Models of hot star decretion disks

Massive stars can during their evolution reach the phase of critical (or very rapid, near-critical) rotation when further increase in rotation rate is no longer kinematically allowed. The mass ejection and angular momentum outward transport from such rapidly rotating star's equatorial surface may lead to formation and supports further existence of a circumstellar outflowing (stellar decretion) disk. The efficient mechanism for the outward transport of the mass and angular momentum is provided by the anomalous viscosity. The outer supersonic regions of the disks can extend up to a significantly large distance from the parent star, the exact radial extension is however basically unknown, partly due to the uncertainties in radial variations of temperature and viscosity. We study in detail the behavior of hydrodynamic quantities, i.e., the evolution of density, radial and azimuthal velocity, and angular momentum loss rate in stellar decretion disks out to extremely distant regions. We investigate the dependence of these physical characteristics on the distribution of temperature and viscosity. We also study the magnetorotational instability, which we regard to be the source of anomalous viscosity in such outflowing disks and to some extent we provide the preliminary models of the two-dimensional radially-vertically correlated distribution of the disk density and temperature. We developed our own two-dimensional hydrodynamic and magnetohydrodynamic numerical code based on an explicit Eulerian finite difference scheme on staggered grid, including full Navier-Stokes viscosity. We use semianalytic approach to investigate the radial profile of magnetorotational instability, where on the base of the numerical time-dependent hydrodynamic model we analytically study the stability of outflowing disks submerged to the magnetic field of central star.Comment: PhD Thesi

Time-dependent modeling of extended thin decretion disks of critically rotating stars

During their evolution massive stars can reach the phase of critical rotation when a further increase in rotational speed is no longer possible. Direct centrifugal ejection from a critically or near-critically rotating surface forms a gaseous equatorial decretion disk. Anomalous viscosity provides the efficient mechanism for transporting the angular momentum outwards. The outer part of the disk can extend up to a very large distance from the parent star. We study the evolution of density, radial and azimuthal velocity, and angular momentum loss rate of equatorial decretion disks out to very distant regions. We investigate how the physical characteristics of the disk depend on the distribution of temperature and viscosity. We calculated stationary models using the Newton-Raphson method. For time-dependent hydrodynamic modeling we developed the numerical code based on an explicit finite difference scheme on an Eulerian grid including full Navier-Stokes shear viscosity. The sonic point distance and the maximum angular momentum loss rate strongly depend on the temperature profile and are almost independent of viscosity. The rotational velocity at large radii rapidly drops accordingly to temperature and viscosity distribution. The total amount of disk mass and the disk angular momentum increase with decreasing temperature and viscosity. The time-dependent one-dimensional models basically confirm the results obtained in the stationary models as well as the assumptions of the analytical approximations. Including full Navier-Stokes viscosity we systematically avoid the rotational velocity sign change at large radii. The unphysical drop of the rotational velocity and angular momentum loss at large radii (present in some models) can be avoided in the models with decreasing temperature and viscosity

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

Context. Evolution of massive stars is affected by a significant loss of mass either via (nearly) spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely the outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around massive stars is still under debate. Aims. We study the hydrodynamic and thermal structure of optically thick, dense parts of outflowing circumstellar disks that may be formed around various types of critically rotating massive stars, for example, Be stars, B[e] supergiant (sgB[e]) stars or Pop III stars. Methods. We specify the optical depth of the disk along the line-of-sight from stellar poles. Within the optically thick dense region we calculate the vertical disk thermal structure using the diffusion approximation while for the optically thin outer layers we assume a local thermodynamic equilibrium with the impinging stellar irradiation. We use two of our own types of hydrodynamic codes: two-dimensional operator-split numerical code and unsplit code based on the Roe's method. Results. Our models show the geometric distribution and contribution of viscous heating that begins to dominate in the central part of the disk. In the models of dense viscous disks the viscosity increases the central temperature up to several tens of thousands of Kelvins. The high mass-loss rates and high viscosity lead to instabilities with significant waves or bumps in density and temperature in the very inner disk region. Conclusions. The two-dimensional radial-vertical models of dense outflowing disks including the full Navier-Stokes viscosity terms show very high temperatures that are however limited to only the central disk cores inside the optically thick area, while near the edge of the optically thick region the temperature may be low enough for the existence of neutral hydrogen.Comment: 24 pages, 21 figure

