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Stability of Accretion Discs around Magnetized Stars

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International Journal of Astronomy and Astrophysics 2014年第2期4卷 319-331页

作者： solomon belay tessema Department of Physics Kotebe University College Addis Ababa Ethiopia

We study the stability of accretion disc around magnetised stars. Starting from the equations of magnetohydrodynamics we derive equations for linearized perturbation of geometrically thin, optically thick axisymmetric accretion disc with an internal dynamo around magnetized stars. The structure and evolution of such discs are governed by an evolution equation for matter surface density ∑(R,T). Using the time-dependent equations for an accretion disc we do a linear stability analysis of our steady disc solutions in the presence of the magnetic field generated due to an internal dynamo.

关键词： Accretion Accretion Discs Instabilities Stability Magnetic Fields MHD Stars Neutron X-Rays Binaries

Evolution of Angular Momentum and Orbital Period Changes in Close Double White Dwarf Binaries

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International Journal of Astronomy and Astrophysics 2018年第3期8卷 275-298页

作者： Seblu Humne Negu solomon belay tessema Astronomy and Astrophysics Research Development Department Ethiopian Space Science and Technology Institute (ESSTI)-Entoto Observatory and Research Center (EORC) Addis Ababa Ethiopia

We have presented the evolution of angular momentum and orbital period changes between the component spins and the orbit in close double white dwarf binaries undergoing mass transfer through direct impact accretion over a broad range of orbital parameter space. This work improves upon similar earlier studies in a number of ways: First, we calculate self-consistently the angular momentum of the orbit at all times. This includes gravitational, tides and mass transfer effects in the orbital evolution of the component structure models, and allow the Roche lobe radius of the donor star and the rotational angular velocities of both components to vary, and account for the exchange of angular momentum between the spins of the white dwarfs and the orbit. Second, we investigate the mass transfer by modeling the ballistic motion of a point mass ejected from the center of the donor star through the inner Lagrangian point. Finally, we ensure that the angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these improvements, we calculate the angular momentum and orbital period changes of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum and orbital period of the orbit at the expense of the spin angular momentum of the donor and accretor. We find that our analysis yields an increase in the predicted number of stable systems compared to that in the previous studies, survive the onset of mass transfer, even if this mass transfer is initially unstable. In addition, as a consequence of the tidal coupling, systems that come into contact near the mass transfer instability boundary undergo a phase of mass transfer with their orbital period.

关键词： Stars: DWD Binaries Mass Transfer Methods: Numerical

Star Formation in Magnetized, Turbulent and Rotating Molecular Cloud: The Critical Mass

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International Journal of Astronomy and Astrophysics 2018年第4期8卷 347-367页

作者： Gemechu Muleta Kumssa solomon belay tessema Astronomy and Astrophysics Research Division Entoto Observatory and Research Center (EORC) Ethiopian Space Science and Technology Institute (ESSTI) Addis Ababa Ethiopia

In this paper, we present the critical mass of magnetized, turbulent and rotating star-forming molecular cloud core (MCc) in the presence of magnetic tension. The critical mass of star-forming magnetized cloud is influenced by the magnetic tension, magnetic pressure and other pressures. Applying the method of theoretical modelling by taking into account the basic equations and assumptions, we formulate the critical mass of magnetized MCc in different cases. Accordingly, the minimum critical masses we find in both cases are different. Energy due to magnetic tension significantly triggers the collapse at relatively larger radius of the core. The model shows that when the initial radius of the parent cloud (Ro) is larger than that of collapsing core radius (Rcore) the magnetic tension also has the larger radius of curvature, so it plays a significant role in supporting gravity to collapse the core. The results indicate gravity without magnetic tension may not overcome magnetic pressure, turbulence pressure and pressure due to rotation. This shows the critical mass of MCc for the collapse depends on the tension force that magnetic field lines apply on the envelope. We conclude that if there is magnetic pressure in star-forming MCc, there is also unavoidable magnetic tension, which triggers the collapse of the core. If there is no magnetic tension, the magnetized MCc needs relatively larger mass and higher density within the small size to collapse.

关键词： Star Formation Molecular Cloud Critical Mass Magnetic Field Turbulence Magnetic Tension

Mass Transfer in Binary Stellar Evolution and Its Stability

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International Journal of Astronomy and Astrophysics 2015年第3期5卷 222-241页

作者： Seblu Humne Negu solomon belay tessema Astronomy and Astrophysics Research Division Entoto Observatory and Research Center (EORC) Addis Ababa Ethiopia

The evolution of a binary star system by various analytical and numerical approximations of mass transfer rate normalized to the equilibrium rate and its stability conditions are investigated. We present results from investigations of mass transfer and stability in close binary star systems using the different orbital parameters. The stability and instability of mass transfer in binary star evolution depends on the exchange of material which the response of the binary to the initial Roche lobe overflow causes the donor to loose even more material. Our work is mainly focused on basic mathematical derivations, analytical and numerical solutions in order to explain the mass transfer system in different orbital parameters as well as the results are compared with previous studies in both cases. Mass transfer is usually stable, as long as the winds specific angular momentum does not exceed the angular momentum per reduced mass of the system. This holds for both dynamical and thermal time scales. Those systems which are not stable will usually transfer mass on the thermal time scale. The variation of Roche lobe radius with mass ratio in the binary, for various orbital parameters in the conservative and non-conservative mass transfer, as well as the evolution equations, orbital angular momentum of the binary system and the corresponding analytical and numerical solutions for different cases, under certain restrictive approximations is derived, simulated and discussed.

