


Volume 62, Nº 1 (2018)
- Ano: 2018
- Artigos: 8
- URL: https://journals.rcsi.science/1063-7729/issue/view/12012
Article
The Motion of Magnetic Elements in and around Sunspot Penumbrae
Resumo
Structural magnetic elements observed in sunspot penumbrae are employed as indicators of motions occurring in and around penumbrae. The analysis presented here is base on SDO/HMI continuum images and magnetograms of the line-of-sight field obtained for the active region NOAA 11117. In a first approximation, the penumbral magnetic fields can be considered alternating spines and interspine filaments. In the plane of the sky, spines are thin radial elements with higher field strengths and lower magnetic-field inclinations compared with those in surrounding areas. It is confirmed that spines first appear as protrusions of the umbra magnetic fields visible in magnetograms, and then develop simultaneously with the growth of the penumbra. The departure of magnetic elements from penumbrae as a result of the detachment of the ends of spines begin 1–1.5 h after the spine formation. Inmature penumbrae, magnetic elements emerge fairly often, and the departure of groups of field elements sometimes generates structures resembling moving ribbons. The velocities of magnetic elements that have separated from spines are a factor of two to three lower than those of elements that have separated from inter-spine filaments. The results obtained agree well with an “uncombed” model for the penumbral magnetic fields.



Determination of the Spins of Supermassive Black Holes in FR I and FR II Radio Galaxies
Resumo
The spins of supermassive black holes in FR I and FR II radio galaxies are estimated using two models for the generation of the relativisitic jets, based on the Blandford–Znajek and Blandford–Payne mechanisms: the hybrid model of Meier and a flux-trapping model. The magnetic field at the event horizon is estimated assuming equipartition between the energy densities of the magnetic field and the accreting material. The magnetic field near the inner edge of the accretion disk is estimated assuming equipartition between the magnetic pressure and the radiation pressure, and also assuming proportionality between the magnetic field and the spin. In the case of FR I objects, both mechanisms for the generation of the jets (the hybrid model of Meier and a flux-trapping model) are efficient. For the FR II objects, equipartition between the energy densities of the magnetic field and the accretion flow facilitates stronger retrograde rotation of the supermassive black hole. Plots of spin versus mass suggest a predominantly chaotic character for the accretion in both types of radio galaxies.



Hydrodynamical Aspects of the Formation of Spiral–Vortical Structures in Rotating Gaseous Disks
Resumo
This paper is dedicated to numerical simulations of spiral–vortical structures in rotating gaseous disks using a simple model based on two-dimensional, non-stationary, barotropic Euler equations with a body force. The results suggest the possibility of a purely hydrodynamical basis for the formation and evolution of such structures. New, axially symmetric, stationary solutions of these equations are derived that modify known approximate solutions. These solutions with added small perturbations are used as initial data in the non-stationary problem, whose solution demonstrates the formation of density arms with bifurcation. The associated redistribution of angular momentum is analyzed. The correctness of laboratory experiments using shallow water to describe the formation of large-scale vortical structures in thin gaseous disks is confirmed. The computations are based on a special quasi-gas-dynamical regularization of the Euler equations in polar coordinates.



The Problem of Spectral Mimicry of Supergiants
Resumo
The phenomenon of spectral mimicry refers to the fact that hypergiants and post-AGB supergiants—stars of different masses in fundamentally different stages of their evolution—have similar optical spectra, and also share certain other characteristics (unstable extended atmospheres, expanding dust–gas envelopes, high IR excesses). As a consequence, it is not always possible to distinguish post-AGB stars from hypergiants based on individual spectral observations in the optical. Examples of spectral mimicry are analyzed using uniform, high-quality spectral material obtained on the 6-m telescope of the Special Astrophysical Observatory in the course of long-term monitoring of high-luminosity stars. It is shown that unambiguously resolving the mimicry problem for individual stars requires the determination of a whole set of parameters: luminosity, wind parameters, spectral energy distribution, spectral features, velocity field in the atmosphere and circumstellar medium, behavior of the parameters with time, and the chemical composition of the atmosphere.



