The role of shear in dissipative gravitational collapse

In this paper we investigate the physics of a radiating star undergoing dissipative collapse in the form of a radial heat flux. Our treatment clearly demonstrates how the presence of shear affects the collapse process; we are in a position to contrast the physical features of the collapsing sphere in the presence of shear with the shear-free case. By employing a causal heat transport equation of the Maxwell-Cattaneo form we show that the shear leads to an enhancement of the core temperature thus emphasizing that relaxational effects cannot be ignored when the star leaves hydrostatic equilibrium.Comment: 15 pages, To appear in Int. J. Mod. Phys.

The effect of a two-fluid atmosphere on relativistic stars

We model the physical behaviour at the surface of a relativistic radiating star in the strong gravity limit. The spacetime in the interior is taken to be spherically symmetrical and shear-free. The heat conduction in the interior of the star is governed by the geodesic motion of fluid particles and a nonvanishing radially directed heat flux. The local atmosphere in the exterior region is a two-component system consisting of standard pressureless (null) radiation and an additional null fluid with nonzero pressure and constant energy density. We analyse the generalised junction condition for the matter and gravitational variables on the stellar surface and generate an exact solution. We investigate the effect of the exterior energy density on the temporal evolution of the radiating fluid pressure, luminosty, gravitational redshift and mass flow at the boundary of the star. The influence of the density on the rate of gravitational collapse is also probed and the strong, dominant and weak energy conditions are also tested. We show that the presence of the additional null fluid has a significant effect on the dynamical evolution of the star.Comment: 31 pages, Minor corrections implemente

Mixed potentials in radiative stellar collapse

We study the behaviour of a radiating star when the interior expanding, shearing fluid particles are traveling in geodesic motion. We demonstrate that it is possible to obtain new classes of exact solutions in terms of elementary functions without assuming a separable form for the gravitational potentials or initially fixing the temporal evolution of the model unlike earlier treatments. A systematic approach enables us to write the junction condition as a Riccati equation which under particular conditions may be transformed into a separable equation. New classes of solutions are generated which allow for mixed spatial and temporal dependence in the metric functions. We regain particular models found previously from our general classes of solutions.Comment: 10 pages, To appear in J. Math. Phy

Perception of risk of HIV infections and sexual behaviour of the sexually active university students in Zimbabwe

The study sought to establish university students’ perceptions of risk of HIV infections. A cross-sectional survey was conducted on 345 sexually active students at two universities in Zimbabwe (one state and one private).  Results revealed that above a quarter of the respondents felt at risk of getting HIV due to their regular partners’ sexual behaviours and more than half felt at risk of getting HIV due to their casual partners’ sexual behaviours. In addition, a third of the respondents acknowledged the HIV risk due to their own sexual behaviours. More state university respondents felt exposed to HIV infections due to own sexual behaviours than their private university counterparts. Despite these revelations, only 66.56% had earlier thought of their chances of getting infected with HIV. Personal HIV risk perceptions were low, reported by 27.76% of the sexually active respondents. Almost all respondents described their fellows’ sexual behaviours as either risky or very risky.Keywords: university students, HIV risk perceptions, sexual behaviour, ZimbabweL’e´tude visait a` e´valuer la perception du risque d’infection a` VIH des e´tudiants de l’universite´. Une ’e´tude transversale a e´te´ mene´e sur 345 e´tudiants sexuellement actifs dans deux universite´s du Zimbabwe (une e´tatique et une prive´e). Les re´sultats ont re´ve´le´ que plus d’un quart des re´pondants s’estimaient eˆtre expose´s au risque de contracter le VIH a` cause du comportement sexuel de leurs partenaires re´guliers et plus de la moitie´ se sentait exposer au risque de contracter le VIH a` cause du comportement sexuel de leurs partenaires occasionnels. En outre, un tiers des re´pondants ont reconnu qu’ils e´taient expose´s au risque de contracter le VIH a` cause de leur propre comportement sexuel. Il y a eu plus des re´pondants de l’universite´ de l’E´tat qui ont estime´ qu’ils e´taient expose´s au risque d’infection a` VIH a` cause de leur comportement sexuel que leurs homologues de l’universite´ prive´e. Malgre´ ces affirmations, seulement 66,56% s’e´taient de´ja` imagine´ qu’ils e´taient expose´s au risque d’eˆtre infecte´ par le VIH. La perception de contracter le VIH au niveau individuel e´tait faible, c’e´tait reconnu par 27,76% des re´pondants sexuellement actifs. Presque tous les re´pondants de´crivent le comportement sexuel de leurs semblables comme e´tant a` risque ou tre`s a` risque.Mots-cle´s: e´tudiants de l’universite´, risque d’infection a` VIH, comportement sexuel, Zimbabw

