Galactic dynamo with helicity fluxes

We construct an approximation for the magnetic field of galaxies that takes into account the magnetic helicity conservation law. In our calculations, we use the fact that the galactic disk is fairly thin and, therefore, the magnetic field component perpendicular to the galactic disk can be neglected (the so-called no-z approximation). However, an averaging of the magnetic field over the entire galaxy, as was done in previous works, is not performed. Our results are compared both with the approximation that disregards the helicity flux and with the results obtained in models with helicity fluxes but without averaging. We show that, compared to the classical model, there are a number of new effects (for example, magnetic field oscillations) and, compared to the model with averaging, the behavior of the magnetic field “softens”: its stationary value is reached more slowly and the oscillation amplitude decreases. This is because the dissipative processes changing the magnetic field growth rate are taken into account in our model. In contrast to the model with averaging, here it becomes possible to construct the dependence of the magnetic field and helicity on the distance from the galactic center.     

Star formation regions as galactic dissipative structures

The theory of dissipative structures, applied to star formation systems, provides a conceptual framework for the study of the behaviour and evolution of these systems. As shown by an analysis of a model star formation process system, prolonged stationary star formation in localized areas and repetitive bursting star formation events can be understood as different behavioural modes of galactic dissipative structures. Young stellar associations with theirHII regions and molecular clouds are manifestations of the ordered distribution of matter participating in the star formation processes. A self-organization with the appearance of ordered structures is, in general, to be expected in nonequilibrium systems in which nonlinear processes occur. However, lacking a thermodynamic theory that can be applied to self-gravitating systems, the behaviour of star-forming regions can only be studied by model calculations simulating the process system within the region.     

A thin disc model of the galactic dynamo

An axisymmetric αω-dynamo model of the Galactic disc is investigated. The disc is thin and its width, 2h^*, varies slowly with distance, ωT^*, from the axis of symmetry. The strength of the α^*, varies linearly with axialdistance, z^*, while differential rotation, dΩ^*/dωT^*, remains constant. Otherwise α^* and dΩ^*/dωT^* are arbitrary functions of ωT^*. The results are applied to the special case of the oblate spheroid first investigated by STIX (1976) and later by WHITE (1977). In the limit of small aspect ratio the new results agree with those obtained by WHITE and confirm that the dynamo numbers computed by STIX are too small.     

Star formation efficiency in low-density regions in galactic disks

The radial dependences of the star formation efficiency??_SFE = ??_SFR/??_gas (per unit disk surface area) in normal surface brightness spiral galaxies and low surface brightness (LSB) galaxies are compared with the radial variations of the gas and stellar disk surface and volume densities. The volume density of the components in the disk midplane is found through a self-consistent solution of the disk equilibrium equations by taking into account the dark halo. The disk thickness variation with radius R is calculated within the model of a galaxy with a marginally stable disk by taking into account the increase of the stability parameter Q _T,c along the radius. We show that the star formation efficiency depends weakly (for LSB galaxies, does not depend at all) on the gas density but correlates well with the disk surface and volume density, with the normal and LSB galaxies forming a single sequence. The dependence vanishes only at extremely low disk densities (?? _disk ? (1?3) M _?? pc^?2, ?? _stars ?? (1?3) × 10^?24 g cm^?3), where star formation probably ceases to be related to disk properties. Estimations of the gas volume density allow us to check the expected form of the ??_SFR-?? _disk relationship that follows from the model by Ostriker et al., which relates the star formation rate to the pressure of the diffuse gas medium. For most of the galaxies considered, there is satisfactory agreement with the model, except for the densest (of the order of several hundred M _?? pc^?2) and least dense (several M?? pc^?2 or less) disk regions.     

