A Comparison of Global Magnetic Field Skeletons and Active-Region Upflows     

S. J. Edwards  C. E. Parnell  L. K. Harra  J. L. Culhane  D. H. Brooks 《Solar physics》2016,291(1):117-142

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Smearing of mass accretion rate variation by viscous processes in accretion disks in compact binary systems     

A. Ghosh  Sandip K. Chakrabarti 《Astrophysics and Space Science》2016,361(9):310

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Cluster analysis on the bulk elemental compositions of Antarctic stony meteorites          下载免费PDF全文

Hideaki Miyamoto  Takafumi Niihara  Takeshi Kuritani  Peng K. Hong  James M. Dohm  Seiji Sugita 《Meteoritics & planetary science》2016,51(5):906-919

Remote sensing observations by recent successful missions to small bodies have revealed the difficulty in classifying the materials which cover their surfaces into a conventional classification of meteorites. Although reflectance spectroscopy is a powerful tool for this purpose, it is influenced by many factors, such as space weathering, lighting conditions, and surface physical conditions (e.g., particle size and style of mixing). Thus, complementary information, such as elemental compositions, which can be obtained by X‐ray fluorescence (XRF) and gamma‐ray spectrometers (GRS), have been considered very important. However, classifying planetary materials solely based on elemental compositions has not been investigated extensively. In this study, we perform principal component and cluster analyses on 12 major and minor elements of the bulk compositions of 500 meteorites reported in the National Institute of Polar Research (NIPR), Japan database. Our unique approach, which includes using hierarchical cluster analysis, indicates that meteorites can be classified into about 10 groups purely by their bulk elemental compositions. We suggest that Si, Fe, Mg, Ca, and Na are the optimal set of elements, as this set has been used successfully to classify meteorites of the NIPR database with more than 94% accuracy. Principal components analysis indicates that elemental compositions of meteorites form eight clusters in the three‐dimensional space of the components. The three major principal components (PC1, PC2, and PC3) are interpreted as (1) degree of differentiations of the source body (i.e., primitive versus differentiated), (2) degree of thermal effects, and (3) degree of chemical fractionation, respectively.  相似文献   

A new model for charged anisotropic compact star     

S. K. Maurya  M. K. Jasim  Y. K. Gupta  T. T. Smitha 《Astrophysics and Space Science》2016,361(5):163

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Anisotropic generalization of Matese & Whitman solution for compact star models in general relativity     

Baiju Dayanandan  S. K. Maurya  Y. K. Gupta  Smitha T.T. 《Astrophysics and Space Science》2016,361(5):160

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Optimisation of the Population Monte Carlo algorithm: Application to constraining isocurvature models with cosmic microwave background data     

D. Moodley  K. Moodley 《Astronomische Nachrichten》2016,337(6):672-682

We optimise the parameters of the Population Monte Carlo algorithm using numerical simulations. The optimisation is based on an efficiency statistic related to the number of samples evaluated prior to convergence, and is applied to a D ‐dimensional Gaussian distribution to derive optimal scaling laws for the algorithm parameters. More complex distributions such as the banana and bimodal distributions are also studied. We apply these results to a cosmological parameter estimation problem that uses CMB anisotropy data from the WMAP nine‐year release to constrain a six parameter adiabatic model and a fifteen parameter admixture model, consisting of correlated adiabatic and isocurvature perturbations. In the case of the adiabatic model and the admixture model we find that the number of sample points increase by factors of 3 and 20, respectively, relative to the optimal Gaussian case. This is due to degeneracies in the underlying parameter space. The WMAP nine‐year data constrain the admixture model to have an isocurvature fraction of 36.3 ± 2.8 %. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Probing Individual Sources during Reionization and Cosmic Dawn using Square Kilometre Array HI 21-cm Observations     

Kanan K. Datta  Raghunath Ghara  Suman Majumdar  T. Roy Choudhury  Somnath Bharadwaj  Himadri Roy  Abhirup Datta 《Journal of Astrophysics and Astronomy》2016,37(4):27