关键词： Mass Transfer Binary Star Stellar Evolution Stability Instability

Study of Principal Catalogues of Visual Double Stars

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International Journal of Astronomy and Astrophysics 2015年第4期5卷 267-274页

作者： Yikdem Mengesha Gebrehiwot solomon belay tessema Oleg Malkov Astronomy and Astrophysics Research Division Entoto Observatory and Research Center (EORC) Addis Ababa Ethiopia Institute of Astronomy Russian Academy Sciences Moscow Russia

The goal of this study is to develop a tool for detection of errors and misprints in principal catalogues of visual double stars. Modern statistical investigations of visual doubles (which are about 130 thousand, and they represent the most numerous observational type of binaries) are based almost exclusively on data, listed in those catalogues. So their correction is a challenging task, otherwise the statistics of visual binaries can be biased. Study of individual stars should also be based on correct data. We have presented tools to detect some of errors in the catalogues, especially, to indicate cases i) when positional information on components of a pair is contradictory, and ii) when a double star or its component is included in a catalogue twice, under different names.

关键词： Visual Binary Star Positional Inconsistency Data Duplication Reference Coordinates Additional Component Coordinates

Vulnerability analysis of smallholder farmers to climate variability and change: an agroecological system-based approach in the Fincha’a sub-basin of the upper Blue Nile Basin of Ethiopia

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Ecological Processes 2019年第1期8卷 49-66页

作者： Israel tessema belay Simane Center for Environment and Development College of Development StudiesAddis Ababa UniversityAddis AbabaEthiopia Environmental Engineering College of Biological and Chemical EngineeringAddis Ababa Science and Technology UniversityAddis AbabaEthiopia

Background:Ethiopia is frequently cited as a country that is highly vulnerable to climate variability and *** country’s high vulnerability arises mostly from climate-sensitive agricultural sector that suffers a lot from risks associated with rainfall *** vulnerability factors(exposure,sensitivity,and adaptive capacity)of the agricultural livelihoods to climate variability and change differ across agro-ecological systems(AESs).Therefore,the aim of this study was to analyze AES-specific vulnerability of smallholder farmers to climate variability and change in the Fincha’a *** surveyed 380 respondents from 4 AESs(highland,midland,wetland,and lowland)randomly ***,focus group discussion and key informant interviews were also performed to supplement and substantiate the quantitative *** vulnerability index was employed to analyze the levels of smallholders’agriculture vulnerability to climate variability and *** on socioeconomic and biophysical attribute were collected and combined into the indices and vulnerability score was calculated for each agro-ecological ***:Considerable variation was observed across the agro-ecological systems in profile,indicator,and the three livelihood vulnerability indices-Intergovernmental Panel on Climate Change dimensions(exposure,sensitivity,and adaptive capacity)of *** lowland AES exhibited higher exposure,low adaptive capacity,and high vulnerability,while the midland AES demonstrated lower exposure,higher adaptive capacity,and lower *** wetland and highland AESs scored intermediate ***:Our results suggest that resilience-building adaptation strategies are vital to reduce the vulnerability of smallholder *** measures taken should consider site-specific agro-ecological system requirements to reduce the vulnerability of smallholder mixed crop-livestock agriculture *** the approach based on the long-term rea

关键词： Vulnerability index Exposure Sensitivity Adaptation capacity Fincha’a sub-basin

Tree species determine soil seed bank composition and its similarity with understory vegetation in a semi-arid African savanna

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Ecological Processes 2017年第1期6卷 84-99页

作者： Zewdu Kelkay tessema belay Ejigu Lisanework Nigatu Rangeland Ecology and Biodiversity Program School of Animal and Range SciencesCollege of Agriculture and Environmental SciencesHaramaya UniversityPO Box 138Dire DawaEthiopia Department of Animal Sciences Faculty of AgricultureWollega UniversityPO Box 38Shambu CampusEthiopia School of Natural Resource Management and Environmental Sciences College of Agriculture and Environmental SciencesHaramaya UniversityPO Box 138Dire DawaEthiopia

