Stability of viscous fluid in Bianchi type-VI model with cosmological constant     

J. Sadeghi  Ali R. Amani  N. Tahmasbi 《Astrophysics and Space Science》2013,348(2):559-564

In this paper, we investigate Bianchi type-VI cosmological model for the universe filled with dark energy and viscous fluid in the presence of cosmological constant. Also, we show accelerating expansion of the universe by drawing volume scale, pressure and energy density versus cosmic time. In order to solve the Einstein’s field equations, we assume the expansion scalar is proportional to a component of the shear tensor. Therefore, we obtain the directional scale factors and show the EOS parameter crosses over phantom divided-line.  相似文献   

Hot plasma waves in Veselago medium around Reissner-Nordström black hole     

M. Hossain Ali  M. Ashrafuzzaman Khan 《Astrophysics and Space Science》2013,348(1):163-178

We investigate the general relativistic magnetohydronadynamic (GRMHD) equations for hot plasmas in a Veselago medium around the Reissner-Nordström (RN) black hole. Using the 3+1 formalisms of spacetime, we write the GRMHD equations and perturb them linearly. These are then Fourier analyzed for the magnetized and nonmagnetized plasmas in rotating and nonrotating backgrounds. We derive dispersion relations and analyze the wave properties by the graphs of wave vector, refractive index and change in refractive. The results confirm the presence of Veselago medium for rotating magnetized/nonmagnetized and nonrotating nonmagnetized plasmas.  相似文献   

Spatial galaxy distribution function for a three-component system     

Manzoor?A.?MalikEmail author  Raja?Nisar?Ali  Farooq?Ahmad 《Astrophysics and Space Science》2011,336(2):447-454

We derive the distribution function and the allied thermodynamic quantities for a system of galaxies with three mass species. A new clustering parameter b ₃ that inherently takes into account the masses and the number of galaxies of each kind, emerges directly from the calculations. Our general conclusion is that the inclusion of the third component does not significantly effect the overall features of the distribution function.  相似文献   

An Observational Study of the Strong Single Pulses of PSR J0034-0721     

Buhalqem Nizamdin  Ali Esamdin  Zhi-Yong LiuNa Wang  Cheng-Shi Zhao 《Chinese Astronomy and Astrophysics》2011,35(1):37

Using the 25m radio telescope of the Urumqi Observatory, the strong single pulses of the pulsar PSR J0034-0721 were observed at 1.54 GHz on 6th Aug. 2007. With the technique of single-pulse detection, 116 single pulses with the signal-to-noise ratios of R_SN≥5 were detected from the observed data of 1 h. At 1.54 GHz, the signal-to-noise ratios of the detected single pulses are in the range from 5 to 10.5, and the peak flux densities of these pulses are approximately 14∼29 times that of the average pulse (AP), much less than the ratios between the intensities of typical giant pulses and the intensity of AP. The cumulative distribution of the intensities of these pulses is basically a powerlaw distribution with the spectral index α = −4.3 ± 0.4. The detection rates for the pulses of R_SN≥5 and R_SN≥10 are 3% and 0.08%, respectively. For these pulses, the half-peak width W₅₀ ranges from 1.6 ms to 8 ms, 3.9 ms in average. The phases of the vast majority of the strong single pulses are concentrated around the peak position of AP, but 2 strong pulses of R_SN≥8.5 are detected at the phases about 33 ms earlier than the phase of the AP peak. This implies that there probably exist two emission regions of strong pulses, and this is consistent with the previously observed results at 40 MHz and 111 MHz, except that at 1.54 GHz the profile of AP exhibits only one component.  相似文献   

Period studies of classical Algol-type binaries II: UX Leo,RW Mon,EQ Ori,XZ UMa and AX Vul     

F. Soydugan  A. Erdem  S.S. Doğru  F. Aliçavuş  E. Soydugan  C. Çiçek  O. Demircan 《New Astronomy》2011,16(4):253-264

This study presents an investigation of the orbital period variations of five Algol type binaries, UX Leo, RW Mon, EQ Ori, XZ UMa and AX Vul based on all available minima times. The O–C diagrams of all systems exhibit a periodic variation superimposed on a downward parabolic segment. The mass loss due to magnetic braking effect in the cooler components is assumed to account for the parabolic variation with a downward shape, while it is suggested that the light-time effect (LITE) due to an unseen component around the eclipsing binaries explains the tilted sinusoidal changes in their O–C diagrams. The orbital period decrease rates for the systems are estimated as approximately between about 0.7 and 2.5 s per century. It is clearly seen that mass loss effect is more dominant than the expected mass transfer for classical Algols in this study. The minimum mass of the probable third bodies around the eclipsing pairs was calculated to be ?0.5 M_⊙ except for UX Leo, in which it was estimated to be approximately 0.9 M_⊙. In order to search for third lights in the light curves of five systems, the V-light curves of the systems were analyzed and their physical and photometric parameters were determined. For UX Leo, a significant third light contribution was determined. We found a very small third light that can be tested using multi-color light curves, for RW Mon, EQ Ori and XZ UMa, while a third light for AX Vul could not be exposed.  相似文献   

