• Volume 35, Issue 4

December 2014,   pages  595-773

• Measuring Velocity and Acceleration Using Doppler Shift of a Source with an Example of Jet in SS433

We describe here as to how the Doppler shift of a source needs to be used to measure its velocity and acceleration. We also apply this method, as an example here, to spectral lines of the blue-shifted jet in micro-quasar SS433 and discuss the intricacies of these measurements.

• Lunar-Forming Giant Impact Model Utilizing Modern Graphics Processing Units

Recent giant impact models focus on producing a circumplanetary disk of the proper composition around the Earth and defer to earlier works for the accretion of this disk into the Moon. The discontinuity between creating the circumplanetary disk and accretion of the Moon is unnatural and lacks simplicity. In addition, current giant impact theories are being questioned due to their inability to find conditions that will produce a system with both the proper angular momentum and a resultant Moon that is isotopically similar to the Earth. Here we return to first principles and produce a continuous model that can be used to rapidly search the vast impact parameter space to identify plausible initial conditions. This is accomplished by focusing on the three major components of planetary collisions: constant gravitational attraction, short range repulsion and energy transfer. The structure of this model makes it easily parallelizable and well-suited to harness the power of modern Graphics Processing Units (GPUs). The model makes clear the physically relevant processes, and allows a physical picture to naturally develop. We conclude by demonstrating how the model readily produces stable Earth–Moon systems from a single, continuous simulation. The resultant systems possess many desired characteristics such as an iron-deficient, heterogeneously-mixed Moon and accurate axial tilt of the Earth.

• On Asymmetries in Powerful Radio Sources and the Quasar/Galaxy Unification

We utilize the distributions of fractional separation difference (𝑥) as asymmetry parameter, linear size (𝐷) and core-to lobe luminosity ratio (𝑅) as orientation indicators, to investigate a consequence of radio source orientation and relativistic beaming effects in a sample of powerful non-symmetric extragalactic radio sources. In this scenario, radio sources viewed at small orientation angles to the line-of-sight are expected to show a high degree of asymmetry in observed radio structures due to relativistic beaming, with foreshortened projected linear sizes. A simple consequence of this is the 𝑥 - 𝐷 anti-correlation. Results show a tight correlation (𝑟 &gt; 0.8) between the total and core radio luminosities and a clear 𝑥 - 𝐷 anti-correlation (𝑟 ∼ -0.5). The observed 𝑥 - 𝐷 anti-correlation is consistent with average orientation angle 𝜙 ≈ 48° and a maximum Lorentz factor 𝛾 ∼ 2 for the sample, with minimum angular separation of 26° between radio galaxies and quasars. However, there is no clear 𝑥 - 𝑅 correlation. While the results are consistent with quasar/galaxy unification via orientation, intrinsic asymmetry also seems to play a major role.

• Extension of Cherenkov Light LDF Parametrization for Tunka and Yakutsk EAS Arrays

The Cherenkov light Lateral Distribution Function (LDF) from particles initiated Extensive Air Showers (EAS) with ultrahigh energies (𝐸 &gt; 1016 eV) was simulated using CORSIKA program for configuration of Tunka and Yakutsk EAS arrays for different primary particles (p, Fe and O2) and different zenith angles. By depending on the Breit–Wigner function, a parametrization of the Cherenkov light LDF was reconstructed on the basis of this simulation as a function of the primary energy. The comparison of the approximated Cherenkov light LDF with that measured on Tunka and Yakutsk EAS arrays gives the possibility of identification of energy spectrum and mass composition of particles initiating EAS about the knee region of the cosmic ray spectrum. The extrapolation of approximated Cherenkov light LDF for energies 20, 30 and 50 PeV was obtained for different primary particles and different zenith angles.

• Benford’s Law in Astronomy

Benford’s law predicts the occurrence of the 𝑛-th digit of numbers in datasets originating from various sources all over the world, ranging from financial data to atomic spectra. It is intriguing that although many features of Benford’s law have been proven, it is still not fully understood mathematically. In this paper we investigate the distances of galaxies and stars by comparing the first, second and third significant digit probabilities with Benford’s predictions. It is found that the distances of galaxies follow the first digit law reasonable well, and that the star distances agree very well with the first, second and third significant digit.

• Interplay between Dark Matter and Galactic Structure in Disk and Oblate Elliptical Galaxies

Understanding the regular or chaotic nature of orbits in galaxies is undoubtedly an issue of great importance. We determine the character of orbits of stars moving in the meridional plane (𝑅, 𝑧) of an axially symmetric time-independent galactic model with a spherical central nucleus, and a flat biaxial oblate dark matter halo component. In particular, we try to reveal the influence of the fractional portion of dark matter on the structure and also on the different families of orbits of the galaxy, by monitoring how the percentage of chaotic orbits, as well as the percentages of orbits of the main regular resonant families evolve when the ratio of dark matter to luminous mass varies. The smaller alignment index (SALI) was computed by numerically integrating the equations of motion as well as the variational equations to extensive samples of orbits in order to distinguish between ordered and chaotic motion. In addition, a method based on the concept of spectral dynamics that utilizes the Fourier transform of the time series of each coordinate is used to identify the various families of regular orbits and also to recognize the secondary resonances that bifurcate from them. The investigation is carried out both in the physical (𝑅, 𝑧) and the phase (𝑅, $\dot{R}$) space for a better understanding of the orbital properties of the system. The numerical computations reveal that in both cases, the fractional portion of dark matter influences more or less, the overall orbital structure of the system. It was observed however, that the evolution of the percentages of all types of orbits as a function of the fractional portion of dark matter strongly depends on the particular type of space (physical or phase) in which the initial conditions of orbits are launched. The results are compared with the similar earlier work.

