Stellar dynamics in young clusters: the formation of massive runaways and very massive runaway mergers

In the present paper we combine an N-body code that simulates the dynamics of young dense stellar systems with a massive star evolution handler that accounts in a realistic way for the effects of stellar wind mass loss. We discuss two topics.

1. The formation and the evolution of very massive stars (with masses >120 M_⊙) is followed in detail. These very massive stars are formed in the cluster core as a consequence of the successive (physical) collisions of the 10–20 most massive stars in the cluster (this process is known as ‘runaway merging’). The further evolution is governed by stellar wind mass loss during core hydrogen and core helium burning (the WR phase of very massive stars). Our simulations reveal that, as a consequence of runaway merging in clusters with solar and supersolar values, massive black holes can be formed, but with a maximum mass ≈70 M_⊙. In low-metallicity clusters, however, it cannot be excluded that the runaway-merging process is responsible for pair-instability supernovae or for the formation of intermediate-mass black holes with a mass of several 100 M_⊙.
2. Massive runaways can be formed via the supernova explosion of one of the components in a binary system (the Blaauw scenario), or via dynamical interaction of a single star and a binary or between two binaries in a star cluster. We explore the possibility that the most massive runaways (e.g. ζ Pup, λ Cep, BD+43°3654) are the product of the collision and merger of two or three massive stars.

     

A multiphysics and multiscale software environment for modeling astrophysical systems

We present MUSE, a software framework for combining existing computational tools for different astrophysical domains into a single multiphysics, multiscale application. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for studying generalized stellar systems. We have now reached a “Noah’s Ark” milestone, with (at least) two available numerical solvers for each domain. MUSE can treat multiscale and multiphysics systems in which the time- and size-scales are well separated, like simulating the evolution of planetary systems, small stellar associations, dense stellar clusters, galaxies and galactic nuclei. In this paper we describe three examples calculated using MUSE: the merger of two galaxies, the merger of two evolving stars, and a hybrid N-body simulation. In addition, we demonstrate an implementation of MUSE on a distributed computer which may also include special-purpose hardware, such as GRAPEs or GPUs, to accelerate computations. The current MUSE code base is publicly available as open source at http://muse.li.     

Study of the influence of the physicochemical parameters on microbial abundance in various ambient conditions

Nowadays, the problems of rejections constitute an increasingly important danger in the receiving mediums. Indeed, water is increasingly affected by minerals and organic matters and even by the micro-organisms of which some are pathogenic and thus dangerous for the ecosystem. In this context, the present study was conducted to investigate the microbiological pollution of water of the lake Mellah, located in the national park of El Kala, North-eastern Algerian. This lake is a depression which communicates with the sea and flowing the domestic effluents of the neighbouring localities. The physicochemical parameters (Temperature, pH; Eh; Electric conductivity; Salinity, Turbidity, and O₂) and bacteriological (coliformes total, coliformes thermo tolerant, streptococci) were followed during the year 2011 with a seasonal frequency (February, May, August, November). Seventeen (17) sites of three different habitats were investigated; the waste-water treatment plant (WWTP), Wadis and their upstream to the estuary and the lake. The follow-up of analysis of the physicochemical and biological parameters in the various points of observation was the object of a graphs data processing and statistics (ACP) to determine the bacteriological degree of pollution. Results showed a contamination by the fecal matter with a gradient decreasing from the upstream of the discharge system towards the downstream. Compared to rain waters, microbiological pollution in the lake was selective in time. The CT and the CTT decreased in dry season where the ambient conditions (salinity, pH and temperature) become unfavorable, putting the factor salinity concerned to degrade the water pollution.     

Use of a new method for determining the vulnerability and risk of pollution of major groundwater reservoirs in the region of Annaba–Bouteldja (NE Algeria)

Scientists are deeply concerned by the state of vulnerability of groundwater reservoirs. It is a complex task because of the difficulties in determining the degree of pollution of the groundwater. Many methods have been adopted like (DRASTIC, GOD, SI, SINTACS…). Another method (Kherici et al. in Geogr Tech 1–14, 2010) is added to identify the vulnerability of groundwater reservoirs and control the evolution of pollutants. The present article targets the determination of the vulnerability and risks of pollution of groundwater reservoirs of a climatic Mediterranean region (Annaba–Bouteldja region). The device used is based on the combination of two criteria: natural factors (thickness of the unsaturated zone, geological facies and degree of self-purification) and the causes of vulnerability and the pollution risks entropic factors (caused by man). The application of Kherici’s method has revealed a distinction between the different degrees of pollution and has allowed a neat classification of the different reservoirs in the study. The results lead to a vulnerability map and the risks of pollution of Annaba–Bouteldja different aquifers. It has also led to the installation of protection areas; sustained by an efficient general evacuation plan of the sewerage net and the construction of treatment station of the sewage effluents in the urbanized areas.     

Assessing the organic pollution of surface water of Medjerda watershed (NE Algeria)

Water quality in the Northern part of Mellegue-Medjerda watershed (East Algeria) has been adversely affected by important pollutants discharged into the Medjerda wadi without, in most cases, any treatment. Chemical and physical degradation are due to agricultural and industrial practices and domestic wastewaters. Over a three-month period, a study of the low-flow water quality characteristics throughout Medjerda wadi was undertaken. Longitudinal profiles of water quality were constructed using data from fourteen sites. All sewage, agricultural, and industrial inputs were included. Analyzed properties were nutrients (NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, and PO₄ ³⁻), Biochemical oxygen demand after five days (BOD₅), chemical oxygen demand (COD), and dissolved oxygen (DO). Along Medjerda wadi, all values change because of conditions specific to each sampling station. Nitrate was the most important form of nitrogen-element load (94%). Its concentration reached 34.3 mg L⁻¹ at OM₄ point, downstream of domestic wastewater discharges. The spatial evolution of the organic pollution index (OPI) shows that the wastewater effluent constitutes the main source of pollution. Indeed, water quality goes from a moderate pollution state at some sampling stations not or slightly affected by wastewaters discharges to a very strong pollution state (OPI of about 1.75) downstream of the domestic effluents inputs of Souk-Ahras city.     

Experimental Study of Crushed Granular Materials by the Notion of Fractal Dimension in 2D and 3D

Geotechnical and Geological Engineering - The micro-texture of the aggregates of a pavement layer has a direct influence on their resistance. Whatever the position of these aggregates in a pavement...     

GIS-based GALDIT method for vulnerability assessment to seawater intrusion of the Quaternary coastal Collo aquifer (NE-Algeria)

The overexploitation of groundwater in coastal aquifers is often accompanied by seawater intrusion, intensified by climate change and sea level rise. Heading long-term water quality safety and thus the determination of vulnerable zones to seawater intrusion becomes a significant hydrogeological task for many coastal areas. Due to this background, the present study focussed the established methodology of the GIS-based GALDIT model to assess the aquifer vulnerability to seawater intrusion for the Algerian example of the Quaternary coastal Collo aquifer. According to the result analysis overall, more than half of the total surface of the northern study area can be classified as highly vulnerable. Besides the coastline, the areas nearby the local wadis of Guebli and Cherka occur to be the most vulnerable in the region. In view of further map removal performance as well as single-parameter sensitivity analyses from a coupled perspective respectively the GALDIT parameters, distance from the shore (D) and aquifer hydraulic conductivity (A) have been found to be of key significance regarding the model results (mean effective weightings ~?18–19%). Overall, the study results provide a good approximation basis for future management decisions of the Collo aquifer region, including various perspectives such as identification of suitable settings for prospective groundwater pumping wells.