Large-scale photometric activity of UX Ori type stars

Radical readjustments in the photometric activity of young stars owing to sudden changes in the circumstellar extinction are discussed using the light curves of two UX Ori type stars, CQ Tau and V1184 Tau, as examples. Changes of this sort can be caused by large deviations from axial symmetry in the distribution of circumstellar dust, as well as by large variations in the mass accretion rate in circumstellar disks. A large amount of dust may also appear in the vicinity of a young star owing to collisions of planetesimals. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 5–13 (February 2008).     

Fossil magnetic field of accretion disks of young stars

We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains. Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions. Magnetic field is quasi-poloidal in the dusty “dead” zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozen-in field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a _d=0.1 μm. In the case of large dust grains (a _d>0.1 μm) or enhanced ionization rates, the magnetic field is quasi-radial in the outer regions. It is shown that the inner boundary of dusty “dead” zone is placed at r=(0.1–0.6) AU for accretion disks of stars with M=(0.5–2)?M _⊙. Outer boundary of “dead” zone is placed at r=(3–21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the “dead” zone is more than 3?M _⊕ for stars with M≥1?M _⊙.     

Determination of effective temperatures of the accretion disks in symbiotic stars

The conditions of the excitation of ultraviolet emission lines in symbiotic stars-triple systems with white dwarf and nebular clouds are examined. A new method is suggested for the determination of effective temperatureT _* of the accretion disk around of white dwarf, which is based on the balance between the summary intensitiesE _e =E _i of ultraviolet emission lines, escaped by nebular cloud, and a definite part of the energy of black-body radiation of accretion disk in the region shorter from the some wavelength ₀. This condition brings us to the formula (9) which is used for the determination of the temperatureT _* and radiusr _n of nebular cloud around the symbiotic system. It is shown that practically in all cases ₀=180 Å=const. The results of the application of this method in relation to the five symbiotic triple-star systems are presented in Tables III and IV.     

Non-LTE modeling of supernova-fallback disks

We present a first detailed spectrum synthesis calculation of a supernova-fallback disk composed of iron. We assume a geometrically thin disk with a radial structure described by the classical α-disk model. The disk is represented by concentric rings radiating as plane-parallel slabs. The vertical structure and emission spectrum of each ring is computed in a fully self-consistent manner by solving the structure equations simultaneously with the radiation transfer equations under non-LTE conditions. We describe the properties of a specific disk model and discuss various effects on the emergent UV/optical spectrum. We find that strong iron-line blanketing causes broad absorption features over the whole spectral range. Limb darkening changes the spectral distribution up to a factor of four depending on the inclination angle. Consequently, such differences also occur between a blackbody spectrum and our model. The overall spectral shape is independent of the exact chemical composition as long as iron is the dominant species. A pure iron composition cannot be distinguished from silicon-burning ash. Non-LTE effects are small and restricted to few spectral features.     

Massive accretion disks

Recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging have revealed dense gas and dust accretion disks in nearby ultra-luminous galactic nuclei. In the best studied ultraluminousIR galaxy, Arp 220, the 2m imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gasmasses 10⁹M for each disk. The two gas disks in Arp 220 are counterrotating and their dynamical masses are 2×10⁹ M , that is, only slightly largerthan the gas masses. These disks have radii 100 pc and thickness 10-50 pc. The high brightness temperatures of the CO lines indicatethat the gas in the disks has area filling factors 25-50% and mean densitiesof 10⁴ cm^-3. Within these nuclear disks, the rate of massive star formation is undoubtedly prodigious and, given the high viscosity of the gas, there will also be high radial accretion rates, perhaps 10 M yr ^-1. If this inflow persists to very small radii, it is enough to feed even the highest luminosity AGNs.     

Magnetically tilted accretion disks

Accretion disks around magnetized, compact stars are expected to be tilted near their inner edges, due to the stresses exerted by the corotating magnetosphere of the inclined central rotator. We reassess numerically the results obtained analytically by Lipunovet al. (1981). Four qualitatively different situations occur, depending on the relative orientations of the outer accretion disk, the spin of the central rotator, and its magnetic dipole axis. In at least two of them, the inner part of the disk is expected to be decomposed into massive, magnetically confined clumps.     