• The Astrophysical S-factor for the 2H(𝛼, 𝛾)6Li Nuclear Reaction at Low-Energies

The alpha radiative capture reactions are the key to understand about primordial nucleosynthesis and the observed abundance of light nucleus in stars. The astrophysical S-factor for the process 2H(𝛼, 𝛾)6Li has been calculated at the low-energies relevant to big-bang nucleosynthesis and in comparison with laboratory data. On the basis of the model, the alpha radiative capture process is studied by using the two-and three-body electromagnetic currents. The bound and resonance states of 6Li are calculated via an inverse process, deuteron-𝛼 photodisintegration of a 6Li nucleus. In comparison with other theoretical approaches and available laboratory data, excellent agreement is achieved for the astrophysical S-factor of this process.

• On the Stability of 𝐿4,5 in the Relativistic R3BP with Radiating Secondary

This paper discusses the motion of a test particle in the neighbourhood of the triangular points 𝐿4,5 by considering the less massive primary (secondary) as a source of radiation in the framework of the relativistic restricted three-body problem (R3BP). It is found that the positions and stability of the triangular point are affected by both relativistic and electromagnetic radiation factors. It turns out that both the coordinates of the infinitesimal mass are affected, contrary to the classical where this happens only for one coordinate. A practical application of this model could be the study of dynamical evolution of dust particles in orbits around a binary system with a dark degenerate first primary and a secondary stellar companion.

• Effect of Perturbations in the Coriolis and Centrifugal Forces on the Stability of 𝐿4 in the Relativistic R3BP

The centrifugal and Coriolis forces do not appear as a result of physically imposed forces, but are due to a special property of a rotation. Thus, these forces are called pseudo-forces or `fictitious forces’. They are merely an artifact of the rotation of the reference frame adopted. This paper studies the motion of a test particle in the neighbourhood of the triangular point 𝐿4 in the framework of the perturbed relativistic restricted three-body problem (R3BP) when small perturbations are conferred to the centrifugal and Coriolis forces. It is found that the position and stability of the triangular point are affected by both the relativistic factor and small perturbations in the Coriolis and centrifugal forces. As an application, the Sun–Earth system is considered.

• Ultraviolet Spectroscopic Study of BY Circini and V 1425 Aquilae from IUE Satellite

We present the ultraviolet spectra of two classical novae, BY Circini 1995 and V1425 Aquilae 1995, using observations obtained with the International Ultraviolet Explorer (IUE). The outflow velocity of the ionized ejecta was calculated to be 2100–2700 km s-1 and 1600–1900 km s-1 (FWHM) for BY Cir and V1425 Aql, respectively. The models generated by Jose &amp; Hernanz (1998, ApJ, 494, 680) indicate that BY Cir is a CO4/CO5 nova with WD mass of ∼ 1.15𝑀, while V1425 Aql is a CO2/CO3 nova with WD mass of ∼ 0.9𝑀. Different profiles of BY Cir and V1425 Aql showing variations of line fluxes at different orbital phases are presented. This paper focuses on the calculated spectral line fluxes for the N III emission line at 1750 Å and the C III emission line at 1909 Å, produced in the expanding shell of ionized gas. Our results show that there are variations of line fluxes with time. We attribute these spectral variations in line fluxes to the variations of both density and temperature in the line-emitting regions as a result of the inhomogeneous ejecta. The variable line fluxes, ultraviolet luminosities and accretion rates support a model of the ejecta consisting of an optically thin dust shell and hot ionized gas.

• Effects of Triaxiality, Oblateness and Gravitational Potential from a Belt on the Linear Stability of 𝐿4,5 in the Restricted Three-Body Problem

In this paper we have considered the restricted three body problem (R3BP) in which the more massive primary is triaxial, the less massive primary and infinitesimal body are oblate spheroids, and are encompassed by a belt of homogenous material points. Analytically and numerically, we have studied the effects of triaxiality of the more massive primary, oblateness of both the less massive primary and infinitesimal body and the gravitational potential generated by the belt on the location of the triangular libration points 𝐿4,5 and their linear stability. 𝐿4,5 do not form equilateral triangles with the primaries in the presence of all or any of the aforementioned perturbations. Due to triaxiality of the more massive primary and oblateness of the infinitesimal body the triangular libration points are seen to move away from the line linking the primaries, whereas they shift closer to the line owing to the oblateness of the less massive primary and the potential from the belt. The range 0 &lt; 𝜇 &lt; 𝜇c of stability of the triangular points is reduced in the presence of any of the perturbations, except when considering the potential from the belt the range increases, where 𝜇c is the critical mass ratio. The oblateness of a test particle (of infinitesimal mass) shifts the location of its libration positions away from the primaries and reduces its range of stability.

• Acknowledgments

• Author Index

• Subject Index

• # Journal of Astrophysics and Astronomy

Current Issue
Volume 40 | Issue 2
April 2019

• # Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

© 2017-2019 Indian Academy of Sciences, Bengaluru.