61 research outputs found
The effects of double folding cluster model potential on some astrophysical reactions
The Double Folding Cluster Model Potential is constructed using the α - cluster structure of nuclei. It can be derived by folding an α − α interaction with density distributions of α - clusters inside the projectile and target nuclei. This potential has been successfully tested on elastic scattering data of some selected nuclei. In this work, we are interested to investigate the implications of this potential on astrophysical aspects
Prediction of moment of inertia of rotating nuclei
In this study, the mathematical expression formulated by Bohr for the moment of inertia of even-even nuclei based on the hydrodynamical model is modified. The modification pertains to the kinetic energy of the surface oscillations, including the second and third terms of the R-expansion as well as the first term, which had already been modified by Bohr. Therefore, this work can be considered a continuation and support of Bohr's hydrodynamic model. The procedure yields a Bohr formula to be multiplied by a factor that depends on the deformation parameter. Bohr's (modified) formula is examined by applying it on axially symmetric even-even nuclei with atomic masses ranging between 150 and 190 as well as on some triaxial symmetry nuclei. In this paper, the modification of Bohr's formula is discussed, including information about the stability of this modification and the second and third terms of the R-expansion in Bohr's formula. The results of the calculation are compared with the experimental data and Bohr's results recorded earlier. The results obtained are in good agreement with experimental data, with a ratio of approximately 0.7, and are better than those of the unmodified ones
Adriaan Moorrees : vorming, opleiding en gemeentelike bediening, 1855-1907
Thesis (MTh)--Stellenbosch University, 1984.ENGLISH ABSTRACT: No abstractAFRIKAANSE OPSOMMING: Geen opsommingMaster
Teknologi komunikasi dan proses pembentukan masyarakat bermaklumat: satu analisis semula peranan media elektronik di Malaysia / Mokhtar Muhammad , Abdullah @ Kassim Mohamad , Darusalam Abu Bakar , Jamaludin Abdul Rashid , Haliza Hasan
Tujuan kajian ini dibuat ialah untuk melihat sama ada terdapat pertalian di antara teknologi
komunikasi dari proses pembentukan masyarakat bermaklumat. Kajian ini meliputi semua
penduduk Malaysia yang terdedah kepada salah satu daripada media informasi elektronik
(radio; televisyen, briteks dan internet). Mereka juga terdiri dari berbagai kaum dan bangsa.
Penganalisaan kajian mengenai impak media ini adalah berdasarkan kepada maklumat yang
didapati dari kumpulan responden tersebut. Perbincangan tentang hasil kajian dibuat dengan
terperinci meliputi aspek-aspek seperti (i) Penggunaan Media Elektronik, (ii) Keberkesanan
Siaran Pendidikan di Radio dan Televisyen, (iii) Peranan Teknologi Komunikasi Dalam
Pembentukan Masyarakat Bermaklumat dan (iv) Faktor Demografi Responden
Secara keseluruhan kajian mendapati bahawa majoriti responden memiliki radio'dan/atau
televisyen dan lebih kerap mengikuti rancangan di televisyen berbanding radio. Kajian ini juga
menunjukkan bahawa majoriti responden tidak menggunakan internet, lebih-lebih lagi briteks.