Torsional oscillations and observed rotational period variations in early-type stars

Some chemically peculiar stars in the upper main sequence show rotational period variations of unknown origin. We propose that these variations are a consequence of the propagation of internal waves in magnetic rotating stars that lead to the torsional oscillations of the star. We simulate the magnetohydrodynamic waves and calculate resonant frequencies for two stars that show rotational variations: CU Vir and HD 37776. We provide updated analyses of rotational period variations in these stars and compare our results with numerical models. For CU Vir, the length of the observed rotational period cycle, | $\mathit {\Pi }=67.6(5)$ | yr, can be well reproduced by the models, which predict a cycle length of 51 yr. However, for HD 37776, the observed lower limit of the cycle length, | $\mathit {\Pi }\ge 100$ | yr, is significantly longer than the numerical models predict. We conclude that torsional oscillations provide a reasonable explanation at least for the observed period variations in CU Vir

A search for peculiar stars in the open cluster Hogg 16

The study of chemically peculiar (CP) stars in open clusters provides valuable information about their evolutionary status. Their detection can be performed using the $\Delta a$ photometric system, which maps a characteristic flux depression at $\lambda \sim 5200$ \AA. This paper aims at studying the occurrence of CP stars in the earliest stages of evolution of a stellar population by applying this technique to Hogg 16, a very young Galactic open cluster (about 25 Myr). We identified several peculiar candidates: two B-type stars with a negative $\Delta a$ index (CD-60 4701, CPD-60 4706) are likely emission-line (Be) stars, even though spectral measurements are necessary for a proper classification of the second one; a third object (CD-60 4703), identified as a Be candidate in literature, appears to be a background B-type supergiant with no significant $\Delta a$ index, which does not rule out the possibility that it is indeed peculiar as the normality line of $\Delta a$ for supergiants has not been studied in detail yet. A fourth object (CD-60 4699) appears to be a magnetic CP star of 8 M$_\odot$, but obtained spectral data seem to rule out this hypothesis. Three more magnetic CP star candidates are found in the domain of early F-type stars. One is a probable nonmember and close to the border of significance, but the other two are probably pre-main sequence cluster objects. This is very promising, as it can lead to very strong constraints to the diffusion theory. Finally, we derived the fundamental parameters of Hogg 16 and provide for the first time an estimate of its metal content.Comment: Accepted for publication in New Astronomy. 9 pages, 4 figures, and 4 tables. A complete data table is available via CDS or upon request from the corresponding autho

Time-dependent numerical modeling of large-scale astrophysical processes: from relatively smooth flows to explosive events with extremely large discontinuities and high Mach numbers

summary:We calculate self-consistent time-dependent models of astrophysical processes. We have developed two types of our own (magneto) hydrodynamic codes, either the operator-split, finite volume Eulerian code on a staggered grid for smooth hydrodynamic flows, or the finite volume unsplit code based on the Roe's method for explosive events with extremely large discontinuities and highly supersonic outbursts. Both the types of the codes use the second order Navier-Stokes viscosity to realistically model the viscous and dissipative effects. They are transformed to all basic orthogonal curvilinear coordinate systems as well as to a special non-orthogonal geometric system that fits to modeling of astrophysical disks. We describe mathematical background of our codes and their implementation for astrophysical simulations, including choice of initial and boundary conditions. We demonstrate some calculated models and compare the practical usage of numerically different types of codes

The Educational role of literature in teaching English as a foreign language

Práce předkládá průzkum dostupných materiálů napsaných renomovanými metodiky, ve kterých se navrhují způsoby využití literatury ve výuce cizím jazykům.Dokončená práce s úspěšnou obhajobo

Početní praktikum

Výuková publikace Početní praktikum představuje studijní materiál pro předměty PřF: F1422 Početní praktikum 1 a F2423 Početní praktikum 2, pokrývající veškerou přednášenou látku. Jednotlivé kapitoly vždy obsahují stručnou teorii a soubor příkladů s uvedeným výsledkem.This textbook provides the study material for courses Computing Practice 1 and 2, covering all lectured topics. Each chapter contains brief theory and set of examples with the result