Two-dimensional numerical models of the boundary layer of accretion disks in cataclysmic variables

Nearly half of the total available accretion energy can be released in the boundary layer (BL) if the accreting object is slowly rotating. The spectral distribution of the emitted radiation depends crucially on the internal structure of the BL. Up to now no detailed models concerning the BL exist. We have developed an explicit two-dimensional numerical method written for axisymmetric accretion flows including viscosity effects and angular momentum. We display our first models concerning the BL structure incorporating variation of the stellar rotation and of the fraction of the released energy. The first results show a strong dependence of the BL structure on the local rate of cooling and on the rotation of the primary.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Analysis of the Historical Light Curve of the UX Ori Star CQ Tau

An historical light curve for the UX Ori star CQ Tau is constructed for the period from 1939 through 2003. The star’s photometric behavior includes a cyclical component with a period of about 21 years. An analysis of the periodogram reveals a shorter cycle with a duration of about 1020 days after this cycle is subtracted. Since the photometric activity of UX Ori type stars is caused by variations in the circumstellar extinction, both of these cycles indicate the existence of large-scale deviations from axial symmetry in the distribution of matter in the circumstellar disk of CQ Tau. The orbiting of these inhomogeneities about the star also causes oscillatory variations in its brightness. An analysis of the color-magnitude diagrams reveals differences in the optical parameters of the circumstellar dust along the line of sight in various phases of the 21-year cycle: absorption by larger particles predominates near the brightness maximum of CQ Tau. This means that the reduced circumstellar extinction in this part of the circumstellar disk is the result of a smaller contribution from small particles; this may be evidence of the onset of a process of coagulation of the dust particles and their conversion into large-scale bodies and planetesimals.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 165–174 (May 2005).     

Dissipation of thick accretion disks

A thick accretion disk which is isentropic cannot have simple laminar flow because fluid elements follow orbits which intersect the orbits of other fluid elements, leading to turbulence in astrophysical disks which have very large Reynolds numbers. The turbulence in such disks is estimated using molecular analogies for the behavior of the fluid elements. The usual empirical dissipation parameter ‘α’ is found to be equal to 0.25 under normal circumstances. Characteristic local disk parameters are calculated for a variety of conditions at different distances from a central star of one solar mass. Circumstances involving low midplane optical depths or external heating which can lead to large reductions in the turbulence are discussed.     

Chaotic behavior in accretion disks

The eccentric luminosity variations of quasars are still a mystery. Analytic results of this behavior ranged from multi-periodic behavior to a purely random process. Recently, we have used the non-linear time-series analysis techniques to analyze the optical light curve of 3C 273 and found that its eccentric behavior may be a low-dimensional chaos. This result induces us to seek some non-linear mechanism for the eccentric luminosity variation. In this paper, we propose a simple non-linear accretion disk model and find that under some circumstances, the chaos appears both in the disk and in the light curve. Then we compute the outburst energy ΔI, defined by the difference between the maximum luminosity and the minimum luminosity, and the mean luminosity 〈I〉 and find that when chaos appears, ΔI～〈I〉 ^α ～M ^0.5α , where M is the mass of black hole and α≈1. These results are confirmed by and/or compatible with the observational data analysis.     

Theoretical restrictions on accretion disks

We construct a standard thin disk model taking into account the pressure from both gas and radiation, the opacity contributed by both electron scattering and absorption, and the gravity from both a central object and a disk. A simple and powerful technique for solving the non-linear equations is presented. Through a numerical algorithm for the two equationl for, , , all the disk quantities are expressed as the analytical function of, , . We also discuss the solutions in the limit cases 0, 1 and the parameter range of the linear approximation. From the numerical solutions and limit analyses, we found that it is not necessary to include the self-gravity of the disk.     

Meridional flow in thick accretion disks

We have studied the effect of the flow in the accretion disk. The specific angular momentum of the disk is assumed to be constant and the polytropic relation is used. We have solved the structure of the disk and the flow patterns of the irrotational perfect fluid.As far as the obtained results are concerned, the flow does not affect the shape of the configuration in the bulk of the disk, although the flow velocity reaches even a half of the sound velocity at the inner edge of the disk. Therefore, in order to study accretion disk models with the moderate mass accretion rate—i.e.,

Hydrodynamical activity in thin accretion disks

An asymptotic treatment of thin accretion disks, introduced by Kluźniak and Kita [Kluźniak, W., Kita, D., 2000. Three-dimensional structure of an alpha accretion disk. Available from: <arXiv:astro-ph/0006266v1> (KK)] for a steady-state disk flow, is extended to a time-dependent problem. Transient growth of axisymmetric disturbances is analytically shown to occur on the global disk scale. The implications of this result on the theory of hydrodynamical thin accretion disks, as well as future prospects, are discussed.     

Linkage between accretion disks and blazars

The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission region using a model for a relativistic axisymmetric jet combined with some simplifying assumptions based on the relativistic nature of the flow. The extrapolated magnetic field is compared with estimates based upon the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN 421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-Compton models are inconsistent with the magnetic fields extrapolated in this way. However, in two cases inverse Compton models in which a substantial part of the soft photon field is generated locally agree well, mainly because these models imply magnetic field strengths consistent with an important Poynting Flux component. This comparison is based on estimating the mass accretion rate from the jet energy flux. Further comparisons along these lines will be facilitated by independent estimates of the mass accretion rate in blazars and by more detailed models for jet propagation near the black hole.