Sami Dib

The Virial Balance of Clumps and Cores in Molecular Clouds

We study the instantaneous virial balance of CCs in numerical models of MCs. The models represent a range of magnetic field strengths in MCs from subcritical to nonmagnetic regimes. We identify CCs at different density thresholds and calculate, for each object, the terms that enter the EVT. A CC is gravitationally bound when the gravitational term in the EVT is larger than the amount for the system to be virialized, which is more stringent than the condition that it be large enough to make the total volume energy negative. We also calculate other quantities used to indicate the state of gravitational boundedness: Jeans number Jc, mass-to-magnetic flux ratio μc, and virial parameter αvir. Our results suggest the following: (1) CCs are dynamical out-of-equilibrium structures. (2) The surface energies are of the same order as their volume counterparts. (3) CCs are either in the process of being compressed or dispersed by the velocity field. Yet, not all CCs that have a compressive net kinetic energy are gravitationally bound. (4) There is no one-to-one correspondence between the states of gravitational boundedness as described by the virial analysis or by the other indicators. In general, in the virial analysis, only the inner regions of the objects are gravitationally bound, whereas Jc, αvir, and μc estimates tend to show that they are more bound at the lowest threshold levels and more magnetically supercritical. (5) We observe, in the nonmagnetic simulation, the existence of a bound core with structural and dynamical properties that resemble those of Barnard 68. This suggests that such cores can form in a larger MC and then be confined by the warm gas of a newly formed, nearby H II region.

ON THE ORIGIN OF THE H I HOLES IN THE INTERSTELLAR MEDIUM OF DWARF IRREGULAR GALAXIES

We suggest that large H I holes observed in the interstellar medium (ISM) of galaxies such as the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), and Holmberg II (Ho II, DDO 50, UGC 4305) can form as the combined result of turbulence coupled with thermal and gravitational instabilities. We investigate this problem with three-dimensional hydrodynamic simulations, taking into account cooling and heating processes and the action of the self-gravity of the gas. We construct an algorithm for radiative transfer to postprocess the simulated data and build emission maps in the 21 cm neutral hydrogen line. With this approach, we are able to reproduce the structure of the shells and holes as observed in regions of the ISM where no stellar activity is detected. In order to quantify the comparison of our synthetic maps to the observations, we calculate the physical scale-autocorrelation length relation (L-Lcr relation) both on the synthetic H I maps and the H I map of Ho II. The L-Lcr relation shows a linear increase of the autocorrelation length with the physical scale up to the scale of energy injection and flattens for larger scales. The comparison of the L-Lcr relation between the observations and the synthetic maps suggests that turbulence is driven in the ISM of Ho II on large scales (~6 kpc). The slope of the L-Lcr relation in the linear regime in Ho II is better reproduced by models where turbulence is coupled with a low-efficiency cooling of the gas. These results demonstrate the importance of the interplay between turbulence and the thermodynamics of the gas for structure formation in the ISM. Our analysis can be used to determine the scale on which kinetic energy is injected into the ISM of dwarf irregular galaxies and to derive, in a first approximation, the cooling rate of the gas.

Testing the universality of the IMF with Bayesian statistics: young clusters

The universality of the stellar initial mass function (IMF) is tested using Bayesian statistics with a sample of eight young stellar clusters (IC 348, ONC, NGC 2024, NGC 6611, NGC 2264, ρ Ophiuchi, Chameleon I, and Taurus). We infer the posterior probability distribution function (pPDF) of the IMF parameters 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 inter-cluster comparison of the pPDFs of the IMF parameters for each likelihood function shows that these distributions do not overlap within the 1σ 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. We also quantify the effects of taking into account the completeness correction as well as the uncertainties on the measured masses. The inclusion of the former affects the inferred pPDFs of the slope of the IMF at the low mass end while considering the latter affects the pPDFs of the slope of the IMF in the intermediate- to high mass regime. As variations are observed in all of the IMF parameters at once and for each of the considered likelihood functions, even for completeness corrected samples, we argue that the observed variations are real and significant, at least for the sample of eight clusters considered in this work. The results presented here clearly show that the IMF is not universal.

Star formation rates from young-star counts and the structure of the ISM across the NGC 346/N66 complex in the SMC

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 6×6pc 2 . 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 36pc 2 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 (90pc 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 15pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15pc 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.

Massive stars reveal variations of the stellar initial mass function in the Milky Way stellar clusters

We investigate whether the stellar initial mass function (IMF) is universal, or whether it varies significantly among young stellar clusters in the Milky Way. 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,

H-ATLAS/GAMA: the nature and characteristics of optically red galaxies detected at submillimetre wavelengths

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NGC 7538: multiwavelength study of stellar cluster regions associated with IRS 1–3 and IRS 9 sources

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Magnetic field structure around cores with very low luminosity objects

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Structure and mass segregation in Galactic stellar clusters

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The extended law of star formation: the combined role of gas and stars

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