Dari segi peranan media dalam membentuk masyarakat bermaklumat pula, kajian
menunjukkan bahawa majoriti responden berpendapat bahawa televisyen memainkan
peranan terpenting berbanding dengan media lain. Responden juga telah menyatakan tentang
keupayaan maklumat yang diterima melalui media-media dalam mempengaruhi mereka
dalam menerima sesuatu perubahan
Evolution and fate of very massive stars
There is observational evidence that supports the existence of very massive stars (VMS) in the local universe. First, VMS (Mini ≲ 320 M⊙) have been observed in the Large Magellanic Clouds (LMC). Secondly, there are observed supernovae (SNe) that bear the characteristics of pair creation supernovae (PCSNe, also referred to as pair instability SN) which have VMS as progenitors. The most promising candidate to date is SN 2007bi. In order to investigate the evolution and fate of nearby VMS, we calculated a new grid of models for such objects, for solar, LMC and Small Magellanic Clouds (SMC) metallicities, which covers the initial mass range from 120 to 500 M⊙. Both rotating and non-rotating models were calculated using the GENEVA stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since VMS have very large convective cores during the main-sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO)-type Wolf-Rayet stars. Because of very important mass losses through stellar winds, these stars may have luminosities during the advanced phases of their evolution similar to stars with initial masses between 60 and 120 M⊙. A distinctive feature which may be used to disentangle Wolf-Rayet stars originating from VMS from those originating from lower initial masses would be the enhanced abundances of Ne and Mg at the surface of WC stars. This feature is however not always apparent depending on the history of mass loss. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M⊙ are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M⊙. All VMS in the metallicity range studied here produce either a Type Ib SN or a Type Ic SN but not a Type II SN. We estimate that the progenitor of SN 2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M⊙. None of models presented in this grid produces gamma-ray bursts or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a black hole by producing a completely disruptive PCSN
The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters
NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high
spatial resolution near-IR photometry. We derive high T* for the WN stars in
NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~
2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars
make a disproportionate contribution to the global budget of their host
clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region.
Comparisons with stellar models calculated for the main-sequence evolution of
85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3
systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar
masses are supported by dynamical mass determinations for the components of NGC
3603 A1. We consider the predicted L_X of the R136 stars if they were close,
colliding wind binaries. R136c is consistent with a colliding wind binary
system. However, short period, colliding wind systems are excluded for R136a WN
stars if mass ratios are of order unity. Widely separated systems would have
been expected to harden owing to early dynamical encounters with other massive
stars in such a dense environment. From simulated star clusters, whose
constituents are randomly sampled from the Kroupa IMF, both clusters are
consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster
is either too old, exhibits a deficiency of very massive stars, or more likely
stellar masses have been underestimated - M_init for the most luminous stars in
the Arches cluster approach 200 Msun according to contemporary stellar &
photometric results. The potential for stars greatly exceeding 150 Msun within
metal-poor galaxies suggests that such pair-instability SNe could occur within
the local universe, as has been claimed for SN 2007bi (abridged).Comment: 20 pages, 14 figures, accepted for MNRAS. Version with higher
resolution figures is available from
http://pacrowther.staff.shef.ac.uk/R136.pdf See also
http://www.eso.org/public/news/eso1030/ from Wed 21 from noon (CEST
The quantum Hall effect : spin-charge locking
In two-dimensional electron gas when a large magnetic field is applied in one direction and an electric field perpendicular to it, there is a current in a direction perpendicular to both. This current is called the Hall effect. It remained without quantization until 1980 when it was found that the quantization leads to correct measurement of h/e2. Therefore the quantized Hall effect was further studied at high magnetic fields where fractional quantization was found. The fractional charge can arise from the “incompressibility� in the flux quantization. Laughlin wrote a wave function, the excitations of which are fractionally charged quasiparticles. This wave function comes in competition with charge density waves but for a few fractions it does give the ground state. If “incompressibility� is not considered and it is allowed to be compressible, the fractional charge can arise from the angular momentum which appears in the Bohr magneton in the form of g values. Usually the positive spin is considered but we consider both the positive as well as the negative values so that there is a spin-charge coupling. The values thus calculated for the fractional charge agree with the experimental data on the quantum Hall effect. We have followed this subject for a long time and hence have reviewed the subject. There are several interesting concepts which we learn from this subject. The concept of the Hall effect is quite clear particularly when combined with the flux quantization. We learn about the Landau levels and hence the boson character of electrons in two dimensions. We learn that charge becomes a vector quantity and there is spincharge coupling
Determination of moment of inertia for162-168Hf and164-176Yb deformed nuclei
In this paper, a method of defining the even-even deformed nuclei inertial parameters is suggested. Calculations for
isotopes 162-168Hf and 164-176Yb are listed. The parameters of inertia of rotational nuclei are also defined. Dependence of
the parameters of inertia on the nucleons number is shown
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