Numerical models of the galactic dynamo

Axisymmetric αω-dynamo models are investigated numerically for the galactic dynamo. Contrasting to the eigenvalue formulation of the problem by STIX (1976), an initial-value formulation is developed in a manner which is a generalization of the approach to the solar dynamo by JEPPS (1975). It is found, for STIX's model, that the critical dynamo numbers, P_c, obtained by this approach do not agree with those obtained by STIX . In order to resolve this disagreement SOWARD (1977) has evaluated an asymptotic formula for P_c which confirm the results presented here. Having established this approach, the dependence of the models upon boundary conditions and the relevant astrophysical parameters is investigated, and an attempt is made to simulate nonlinear effects. Finally a comparison is made between predictions of the dynamo models and the observed radiation of certain external galaxies which provides insight into the nature of the intergalactic medium.     

Small-scale magnetic helicity losses from a mean-field dynamo

Using mean-field models with a dynamical quenching formalism, we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity density or the explicit removal which may be associated with the effects of coronal mass ejections in the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes are found to be equally strong for both large- and small-scale fields. In the presence of shear, there is also an additional magnetic helicity flux associated with the mean field, but this flux does not alleviate catastrophic quenching. Outside the dynamo-active region, there are neither sources nor sinks of magnetic helicity, so in a steady state this flux must be constant. It is shown that unphysical behaviour emerges if the small-scale magnetic helicity flux is forced to vanish within the computational domain.     

Hierarchial galactic dynamo and seed magnetic field problem

A new approach to the galactic seed magnetic field problem is briefly discussed. It is shown that, in early stages of galactic evolution, the hierarchial agglomeration and fragmentation processes can account for the generation of a dynamically important magnetic field. The amplification of this field follows an inverse cascade since a non-zero average value of the field amplified on a smaller scale serves as a seed field on the next (earlier) hierarchial scale. In such a scenario, a problem of how to get things started never occurs as any infinitesimally small battery generated seed field (Lazarian 1992a) can be efficiently amplified passing by through a sufficient number of amplification cascades.     

Calculations of infrared fluxes from galactic sources for a polysaccharide grain model

Using transmittance data appropriate for grain material which is predominantly comprised of polysaccharides we have computed infrared fluxes from several types of galactic infrared source. The model used in these computations involves polysaccharide condensation in material flowing out from O-type stars. With the exception of rather minor discrepancies we show that it is possible to match the 2.1–13 observations of a wide range of galactic infrared sources.     

Strong mean field dynamos require supercritical helicity fluxes

Several one and two dimensional mean field models are analyzed where the effects of current helicity fluxes and boundaries are included within the framework of the dynamical quenching model. In contrast to the case with periodic boundary conditions, the final saturation energy of the mean field decreases inversely proportional to the magnetic Reynolds number. If a nondimensional scaling factor in the current helicity flux exceeds a certain critical value, the dynamo can operate even without kinetic helicity, i.e. it is based only on shear and current helicity fluxes, as first suggested by Vishniac & Cho (2001, ApJ 550, 752). Only above this threshold is the current helicity flux also able to alleviate catastrophic quenching. The fact that certain turbulence simulations have now shown apparently non‐resistively limited mean field saturation amplitudes may be suggestive of the current helicity flux having exceeded this critical value. Even below this critical value the field still reaches appreciable strength at the end of the kinematic phase, which is in qualitative agreement with dynamos in periodic domains. However, for large magnetic Reynolds numbers the field undergoes subsequent variations on a resistive time scale when, for long periods, the field can be extremely weak. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the parametric resonance in a thin disk galactic dynamo

The absence of a parametric resonance by the generation of a large scale bisymmetric magnetic field in a thin galactic disk with periodically time dependent properties in the frames of the simplest thin disk galactic dynamo model is shown.     

A galactic formation model based on post-big bang fragmentation

A new model of galactic formation is presented. A primeval distribution of angular momentum is derived, which rests on the postulated presence of mass flows at the early stage of the post-big bang universal expansion, when fragmentation occurs. The shape of any particular fragment and its orientation with respect to the mass flow predestine the morphology of the galaxy that will be produced in the subsequent collapse.A fragment-to-disk mapping transform allows examination of the galactic disk mass distributions that result from the postulates of the model. One class of distribution adequately describes the observed luminosity profiles of elliptical galaxies. Another class indicates the allocation of a large mass fraction to an extended disk, and the intrinsic two-sided symmetry of the model further indicates that the disk formation process will begin with the establishment of two major spiral arms.     