Detection of individual luminous sources during the reionization epoch and cosmic dawn through their signatures in the HI 21-cm signal is one of the direct approaches to probe the epoch. Here, we summarize our previous works on this and present preliminary results on the prospects of detecting such sources using the SKA1-low experiment. We first discuss the expected HI 21-cm signal around luminous sources at different stages of reionization and cosmic dawn. We then introduce two visibility based estimators for detecting such signals: one based on the matched filtering technique and the other relies on simply combing the visibility signal from different baselines and frequency channels. We find that the SKA1-low should be able to detect ionized bubbles of radius \(R_{\mathrm {b}} \gtrsim 10\) Mpc with ～100 h of observations at redshift z～8 provided that the mean outside neutral hydrogen fraction \(\mathrm {x}_{\text {HI}} \gtrsim 0.5\). We also investigate the possibility of detecting HII regions around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock et al. (Nature 474, 7353 (2011)). We find that a 5σ detection is possible with 600 h of SKA1-low observations if the QSO age and the outside x_HI are at least ～2×10⁷ Myr and ～0.2 respectively. Finally, we investigate the possibility of detecting the very first X-ray and Ly- α sources during the cosmic dawn. We consider mini-QSOs like sources which emits in X-ray frequency band. We find that with a total ～ 1000 h of observations, SKA1-low should be able to detect those sources individually with a ～ 9σ significance at redshift z=15. We summarize how the SNR changes with various parameters related to the source properties.  相似文献   

From Nearby Low Luminosity AGN to High Redshift Radio Galaxies: Science Interests with Square Kilometre Array     

P. Kharb  D. V. Lal  V. Singh  J. Bagchi  C. H. Ishwara Chandra  A. Hota  C. Konar  Y. Wadadekar  P. Shastri  M. Das  K. Baliyan  B. B. Nath  M. Pandey-Pommier 《Journal of Astrophysics and Astronomy》2016,37(4):34

We present detailed science cases that a large fraction of the Indian AGN community is interested in pursuing with the upcoming Square Kilometre Array (SKA). These interests range from understanding low luminosity active galactic nuclei in the nearby Universe to powerful radio galaxies at high redshifts. Important unresolved science questions in AGN physics are discussed. Ongoing low-frequency surveys with the SKA pathfinder telescope GMRT, are highlighted.  相似文献   

Investigations of space debris at Terskol observatory     

N. S. Bakhtigaraev  P. A. Levkina  L. V. Rykhlova  A. V. Sergeev  V. K. Taradiy  N. V. Karpov  V. V. Chazov 《Kinematics and Physics of Celestial Bodies》2016,32(5):237-240

Within the framework of the “Astronomy in the Elbrus Region” international program, investigations of space debris objects using optical measurements are carried out at the observation complex of the Zeiss-2000 telescope (developed by the International Center for Astronomical and Medico-Ecological Research, National Academy of Sciences, Ukraine) at Terskol Observatory. The objectives of this study are to detect, catalog, and determine the physical characteristics of small-sized space debris fragments, maintain orbit catalogs, and investigate the influence of nongravitational perturbations on the orbital motion and the evolution of orbits of these fragments.  相似文献   

Indices of growth of danger for space activities from orbital debris and the related mitigation measures     

S. S. Veniaminov  I. I. Oleynikov  E. K. Melnikov 《Kinematics and Physics of Celestial Bodies》2016,32(5):227-232

Intensive development of near-earth space, often followed by breakups of spacecraft, has led to its significant and progressive contamination. Investigation of this problem seems to be of current importance. Collision of space objects (SO), whose relative velocities reaching up to 17.5 km/s, implies catastrophic consequences, especially for active SOs. Particular trouble is caused concerning piloted spacecraft, in particular for the International Space Station (ISS). In this paper, taking ISS as an example, the dangerous progressive growth of the contamination of near space is shown.  相似文献