping the structure and dynamics of the ISM in those regions. The SN explosions we postulate here are likely associated with the existence of young stellar clusters in the far outer regions of the Galaxy and the ultraviolet emission and low levels of star formation observed with the Galaxy Evolution Explorer (GALEX) satellite in the outer regions of local galaxies. Ref: Dib et al. (2009)

The star formation rate and efficiency in molecular clouds and in galaxies

direct young star counts in the most massive star forming region of the Small Magellanic Cloud (NGC 346) -- The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a comparison of the surface density of young stars (Σ) and dust surface density (Σdust) across NGC 346 (N66) in 115 independent pixels of 6 × 6 pc2. We find a correlation between Σ and Σdust with a considerable scatter. A power-law fit to the data yields a steep relation with an exponent of 2.6 ± 0.2. We convert Σdust to gas surface density (Σgas) and Σ to star formation rate (SFR) surface densities (ΣSFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4 ± 1×10-3 M yr-1) is consistent with SFR estimated from the Hα emission integrated over the Hα nebula. On small scales the ΣSFR derived using Hα systematically underestimates the count-based ΣSFR, by up to a factor of 10. This is due to ionizing photons escaping the area, where the stars are counted. We find that individual 36 pc2 pixels fall systematically above integrated disc galaxies in the Schmidt-Kennicutt diagram by on average a factor of ˜7. The NGC 346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of ˜3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (˜10 per cent) within the central 15 pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15 pc is dominated by young stars belonging to a centrally condensed cluster, while the outer parts are dominated by a dispersed population. Therefore, the observed trend could reflect a change of star formation efficiency between clustered and non-clustered star formation. Ref: Hony, .., Dib et al. (2015).

The origin of dust in elliptical galaxies

The nature and characteristics of optically red galaxies detected at submillimetre wavelengths -- We combine Herschel/SPIRE submm observations with existing multiwavelength data to investigate the characteristics of low-redshift 0.01 ≤ z ≤ 0.2, optically red galaxies detected in submm bands. Sources are divided into 2 sub-samples of red and blue galaxies, based on their UV-optical colors. Galaxies in the red sample account for ≈4.2 per cent of the total number of sources with stellar masses M* ≳ 1010 M. We find that ≳30 per cent of them are early-type galaxies and ≳40 per cent are spirals. The color of the red-spiral galaxies could be the result of their highly inclined orientation and/or a strong contribution of the old stellar population. . From the analysis of their SEDs, we find that galaxies in the red sample (of any morphological type) have dust masses similar to those in the blue sample (i.e. normal spiral/star-forming systems). However, in comparison to the red-spirals and in particular blue systems, red-ellipticals have lower mean dust-to-stellar mass ratios. Besides galaxies in the red-elliptical sample have much lower mean star formation/specific star formation rates in contrast to their counterparts in the blue sample. Our results support a scenario where dust in early-type systems is likely to be of an external origin. Ref: Dariush, Dib, Hony et al. 2016.