Coupled dispersive drift acoustic modes in inhomogeneous dusty plasmas with different nonthermal distributions for electrons and ions     

W. Masood  Ali Ahmad 《Astrophysics and Space Science》2012,340(2):367-372

Linear dispersion characteristics of the coupled drift acoustic modes are investigated in inhomogeneous dusty magnetoplasmas both when the dust is considered immobile and when the dust dynamics is taken into account in the presence of nonthermal population of electrons and ions. In this regard, Cairns and Kappa distributed electrons and ions are considered. It is found that the nonthermal distributions affect the phase velocities and the fundamental scalelengths of the plasma. It is found that for both the ion and dust dynamics driven waves, the phase velocities are highest for Cairns, intermediate for Kappa, whereas they are minimum for the Maxwellian distributed electrons. The work presented here may be useful to understand the low frequency electrostatic modes in inhomogeneous dusty plasmas such as those found in planetary environments.  相似文献   

Lithological Mapping in the Eastern Part of the Central Iranian Volcanic Belt Using Combined ASTER and IRS data   总被引：1，自引：0，他引：1  

Seyed Mohammad Hadi Hadigheh  Hojjatollah Ranjbar 《Journal of the Indian Society of Remote Sensing》2013,41(4):921-931

The study area is located in the eastern part of the central Iranian volcanic belt. Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) and Indian Remote Sensing Satellite (IRS ) pan images were used for applying several image classification methods for lithological mapping. ASTER visible-near infrared and shortwave infrared bands were sharpened using IRS pan image. We used classification methods such as Maximum likelihood, Spectral Angle Mapper (SAM) and Spectral Information Divergence (SID) in order to evaluate the usefulness of these methods for geological mapping. The classification results showed that MLC has the best accuracy and the classified image closely resembles the previously prepared geology map of the area.  相似文献   

Erratum to: On weighted total least-squares with linear and quadratic constraints     

Vahid Mahboub  Mohammad Ali Sharifi 《Journal of Geodesy》2013,87(6):607-608

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南山并置站本地连接测量GPS控制网数据精度分析     

张阿丽  熊福文  朱文耀 《测绘通报》2013,(12):4-7,35

2011年8月在新疆天文台南山观测站首次进行了GPS与VLBI空间大地测量技术并置站的本地连接测量。针对南山并置站,本文在建立、优化南山GPS控制网的基础上,探讨本地连接测量中GPS数据处理的方法、精度。通过对GPS控制网7 d连续观测和基于IGS精密星历的GAMIT基线处理、三维空间平差,结果表明:基线在N、E和长度方向的重复性在0.3~0.4 mm,U方向分量重复性为1.4 mm,基线相对精度为0.8×10-8,达到《全球定位系统(GPS)测量规范》(GB/T 18314—2009)中A级网的精度要求,GPS控制点在ITRF2008系统下的坐标精度均优于0.5 mm。  相似文献   

SDI planning using the system dynamics technique within a community of practice: lessons learnt from Tanzania     

Ali Mansourian  Alex Lubida  Petter Pilesjö  Ehsan Abdolmajidi  Monica Lassi 《地球空间信息科学学报》2013,16(2-3):97-110

There exist major challenges in accelerating the spatial data infrastructure (SDI) planning process in the developing countries as well as advocating for politicians to support the development of SDI, due to the high complexity of SDI, lack of knowledge and experience, and limited insight in the benefits. To address these challenges, a methodology for SDI planning in Tanzania, based on the system dynamics technique and the communities of practice concept, was adopted and applied within a community consisting of experts from stakeholder organizations. The groups gathered to develop an SDI plan, while they shared their knowledge and discussed their ideas that helped their understanding of SDI. By running the system dynamics model, the development of SDI over time could be simulated that gave the planning community an insight about the future effects of today’s plans and decisions. Finally, an optimum model could be developed by refinements and improvements done with the consensus of the SDI stakeholders. This model included the components and policies that are essential for a successful SDI implementation in Tanzania and can be used as a basis for SDI planning and help to gain political support. Lessons learnt from this research were promising regarding the usability of the methodology for SDI planning in comparable countries.  相似文献