A note on the magnetic perfect fluid Bianchi type-VIo solutions

We make some comments concerning the magnetic perfect fluid Bianchi type-VI_o solutions given recently by Royet al. (1985a).     

A note on MHD instability of a compressible gravitational streaming fluid cylinder

The fundamental equations are formulated using cylindrical polar coordinates and then solved in the unperturbed state. The perturbation equations are determined, simplified, integrated and the constants of integrations are identified by applying appropriate boundary conditions across the perturbed fluid interface. A cumbersome stability criterion for MHD inviscid compressible self-gravitating streaming fluid cylinder is derived. The magnetic field is stabilizing, the streaming is destabilizing while both of the self-gravitating and compressibility are stabilizing or not according to restrictions and that the gravitational instability of sufficiently long waves will persist. Several approximations are required to obtain Chandrasekhar's and Fermi's dispersion relation (Chandrasekhar and Fermi, 1953).     

Cosmological dynamics of EDGP model with a tachyon field on the brane

We consider the cosmological dynamics of a tachyon field localized on the extended DGP braneworld scenario. We present a detailed analysis of the critical points in the phase space of the model, their stability and late-time cosmological viability of the solutions. We study the luminosity distance behavior of this ?EDGP model and compare it with ΛCDM model. Also we show that the EDGP solutions in the presence of tachyon field can explain late time acceleration of the universe.     

A note on the averaging method

When time-averaged equations are used to discuss the secular behavior of dynamical systems, the action-angle variables conjugate to the action variables of the unperturbed motion of the system should be chosen as dependent variables; otherwise, the results are not correct.Presented at the Division of Dynamical Astronomy Meeting of the American Astronoming Society, Chapel Hill, N.C., U.S.A., December 4, 1976.     

A note on the linear stellar model

For a purely gaseous self-gravitating stellar configuration with linear matter density distribution the total power generated by nuclear reactions is considered. The analytic connection between physical parameters of the macroscopic and microscopic levels of the stellar equilibrium configuration is revealed.     

A note on the Hori-Lie perturbation technique

In this paper we describe how to avoid the introduction of Hori's pseudo time in general planetary theory in the case of two planets.     

A note on the BBSAG visual observations

This Letter contains some comments on the communication by Diethelm and Locher (1988) concerning the period variations in the eclipsing binary systems EE Aqr, BZ Eri, IZ Per, and VZ Hya. Significance of the BBSAG visual observations has been discussed.     

A note on tidal deceleration of the moon

The discrepancy between the observed apparent acceleration of the Moon in longitude (1) and the actual lunar laser ranging data (3), (4) is of the order of ~ 9 × 10^–23 rad s^–2. It cannot be explained by the rms errors in (1) and (3), (4); processes connected with the internal Earth's dynamics and accelerating the Earth in its rotation might be responsible for the phenomenon, leading to the decreasing of the principal moment of the Earth's inertia ~ – 3.2 × 10²⁹m² kg cy^–1.     

A note on the algorithm of symplectic integrators

For a Hamiltonian that can be separated into N+1(N\geq 2) integrable parts, four algorithms can be built for a symplectic integrator. This research compares these algorithms for the first and second order integrators. We found that they have similar local truncation errors represented by error Hamiltonian but rather different numerical stability. When the computation of the main part of the Hamiltonian, H ₀, is not expensive, we recommend to use S ^* type algorithm, which cuts the calculation of the H ₀ system into several small time steps as Malhotra(1991) did. As to the order of the N+1 parts in one step calculation, we found that from the large to small would get a slower error accumulation. This revised version was published online in July 2006 with corrections to the Cover Date.     

A note on the gravitational field of Phobos

Phobos' proximity to the Roche limit, and some of its consequences deserving attention by the Phobos' mission, are the subject of this note.     

A note on the physical parameters of the Moon

Using the Stokes coefficients of the Moon recommended by IERS Standards (1992), we determined the expression of lunar equipotential surface (selenoid), then calculated the parameters of best-fitting lunar ellipsoid. Moreover, we derived a set of hydrostatic values of lunar physical parameters by solving the Clairaut equation, and discussed the feature of nonhydrostatic component in the figure parameters of the Moon. Finally, we suggest that the lunar physical parameters should be divided into three kinds: primary constants, derived constants, and estimated constants (see Table III); as well that the second-degree Love number,k ₂, of the Moon should belong to the estimated constants. At present, in view of the accuracy in reductions of LLR data, a value ofk ₂ obtained from model calculations should be defined as its adopted value. For example, the value ofk ₂, 0.0266, in this paper can replace 0.0222 in IERS Standards (1992).     

