Hoag’s object,remnant of a vanished bar?

The beautiful ringed Hoag’s object, named after its discoverer, is an interesting galaxy. Because of the roundness of its ring-like structure, it has been proposed to be a collisional ring galaxy; however, there is no obvious nearby culprit galaxy that could have collided with it. Considering an alternative, much gentler hypothesis, we study the development of the observed structure via a turning, bar perturbation in the disk potential. However, there is currently no obvious bar present, and rings produced by bars are typically oval. On the basis of much recent work improving our understanding of bar evolution, we assume the bar grows and then vanishes. In simulations of a disk of particles, under such a bar turning in the disk plane, we obtain a bulge core, empty void, and circular ring in the disk that mimic the observations of Hoag’s object. We conclude the inner edge of the ring is just beyond the outer Lindblad resonance (OLR) with the bar pattern speed. We estimate the amount of gas mass in the bulge core to be twice that of the ring. Our simulations indicate that the Hoag Object ring could survive at least 6 billion years after the bar vanishes.     

Stability of the triangular Lagrange points beyond Gascheau��s value

We examine the stability of the triangular Lagrange points L ₄ and L ₅ for secondary masses larger than the Gascheau??s value ${\mu_{\rm G}= (1-\sqrt{23/27}/2)= 0.0385208\ldots}$ (also known as the Routh value) in the restricted, planar circular three-body problem. Above that limit the triangular Lagrange points are linearly unstable. Here we show that between??? _G and ${\mu \approx 0.039}$ , the L ₄ and L ₅ points are globally stable in the sense that a particle released at those points at zero velocity (in the corotating frame) remains in the vicinity of those points for an indefinite time. We also show that there exists a family of stable periodic orbits surrounding L ₄ or L ₅ for ${\mu \ge \mu_G}$ . We show that??? _G is actually the first value of a series ${\mu_0 (=\mu_G), \mu_1,\ldots, \mu_i,\ldots}$ corresponding to successive period doublings of the orbits, which exhibit ${1, 2, \ldots, 2^i,\ldots}$ cycles around L ₄ or L ₅. Those orbits follow a Feigenbaum cascade leading to disappearance into chaos at a value ${\mu_\infty = 0.0463004\ldots}$ which generalizes Gascheau??s work.     

Active galactic nuclei: what’s in a name?

Active galactic nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different “flavours” in the literature that now comprise a complex and confusing AGN “zoo”. It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their “big picture” through observations in each electromagnetic band from radio to \(\gamma \)-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude, we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.     

Towards a new maximum of OJ 287?

The results of a photometric campaign in theB, V, R andI bands of the BL Lac object OJ 287 are presented. The source is found at the same level and spectral slope as in 1991, without any sign forecasting the predicted maximum in August 1994.     

Rotation of a rigid satellite with a fluid component: a new light onto Titan’s obliquity

We revisit the rotation dynamics of a rigid satellite with either a liquid core or a global subsurface ocean. In both problems, the flow of the fluid component is assumed inviscid. The study of a hollow satellite with a liquid core is based on the Poincaré–Hough model which provides exact equations of motion. We introduce an approximation when the ellipticity of the cavity is low. This simplification allows to model both types of satellite in the same manner. The analysis of their rotation is done in a non-canonical Hamiltonian formalism closely related to Poincaré’s “forme nouvelle des équations de la mécanique”. In the case of a satellite with a global ocean, we obtain a seven-degree-of-freedom system. Six of them account for the motion of the two rigid components, and the last one is associated with the fluid layer. We apply our model to Titan for which the origin of the obliquity is still a debated question. We show that the observed value is compatible with Titan slightly departing from the hydrostatic equilibrium and being in a Cassini equilibrium state.     

Did the moon form as a result of a thermonuclear explosion?

The proposed model explains the Moon formation as a result of a thermo-nuclear explosion due to which a big land mass was torn off from the Earth. Within the model framework, on the one hand, the data on the Moon’s physical and chemical parameters are in good agreement. On the other hand, this model corresponds to modern ideas about the dynamism of the Earth’s geological structure which presupposes the presence of a powerful energy source in the Earth’s core, which might have thermonuclear origin.     

