Magnetic field generation by pointwise zero-helicity three-dimensional steady flow of incompressible electrically conducting fluid

We introduce six families of three-dimensional space-periodic steady solenoidal flows, whose kinetic helicity density is zero at any point. Four families are analytically defined. Flows in four families have zero helicity spectrum. Sample flows from five families are used to demonstrate numerically that neither zero kinetic helicity density, nor zero helicity spectrum prohibit generation of large-scale magnetic field by the two most prominent dynamo mechanisms: the magnetic $\alpha$-effect and negative eddy diffusivity. Our computations also attest that such flows often generate small-scale field for sufficiently small magnetic molecular diffusivity. These findings indicate that kinetic helicity and helicity spectrum are not the quantities controlling the dynamo properties of a flow regardless of whether scale separation is present or not.Comment: 37 pages, 11 figures, 54 reference

Evolution of Massive Black Hole Binaries in Rotating Stellar Nuclei and its Implications for Gravitational Wave Detection

According to the currently prevailing cosmological paradigm, mergers between galaxies are an important part of their evolution. Assuming also that most galaxies contain a supermassive black hole at their center, binary supermassive black holes (BSBH) should be common products of galactic mergers. The subject of this dissertation is the dynamical evolution of a BSBH at the center of a galaxy. I calculate the rate of change of a binary\u27s orbital elements due to interactions with the stars of the galaxy by means of 3-body scattering experiments. My model includes a new degree of freedom - the orientation of the BSBH\u27s orbital plane - which is allowed to change due to interaction with the stars in a rotating nucleus. The binary\u27s eccentricity also evolves in an orientation-dependent manner. I find that the dynamics are qualitatively different compared to non-rotating nuclei: 1) The orbital orientation of a BSBH changes towards alignment with the plane of rotation of the nucleus. 2) The orbital eccentricity of a BSBH decreases for aligned BSBHs and increases for counter-aligned ones. I then apply my model to calculate the effects of stellar environment on the gravitational wave background spectrum produced by BSBHs. Using the results of N-body/Monte-Carlo simulations, I account for the different rate of stellar interactions in spherical, axisymmetric and triaxial galaxies. I also consider the possibility that supermassive black hole masses are systematically lower than usually assumed. The net result of the new physical mechanisms included in my model is a spectrum for the stochastic gravitational wave background that has a significantly lower amplitude than in previous treatments, which could explain the discrepancy that currently exists between the models and the upper limits set by pulsar timing array observations

Binary intermediate-mass black hole mergers in globular clusters

We consider the formation of binary intermediate black holes (BIMBH) in globular clusters (GC), which could happen either in situ or due to the mergers between clusters. We simulate the evolution of the BIMBH orbit (and its subsequent merger) due to stellar ejections. We also take into account the evaporation of GCs due to the tidal field of the host galaxy and two-body relaxation. Our results show that if at least $10^{-3}$ of all GCs become BIMBH hosts and the BIMBH masses are $\sim1\%$ of the GC mass, at least one of the inspiralling (or merging) BIMBHs will be detected by LISA during its 4-year mission lifetime. Most of the detected BIMBHs come 1) from heavy GCs ($\gtrsim3\times10^5M_\odot$), as lower-mass GCs end up being disrupted before their BIMBHs have time to merge, and 2) from redshifts $1<z<3$, assuming that most of GCs form around $z\sim4$ and given that the merger timescale for most BIMBHs is $\sim1$ Gyr. If the BIMBH to GC mass ratio is lower ($\sim10^{-3}$) but the fraction of BIMBH hosts among GCs is higher ($\gtrsim10^{-2}$), some of their mergers will also be detected by LIGO, VIRGO, and KAGRA and the proposed Einstein Telescope.Comment: 14 pages, 14 figures, 1 tabl

Orbital evolution of LIGO/Virgo binaries in stellar clusters driven by cluster tides, stellar encounters and general relativity

Origin of LIGO/Virgo gravitational wave events may involve production of binaries with relativistic components in dense stellar systems - globular or nuclear star clusters - and their subsequent evolution towards merger. Orbital parameters of these binaries (the inner orbit) and their motion inside the cluster (the outer orbit) evolve due to both external agents - random encounters with cluster stars and cluster tides due to the smooth cluster potential - and the internal ones - various sources of dissipation and precession within the binary. We present a numerical framework - Binary Evolution in Stellar Clusters (BESC) - that follows the evolution of the binary inner and outer orbits accounting for all these effects simultaneously, enabling efficient Monte Carlo studies. The secular effect of cluster tides is computed in the singly-averaged approximation, without averaging over the outer binary orbit. As to stellar encounters, we include the effects of both close and distant flybys on the inner and outer orbits of the binary, respectively. In particular, this allows us to explicitly account for the dynamical friction sinking the binary towards the cluster centre. Also, given our focus on the LIGO/Virgo sources, we include the general relativistic precession (which suppresses cluster tides at high eccentricities) and the gravitational wave emission (shrinking the binary orbit). We use BESC to illustrate a number of characteristic binary evolutionary outcomes and discuss relative contributions of different physical processes. BESC can also be used to study other objects in clusters, e.g. blue stragglers, hot Jupiters, X-ray binaries, etc.Comment: 20 pages, 16 figures, submitted to MNRA

КНИГА Ж.Р. ФУЛДЖЕР: ОТ РАСПЛАВНЫХ АНОМАЛИЙ К ПЛИТНЫМ ИЛИ ПЛЮМОВЫМ ГИПОТЕЗАМ?

The book «Plates vs. plumes: a geological controversy» (Fig. 1) is intended for the advanced student who is not satisfied by the presentday interpretation of intraplate volcanism. From systematic descriptions of the geological controversy on the plate and plume hypotheses, it follows that, unlike predictions of the former, those of the latter have not been confirmed by observations. Recent intraplate volcanism is explained adequately by models of lithospheric extension and local convection in the upper mantle.Книга «Плиты против плюмов: геологическая полемика» (рис. 1) предназначена для продвинутого студента, неудовлетворенного современной интерпретацией внутриплитного вулканизма. Из систематического изложения геологической полемики по плитной и плюмовой гипотезам следует, что, в отличие от предсказаний первой, предсказания второй не подтверждены наблюдениями. Новейший внутриплитный вулканизм объясняется более правдоподобно моделями растяжения литосферы и локальной конвекции в верхней мантии