A note on the cosmogony of the asteroidal belt

Several arguments based on the orbital dynamics of the asteroids are used to support the idea that the original solar nebula might not necessarily be the Laplacian type of thin disk-like structure. The orbital distribution of the large asteroids is suggested to be a direct result of their accretions of condensed grains in eccentric orbits via the jet stream process.     

A note on variable-G cosmologies

An example of a cosmological model with variable gravitational couplingG and a time-dependent cosmological term , has recently been presented. It has been shown that there is no creation of matter and that the rest mass of particles stays constant in this model. In this paper we will generalize the field equations to the case where bothG and depend both on time and position. It is shown that even in this case there may be no creation.     

A note on antimatter meteors

It is argued here that unless antimatter meteors can be shown to possess some unambiguously unique characteristic not displayed by ordinary koinomatter meteors, it will be difficult to infer their existence given the standard interpretation of meteoroid structure. It is also argued, however, that the existence of antimatter meteors is extremely unlikely.     

A study of the kinematics of the local dark clouds

Lack of reliable estimates of distances to most of the local dark clouds has, so far, prevented a quantitative study of their kinematics. Using a statistical approach, we have been able to extract the average spatial distribution as well as the kinematical behaviour of the local dark clouds from their measured radial velocities. For this purpose, we have obtained radial velocities for 115 southern clouds and used the data from the literature for the northern ones. In this paper we present this new data, analyse the combined data and compare our results with those arrived at by earlier studies. The local clouds are found to be expanding at a speed of ∼ 4 kms^-1 which is in general agreement with the estimates from optical and HI studies. However, it is found that the kinematics of the local clouds is not described by the model proposed for the local HI gas where a ring of gas expanding from a point gets sheared by the galactic rotation. Rather, the observed distribution of their radial velocities is best understood in terms of a model in which the local clouds are participating in circular rotation appropriate to their present positions with a small expansion also superimposed. This possibly implies that cloud-cloud collisions are important. The spatial distribution of clouds derived using such a model is in good agreement with the local dust distribution obtained from measurements of reddening and extinction towards nearby stars. In particular, a region of size ∼ 350 pc in diameter enclosing the Sun is found to be devoid of clouds. Intriguingly, most clouds in the longitude range 100‡ to 145‡ appear to have negative radial velocities implying that they are approaching us. Carried out under the auspices of the Joint Astronomy Program, Department of Physics, Indian Institute of Science, Bangalore in partial fulfillment of the requirements for the Degree of Doctor of Philosophy.     

A note on the Delta Aurigid meteor stream

Seven doubly photographed meteors reported in the literature are shown to be members of the Delta Aurigids, which now appear to extend from September 29 to October 18. Visual observations from the second half of activity in 1980 reveal a zenith hourly rate of no more than 2 or 3. The mean orbital elements of the stream point to an unknown short period (115 years) retrogade comet as a parent.     

A note on the elliptic restricted three-body problem

This work considers periodic solutions, arc-solutions (solutions with consecutive collisions) and double collision orbits of the plane elliptic restricted problem of three bodies for =0 when the eccentricity of the primaries,e _p , varies from 0 to 1. Characteristic curves of these three kinds of solutions are given.     

A note on the energy loss from pulsars

We respond to comments made in a recent letter by Karimet al. (1984), and show that the examples of interstellar absorption at2800 Å that they have presented so far can all be attributed to overexposure of the IUE detectors.     

A note on the Inclination Functions of Satellite Theory

Recurrence relations for the Inclination Functions appearing in Satellite Theory and their derivatives are developed.     

A note on the statistics of early-type close binaries

We define adegree of contact index,I _c, and give its frequency diagram for 16 early-type close binaries. The eventual objective is to see whether the numbers of such systems are continuous or discontinuous across marginal contact. Data for four of the systems-TU Mus, V499 Sco, Pic, and SZ Cam-are from new photometric analyses, whose results are also given. Present results suggest that there is continuity in frequency across marginal contact, although one would like to include more binaries as further observations become available. An interesting point is that the only two systems in the bin of largestI _c are known to be in the rapid phase of mass transfer.