Solution of a Riemann–Hilbert problem for a new expression of Chandrasekhar’s H-function in radiative transfer

The linear singular integral equation derived from the nonlinear integral equation of Chandrasekhar’s H-function in radiative transfer is considered here to develop a new form of H-function as a solution of a Riemann–Hilbert problem using Plemelj and Cauchy integral formulae for complex domain. This new form of H-function is a simple integral of known functions. Forms of H-function both for conservative and nonconservative cases are obtained. Their numerical evaluations are made by Simpson’s one-third rule to arrive at an accuracy to ninth places of decimals.     

Strong lensing of submillimetre galaxies: a tracer of foreground structure?

The steep source counts and negative K -corrections of bright submillimetre galaxies (SMGs) suggest that a significant fraction of those observed at high flux densities may be gravitationally lensed, and that the lensing objects may often lie at redshifts above 1, where clusters of galaxies are difficult to detect through other means. In this case, follow-up of bright SMGs may be used to identify dense structures along the line-of-sight. Here, we investigate the probability for SMGs to experience strong lensing, using the latest N -body simulations and observed source flux and redshift distributions. We find that almost all high-redshift sources with a flux density above 100 mJy will be lensed if they are not relatively local galaxies. We also give estimates of the fraction of sources experiencing strong lensing as a function of observed flux density. This has implications for planning follow-up observations for bright SMGs discovered in future surveys with the Submillimetre Common-User Bolometer Array 2 and other instruments. The largest uncertainty in these calculations is the maximum allowed lensing amplification, which is dominated by the presently unknown spatial extent of SMGs.     

An attempt at a revival of Nordström’s first theory of gravitation

This reading makes the attempt to revive Nordström (1912)’s relativistic theory of gravitation which was rejected on the basis of it contravening observational reality in that it [theory] failed to account for the periastron precession of the planet Mercury and as-well as the gravitational bending of starlight barely grazing the limb of a massive gravitational object such as the Sun. By way of referencing to earlier works (Nyambuya, 2015a; 2015c; Nyambuya and Simango, 2014; Nyambuya, 2010) and putting these works into perspective with the present endaevours, it is argued that Nordström (1912)’s forgotten theory is — in-principle — able to explain these dire shortcomings. With this having been done, the stage is then set for one to re-consider this long forgotten theory. As already demonstrated (in the readings Nyambuya, 2018a; 2018b; 2018c; 2018d; Nyambuya et al., 2018), there is not only merit in doing so — but great hope that, this reconsideration will be able to bear good fruit by way of explaining a number of inexplicable gravitational anomalies and puzzles.     

Explosion of a comet nucleus fragment in the earth’s atmosphere

This paper analyzes data on thermal explosions of large meteoroids in the earth’s atmosphere. The cumulative function of flux of space bodies is corrected with regard to the explosion height, which is determined, according to our approach, by maximum braking. As a result, the integral function of flux in the work [Brown, P., Spalding, R.E., ReVelle, D.O., et al., The Flux of Small Near-Earth Objects Colliding with the Earth, Nature, 2002, vol. 420, pp. 314–316] is consistent with the one we derived earlier. It is found that at least one phenomenon of those discussed in the paper by Brown et al. is a result of explosion of a comet nucleus fragment. It is shown that the Tunguska phenomenon cannot be explained within a monolithic body model.     

Secular dynamics of a lunar orbiter: a global exploration using Prony’s frequency analysis

We study the secular dynamics of lunar orbiters, in the framework of high-degree gravity models. To achieve a global view of the dynamics, we apply a frequency analysis (FA) technique which is based on Prony’s method. This allows for an extensive exploration of the eccentricity ( $e$ )—inclination ( $i$ ) space, based on short-term integrations ( $\sim $ 8 months) over relatively high-resolution grids of initial conditions. Different gravity models are considered: 3rd, 7th and 10th degree in the spherical harmonics expansion, with the main perturbations from the Earth being added. Since the dynamics is mostly regular, each orbit is characterised by a few parameters, whose values are given by the spectral decomposition of the orbital elements time series. The resulting frequency and amplitude maps in ( $e_0,i_0$ ) are used to identify the dominant perturbations and deduce the “minimum complexity” model necessary to capture the essential features of the long-term dynamics. We find that the 7th degree zonal harmonic ( $J_7$ term) is of profound importance at low altitudes as, depending on the initial secular phases, it can lead to collision with the Moon’s surface within a few months. The 3rd-degree non-axisymmetric terms are enough to describe the deviations from the 1 degree-of-freedom zonal problem; their main effect is to modify the equilibrium value of the argument of periselenium, $\omega $ , with respect to the “frozen” solution ( $\omega =\pm 90^{\circ }, \forall \Omega $ , where $\Omega $ is the nodal longitude). Finally, we show that using FA on a fine grid of initial conditions, set around a suitably chosen ‘first guess’, one can compute an accurate approximation of the initial conditions of a periodic orbit.     