Analysis of Photometric Observations of the New Cataclysmic Variable ASASSN-13cx
Resumo
Photometric observations of the variable star ASASSN-13cx acquired in the course of a program of studies of cataclysmic variables and their parameters recently carried out at the Sternberg Astronomical Institute (SAI) are presented. The star was observed with the 50-cm and 60-cm telescopes of the SAI Crimean Astronomical Station and a CCD photometer (∼1800 images in the V and Rc filters) during the variable’s outburst of August–September 2014 and in a period of quiescence in October–November 2016. The ASASSN-13cx system is confirmed to be a SU UMa variable. Parameters of the system are derived from eight light curves using a “composite” model that takes into account the presence of a hot spot on the lateral surface of the geometrically thick disk and of a region of enhanced energy release near the disk edge, at the base of the gas flow (the so-called “hot line”). Parameters of the system for three light curves during the outburst were obtained in the framework of a “spiral” model that additionally takes into account the presence of geometric perturbations on the accretion-disk surface. The parameters of ASASSN-13cx determined using these models provide good accuracy in reproducing the system’s light curves in both states. The basic parameters of the system have been determined for the first time: the component mass ratio q = M1/M2 = 7.0 ± 0.2, the orbital inclination i = 79.9°−80.1°, the distance between the components’ centers of mass a0 = 0.821(1) Rʘ, and the sizes and temperatures of the stars: R1 = 0.0124(5)a0 = 0.0102(4) Rʘ, T1 = 12 500 ± 280 K, 〈R2〉 = 0.236(4)a0 = 0.194(3) Rʘ, T2 = 2550 ± 400 K, corresponding to M4–9V for the spectral type of the secondary. Parameters of the accretion disk have been derived for both activity states. The mass of matter in the accretion disk increased by almost a factor of two during ∼400 orbital periods in quiescence.



Relationship between the Elemental Abundances and the Kinematics of Galactic-Field RR Lyrae Stars
Resumo
Data of our compiled catalog containing the positions, velocities, and metallicities of 415 RR Lyrae variable stars and the relative abundances [el/Fe] of 12 elements for 101 RR Lyrae stars, including four α elements (Mg, Ca, Si, and Ti), are used to study the relationships between the chemical and spatial–kinematic properties of these stars. In general, the dependences of the relative abundances of α elements on metallicity and velocity for the RR Lyrae stars are approximately the same as those for field dwarfs. Despite the usual claim that these stars are old, among them are representatives of the thin disk, which is the youngest subsystem of the Galaxy. Attention is called to the problem of lowmetallicity RR Lyrae stars. Most RR Lyrae stars that have the kinematic properties of thick disk stars have metallicities [Fe/H] < −1.0 and high ratios [α/Fe] ≈ 0.4, whereas only about 10% of field dwarfs belonging to the so-called “low-metallicity tail” have this chemical composition. At the same time, there is a sharp change in [α/Fe] in RR Lyrae stars belonging just to the thick disk, providing evidence for a long period of formation of this subsystem. The chemical compositions of SDSS J1707+58, V455 Oph, MACHO176.18833.411, V456 Ser, and BPSCS 30339–046 do not correspond to their kinematics.While the first three of these stars belong to the halo, according to their kinematics, the last two belong to the thick disk. It is proposed that they are all most likely extragalactic, but the possible appearance of some of them in the solar neighborhood as a result of the gravitational action of the bar on field stars cannot be ruled out.



Detection of Five New RRATs at 111 MHz
Resumo
Results of 111-MHz monitoring observations carried out on the Big Scanning Antenna of the Pushchino Radio Astronomy Observatory during September 1–28, 2015 are presented. Fifty-four pulsating sources were detected at declinations −9° < δ < 42°. Forty-seven of these are known pulsars, five are new sources, and two are previously discovered transients. Estimates of the peak flux densities and dispersion measures are presented for all these sources.



Can Superflares Occur on the Sun? A View from Dynamo Theory
Resumo
Recent data from the Kepler mission has revealed the occurrence of superflares in Sun-like stars which exceed by far any observed solar flares in released energy. Radionuclide data do not provide evidence for occurrence of superflares on the Sun over the past eleven millennia. Stellar data for a subgroup of superflaring Kepler stars are analysed in an attempt to find possible progenitors of their abnormal magnetic activity. A natural idea is that the dynamo mechanism in superflaring stars differs in some respect from that in the Sun. We search for a difference in the dynamo-related parameters between superflaring stars and the Sun to suggest a dynamo mechanism as close as possible to the conventional solar/stellar dynamo but capable of providing much higher magnetic energy. Dynamo based on joint action of differential rotation and mirror asymmetric motions can in principle result in excitation of two types of magnetic fields. First of all, it is well-known in solar physics dynamo waves. The point is that another magnetic configuration with initial growth and further stabilisation can also be excited. For comparable conditions, magnetic field of second configuration is much stronger than that of the first one just because dynamo does not spend its energy for periodic magnetic field inversions but uses it for magnetic field growth. We analysed available data from the Kepler mission concerning the superflaring stars in order to find tracers of anomalous magnetic activity. As suggested in a recent paper [1], we find that anti-solar differential rotation or anti-solar sign of the mirror-asymmetry of stellar convection can provide the desired strong magnetic field in dynamo models. We confirm this concept by numerical models of stellar dynamos with corresponding governing parameters. We conclude that the proposed mechanism can plausibly explain the superflaring events at least for some cool stars, including binaries, subgiants and, possibly, low-mass stars and young rapid rotators.