Solar dynamo model with nonlocal alpha-effect

The first results of the solar dynamo model that allows for the diamagnetic effect of inhomogeneous turbulence and the nonlocal alpha-effect due to the rise ofmagnetic loops are discussed. The nonlocal alpha-effect is not subject to the catastrophic quenching related to the conservation of magnetic helicity. Given the diamagnetic pumping, the magnetic fields are concentrated near the base of the convection zone, although the distributed-typemodel covers the entire thickness of the convection zone. Themagnetic cycle period, the equatorial symmetry of the field, its meridional drift, and the polar-to-toroidal field ratio obtained in the model are in agreement with observations. There is also some disagreement with observations pointing the ways of improving the model.     

Star formation

Various topics on star formation, centered on the observed properties of young stars and their environment, are reviewed. (a) In our Galaxy, young stellar objects are generally associated with giant molecular clouds. (b) Giant molecular clouds cannot be in free-fall collapse. They are probably stabilized by magnetic fields, which are then likely to dominate the dynamical evolution of the clouds themselves. (c) Star formation occurs mostly in spiral arms. The role of spiral density waves is however not yet clearly understood. (d) The formation of massive stars can perturb the evolution of the progenitor cloud, and possibly trigger the sequential formation of OB subgroups. (e) There is a large number of clouds in the Galaxy associated only with low and intermediate mass young stars. These clouds are not perturbed by the presence of massive stars, and are probably the best source of information on the primary triggering mechanism, active on a galactic scale, and on the initial conditions for star formation.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Solar activity forecast with a dynamo model

Although systematic measurements of the Sun's polar magnetic field exist only from mid-1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock–Leighton mechanism of poloidal field generation from decaying sunspots involves randomness, whereas the other aspects of the dynamo process must be reasonably ordered and deterministic. Only if the magnetic diffusivity within the convection zone is assumed to be high (of order  10¹² cm² s⁻¹  ), we can explain the correlation between the polar field at a minimum and the next cycle. We give several independent arguments that the diffusivity must be of this order. In a dynamo model with diffusivity like this, the poloidal field generated at the mid-latitudes is advected toward the poles by the meridional circulation and simultaneously diffuses towards the tachocline, where the toroidal field for the next cycle is produced. To model actual solar cycles with a dynamo model having such high diffusivity, we have to feed the observational data of the poloidal field at the minimum into the theoretical model. We develop a method of doing this in a systematic way. Our model predicts that cycle 24 will be a very weak cycle. Hemispheric asymmetry of solar activity is also calculated with our model and compared with observational data.     

Subcritical dynamo and hysteresis in a Babcock-Leighton type kinematic dynamo model

In the Sun and Sun-like stars,it is believed that cycles of the large-scale magnetic field are produced due to the existence of differential rotation and helici...     

Star formation and AGN

We analyse the data from narrow band H and [O III] CCD imaging of a sample of nearby spiral galaxies with active nuclei. The results show that the large scale star formation depends on the level of nuclear activity, with Seyfert 2 presenting the strongest disk and circum-nuclear star formation, while Seyfert 1 show very little. We present particular results for the galaxies NGC 3982, NGC 4579, NGC 5427 and NGC 7469.     

A planetary dynamo model

Using the Lagrangian approach, the author considered the temporal evolution of an ensemble of interacting magnetohydrodynamic cyclones, obeying equations of the Langevin type, in a rotating medium. The problem is topical for fast-rotating convective objects: cores of planets and a number of stars, where the Rossby numbers are much less than unity and the geostrophic balance of forces is observed. In this work, results of simulation are given both for the two-dimensional case, when axes of cyclones can rotate only in the vertical plane, and for the three-dimensional case when the axes are rotating by two angles. It is shown that a change in the heat flux on the shell boundary impacts the frequency of reversals of the mean dipole magnetic field, which agrees with results of simulation in three-dimensional models of a planetary dynamo. Applications of the model for the giant planets are considered, and an explanation of certain episodes of the geomagnetic field in the past is offered.