HIV protective strategies among college students in Durban, South Africa

There is growing concern about the high level of HIV infection among young people in South Africa. The aim of the study is to examine the HIV protective strategies used by college students with specific emphasis on variations by race group. The data for the study come from a self-administrated survey that was conducted with 3 000 college students in Durban in order to understand the strategies they use to protect themselves against the risk of HIV infection. Overall, students perceived a far greater risk of pregnancy than HIV infection. The results show that abstinence is the most common protective factor among Indian and White students. Among African students, there is great concern about HIV but abstinence is less common. Among sexually active men and women, the majority report having more than one sexual partner (with the exception of Indian females). Female students among all groups were more likely than male students to report that they were faithful to their partners. In Africans the contrast is stark: 25% for women versus 6% for men. Condoms are the most commonly used method by students but are not used in every sexual encounter. Consistent condom use was highest among Indian males (46%) and lowest among White females (13.7%). More effort needs to be directed at promoting correct and consistent condom use in order to avoid the negative consequences associated with unprotected sexual intercourse including unwanted pregnancy and HIV/AIDS

CULTURAL FACTORS AS PREDICTORS OF COGNITIVE TEST PERFORMANCE IN INFORMATION SYSTEMS AND TECHNOLOGY EDUCATION

A scan of the international literature suggests the existence in various countries of a persistent culture-based academicperformance gap across various subjects, including computer science, and at different levels of education. Almost twodecades after the formal demise of Apartheid, this study seeks to investigate whether a culture-based academic achievementgap similarly persists in the South African university classroom in the field of information and systems technology. Thisstudy sought to identify whether performance gaps exist between students of different races, home languages and genders ininformation systems and technology education at a South African university. Pre- and post- assessments were conductedwith first year information systems and technology students at the University of KwaZulu-Natal in respect of three separatecourses (Databases, Networks, Spreadsheets) attended by the same sample of students during the first semester in 2011.Multiple regression analyses were conducted to identify the extent to which the various independent variables (such as race,home language and gender) contributed to the variance of the dependent variables (improvement (gain) score and post testscore). The findings when using post-test scores as the dependent variable suggested that there were significant culture-baseddifferences in cognitive performance among first year South African university students in information systems andtechnology education. However, there were no significant differences in performance improvement (gain) scores oncognitive testing for the same sample. While Black students were significantly out-performed in terms of test scores, therewere no significant differences in the extent to which students improved their marks over the period of the study (one entiresemester). In fact, Black students improved by a slightly better margin than the Indian students, despite their raw test scoresbeing lower than those for their Indian counterparts. This suggests that despite their disadvantaged educational background,Black students are able to respond as effectively as more advantaged students to an equalised educational context once the‘playing fields are levelled at university

Analytical models for quark stars

We find two new classes of exact solutions to the Einstein-Maxwell system of equations. The matter content satisfies a linear equation of state consistent with quark matter; a particular form of one of the gravitational potentials is specified to generate solutions. The exact solutions can be written in terms of elementary functions, and these can be related to quark matter in the presence of an electromagnetic field. The first class of solutions generalises the Mak and Harko model. The second class of solutions does not admit any singularities in the matter and gravitational potentials at the centre.Comment: 10 pages, To appear in Int. J. Mod. Phys.