A family of well behaved charge analogues of?a?well behaved neutral solution in?general relativity

A family of charge analogues of a neutral solution with g ₄₄=(1+Cr ²)⁶ has been obtained by using a specific electric intensity, which involves a parameter K. Both neutral and charged solutions are analysed physically subject to the surface density 2×10¹⁴ gm/cm³ (neutron star). The neutral solution is well behaved for 0.0<Ca ²≤0.10477 while its charge analogues are well behaved for a wide range of a parameter K (0≤K≤72) i.e. pressure, density, pressure-density ratio, velocity of sound is monotonically decreasing and the electric intensity is monotonically increasing in nature for the given range of the parameter K. The maximum mass and radius occupied by the neutral solution are 3.4126M _Θ and 18.9227 km for Ca ²=0.10447 respectively. While the red shift at centre Z ₀=0.9686 and red shift at the surface Z _a =0.4612. For the charged solution, the maximum mass and radius are 5.6111M _Θ and 17.2992 km respectively for K=3.0130 and Ca ²=0.2500, with the red shift Z ₀=3.0113 and Z _a =1.0538.     

Radiation Effect on a Particle’s Periodic Orbits in a Regular Polygon Configuration of N Bodies

The photo-gravitational problems of two or more bodies have attracted much attention during the last decades. In this paper, radiation is considered as an additional factor influencing the particle motion in a regular polygon formation of N big bodies where the ν = Ν ? 1 primary bodies have equal masses and are located at the vertices of a regular polygon and the Nth primary has different mass and is located at the mass center of the system. We assume that some or all the primary bodies are radiation sources and we numerically explore various cases where symmetry of the resultant force field with respect to the same axis is preserved. For the purposes of our investigation we adopt Radzievski’s theory and assumptions. The material gathered helps us to estimate the radiation effect on the evolution of periodic orbits and their characteristics, such as their periods and their stability. Figures and diagrams illustrate these alterations and document our conclusions.     

Solution algorithm of a quasi-Lambert’s problem with fixed flight-direction angle constraint

A two-point boundary value problem of the Kepler orbit similar to Lambert’s problem is proposed. The problem is to find a Kepler orbit that will travel through the initial and final points in a specified flight time given the radial distances of the two points and the flight-direction angle at the initial point. The Kepler orbits that meet the geometric constraints are parameterized via the universal variable z introduced by Bate. The formula for flight time of the orbits is derived. The admissible interval of the universal variable and the variation pattern of the flight time are explored intensively. A numerical iteration algorithm based on the analytical results is presented to solve the problem. A large number of randomly generated examples are used to test the reliability and efficiency of the algorithm.     

Is Cycle 24 the Beginning of a Dalton-Like Minimum?

The unexpected development of cycle 24 emphasizes the need for a better way to model future solar activity. In this article, we analyze the accumulation of spotless days during individual cycles from 1798 – 2010. The analysis shows that spotless days do not disappear abruptly in the transition toward an active Sun. A comparison with past cycles indicates that the ongoing accumulation of spotless days is comparable to that of cycle 5 near the Dalton minimum and to that of cycles 12, 14, and 15. It also suggests that the ongoing cycle has as much as 20±8 spotless days left, from July 2010, before it reaches the next solar maximum. The last spotless day is predicted to be in December 2012, with an uncertainty of 11 months. This trend may serve as input to the solar dynamo theories.     

Optical afterglows of gamma-ray bursts: a bimodal distribution?

The luminosities of the optical afterglows of gamma-ray bursts, 12 h (rest-frame time) after the trigger, show a surprising clustering, with a minority of events being at a significantly smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre- Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic burst is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflect a real bimodality: either the optical luminosity distribution of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength-independent extinction. This population of intrinsically weak or grey-absorbed events can be associated with dark bursts.     

The equatorial expansion of be stars a thin-line wind?

Numerical solutions for the expansion motion equation in the equatiorial plane of a rotating early type Be star are presented. Decreasing terminal velocities are obtained as lower values of the radiative parameter are used.