Petr Kurfürst

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

Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.

Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks

Vývojove modely rychle rotujicich hvězd vedou k výsledku, podle ktereho rotacni rychlost hvězdy může dosahnout kriticke rotacni rychlosti. Kriticky rotujici hvězdy se již nemohou otacet rychleji a proto ztraci nadbytecný moment hybnosti odtekajicim diskem. Radialni dosah disku neni znamý, castecně proto, že nemame informaci o radialni zavislosti viskozity. V clanku studujeme magnetorotacni nestabilitu, ktera je považovana za původ anomalni viskozity v odtekajicich discich. Využivame analyticke výpocty pro studium nestability odtekajicich disků ponořených v magnetickem poli. Magnetorotacni nestabilita se objevi v blizkosti hvězdy v připadě, že plazmatický parametr je dostatecně velký. Ve velkých vzdalenostech od hvězdy nestabilita vymizi v oblasti, kde je oběžna rychlost zhruba rychlosti zvuku. Magnetorotacni nestabilita je vhodným zdrojem viskozity v odtekajicich discich i v oblastech, kde se radialni rychlost latky bliži rychlosti zvuku. Zvukový bod může být tedy považovan za efektivni poloměr disku. Ztrata momentu hybnosti může být pocitana pravě na tomto poloměru. Studojee možne ověřeni vnějsiho poloměru disku na zakladě pozororovani hvězd a rentgenových dvojhvězd typu Be.

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, 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, Pi>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

New inclination changing eclipsing binaries in the Magellanic Clouds

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

photometric system, which maps a characteristic flux depression at

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

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Hydrodynamics of Decretion Disks of Rapidly Rotating Stars

\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

The Strange Case of OGLE-SMC-ECL-0277

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Tracing the B[e] Phenomenon with the Narrow-Band delta a Photometric System

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