Introduction:The presence of larger trees in semi-arid African savannas creates sub-habitats,which influences on herbaceous plant communities grown under their canopies differently from opened *** of seed banks accumulated in the soils over time beneath larger trees could facilitate the recovery of plant communities that might disappear due to sustained heavy grazing,prolonged fire,or other anthropogenic factors in semi-arid African ***,the impact of larger trees on soil seed bank composition and its similarity with plant communities grown under their canopies are less understood in semi-arid African ***,we studied the effect of leguminous and non-leguminous tree species and their canopies on soil seed bank(SSB)composition and its similarity with understory vegetation(USV)in a semi-arid savanna of ***:We selected 20 matured trees from 3 dominant tree species,representing one leguminous(Acacia robusta Burch)and 2 non-leguminous tree species(Ziziphus spina-Christi and Balanites aegyptiaca(L.)Del),found in isolation,a total of 60 trees for this *** each selected individual tree,the species composition of USV were recorded using 1-m2 quadrat in four directions(north,south,east,and west)under the inside and outside tree canopies during the flowering ***,soil samples in a 1-m2 quadrat were also collected under the inside tree canopies and their corresponding outside canopies,in each individual tree,for the determination of SSB composition,using a seed emergence ***,the soil was thoroughly mixed after removal of all roots and plant fragments,and spread over sand in plastic pots to a depth of 20 *** pots were placed at random in a glasshouse,examined every 3 days,for the first 2 months,and thereafter weekly for 6 months.A total of 960 soil samples were used for the determination of SSB composition during this ***:A total of 64 species were emerged from the SSB samples,of which 27 w

关键词： Functional groups Herbaceous layers Plant abundance Seedling density Species composition Species diversity Tree-grass interaction

Predictors of moral distress among nurses working in Jimma University Medical Center,South West Ethiopia

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Frontiers of Nursing 2020年第4期7卷 369-377页

作者： Habtam Abebaw Beyaffers Marta tessema Woldetsadik Admasu belay Gizaw School of Nursing and Midwifery Jimma University Institute of HealthJimma Oromia 378Ethiopia

Background:Moral distress is characterized by biopsychosocial,cognitive,and behavioral effects experienced by clinicians when their values are compromised by internal or external constraints,which results in the inability to give the desired care to ***:To assess predictors of moral distress among nurses working in Jimma University Medical Center,South West ***:An institution-based cross-sectional study design was used.A simple random sampling technique was applied to select a total of 248 study *** were collected using a structured self-administered questionnaire,which contains sociodemographic characteristics,Moral Distress Scale-Revised,personal factors,and organizational *** data were entered into Epi-data version 3.1 and analyzed by SPSS software version *** statistics,bivariate analysis,and multivariable logistic regression analysis were ***,P-value<0.05 was used to declare and include variables with statistically significant in predicting the outcome ***:More than two-thirds of the study participants 170(68.5%)were *** mean age of the respondents is 29 *** the study participants,174(70.16%)nurses had experienced a high level of moral ***,working hours,professional commitment,autonomy,and working environment were statistically significant predictors of moral ***:More than two-thirds of the nurses were experiencing a high level of moral *** will affect the nursing service quality,nurses,the nursing profession,and the organization as a *** finding is critical for the study since the problem is happening in the presence of low nurse to patient ratio and low nursing care ***,working hours per week,professional commitment,autonomy,and working environment were identified as predictors of moral distress.

关键词： moral distress nurses JUMC Ethiopia

Tuberculosis Lymphadenitis in Northwest Ethiopia: Implications for Public Health

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Open Journal of Medical Microbiology 2013年第1期3卷 18-24页

作者： Fantahun Biadglegne Berhanu Anagaw belay Anagaw Weghata Tesfaye belay tessema Ulrich Sack Arne C. Rodloff College of Medicine and Health Sciences University of Gondar Gondar Ethiopia Faculty of Medicine and Health Sciences Bahir Dar University Bahir Dar Ethiopia Faculty of Science Addis Ababa University Addis Ababa Ethiopia Institute of Clinical Immunology University Hospital University of Leipzig Leipzig Germany Institute of Medical Microbiology and Epidemiology of Infectious Diseases University Hospital University of Leipzig Leipzig Germany

Objective: In Ethiopia where there are limited diagnostic facilities, the actual burden of tuberculosis (TB) lymphadenitis is not well known. Therefore, we conducted this study to determine the proportion of TB lymphadenitis (TBL) in childhood and adults in Northwest Ethiopia. Materials and Methods: A prospective cross sectional study was conducted from April to May 2012. Fine needle aspiration cytology (FNAC) for cytological diagnosis of TBL was used. The diagnosis of TBL was established when cytological features from lymph node aspirates are strongly suggestive of TB. Descriptive and multivariate analysis was done using SPSS version 16. Results: Out of 1070 patients attending the cytological diagnosis in the study sites 437 (41%) were positive for TBL. Of the 437 registered TBL, 59 (13.5%) were pediatric patients and 378 (86.5%) were adults. There were more females than males with a male to female ratio of 0.8:1. The cervical region had the most common group of TBL with 321 (73.2%) patients. Most of all these patients 314 (75.3%), were matted with a majority of (250/314) being in the cervical region. Individuals who had contact history with TB patients (P = 0.046) were more likely to have TBL. Conclusions: In the studied region high prevalence of TBL was documented. Screening of TBL particularly for those who had contact with TB patients is recommended. Most patients in our study presented with matted lymph nodes, indicating that late arrival of patients to health institutions. Thus continuous and intensified public health strategies on health education and early referral system have to be done in order to link them to health institutions earlier than at present.

关键词： FNAC Epitheliod Cell Granulouma Caseous Necrosis TBL Northwest Ethiopia