Search

Your search keyword '"Okazaki, A. T."' showing total 191 results
Start Over Author "Okazaki, A. T."
191 results on '"Okazaki, A. T."'

165. Cyclic variability of the circumstellar disk of the Be star ζTauri*

Author

Štefl, S., Rivinius, Th., Carciofi, A. C., Le Bouquin, J.-B., Baade, D., Bjorkman, K. S., Hesselbach, E., Hummel, C. A., Okazaki, A. T., Pollmann, E., Rantakyrö, F., Wisniewski, J. P., Štefl, S., Rivinius, Th., Carciofi, A. C., Le Bouquin, J.-B., Baade, D., Bjorkman, K. S., Hesselbach, E., Hummel, C. A., Okazaki, A. T., Pollmann, E., Rantakyrö, F., and Wisniewski, J. P.

Abstract

Context. Emission lines formed in decretion disks of Be stars often undergo long-term cyclic variations, especially in the violet-to-red ($V/R$) ratio of their primary components. The underlying structural and dynamical variations of the disks are only partly understood. From observations of the bright Be-shell star ζTau, the possibly broadest and longest data set illustrating the prototype of this behaviour was compiled from our own and archival observations. It comprises optical and infrared spectra, broad-band polarimetry, and interferometric observations.

Published
2009
Full Text
View/download PDF

167. Non-axisymmetric Structure of Accretion Disks around the Neutron Star in Be/X-ray Binaries.

Author

Hayasaki, Kimitake and Okazaki, Atsuo T.

Subjects
*NEUTRON stars, *STARS, *DISKS (Astrophysics), *X-ray binaries, *BINARY stars, *PRESSURE
Abstract

A non-axisymmetric structure of accretion disks around the neutron star in Be/X-ray binaries is studied, analyzing the results from 3D SPH simulations performed by [1]. It is found that a ram pressure due to the phase-dependent mass transfer from the Be-star disk excites a one-armed, trailing spiral structure in the accretion disk around the neutron star. The spiral wave has a transient nature; it is excited around the periastron, when the material is transferred from the Be disk, and gradually damped afterwards. We also find that the orbital phase-dependence of the mass-accretion rate is mainly caused by the inward propagation of the spiral wave excited on the accretion disks. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Published
2005
Full Text
View/download PDF

181. Constraining the absolute orientation of η Carinae's binary orbit: a 3D dynamical model for the broad [Fe iii] emission★.

Author

Madura, T. I., Gull, T. R., Owocki, S. P., Groh, J. H., Okazaki, A. T., and Russell, C. M. P.

Subjects
BINARY stars, STELLAR mass, SPECTROGRAPHS, HYDRODYNAMICS, ASTRONOMICAL observations, SIMULATION methods & models, FIELD emission
Abstract

ABSTRACT We present a three-dimensional (3D) dynamical model for the broad [Fe iii] emission observed in η Carinae using the Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS). This model is based on full 3D smoothed particle hydrodynamics simulations of η Car's binary colliding winds. Radiative transfer codes are used to generate synthetic spectroimages of [Fe iii] emission-line structures at various observed orbital phases and STIS slit position angles (PAs). Through a parameter study that varies the orbital inclination i, the PA θ that the orbital plane projection of the line of sight makes with the apastron side of the semimajor axis and the PA on the sky of the orbital axis, we are able, for the first time, to tightly constrain the absolute 3D orientation of the binary orbit. To simultaneously reproduce the blueshifted emission arcs observed at orbital phase 0.976, STIS slit PA =+38° and the temporal variations in emission seen at negative slit PAs, the binary needs to have an i≈ 130° to 145°, θ≈−15° to +30° and an orbital axis projected on the sky at a PA ≈ 302° to 327° east of north. This represents a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula, in 3D. The companion star, ηB, thus orbits clockwise on the sky and is on the observer's side of the system at apastron. This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modelling to determine the stellar masses. [ABSTRACT FROM AUTHOR]

Published
2012
Full Text
View/download PDF

182. Accretion disc formation around the neutron star in Be/X-ray binaries.

Author

Hayasakp, Kimitake and Okazaki, Atsuo T.

Subjects
*X-rays, *NEUTRON stars, *HYDRODYNAMICS, *GALAXIES, *ORBITS (Astronomy), *ASTROPHYSICS
Abstract

We study the accretion on to the neutron star in Be/X-ray binaries, using a three-dimensional smoothed particle hydrodynamics code and data imported from a recent simulation by Okazaki et al. for a coplanar system with a short period and a moderate eccentricity , which targeted the Be/X-ray binary 4U 0115+63. For simplicity, we adopt the polytropic equation of state. We find that a time-dependent accretion disc is formed around the neutron star regardless of the simulation parameters. In the long term, the disc evolves via a two-stage process, which consists of the initial developing stage and the later developed stage. The developed disc is nearly Keplerian. In the short term, the disc structure modulates with the orbital phase. The disc shrinks at the periastron passage of the Be star and restores its radius afterwards. The accretion rate on to the neutron star is also phase dependent, but its peak is broader and much lower than that of the mass transfer rate from the Be disc, unless the polytropic exponent is as large as 5/3. Our simulations show that the truncated Be disc model for Be/X-ray binaries is consistent with the observed X-ray behaviour of 4U 0115+63. [ABSTRACT FROM AUTHOR]

Published
2004
Full Text
View/download PDF

183. GIobaI Trapped Oscillations of Relativistic Accretion Disks

Author

OKAZAKI, Atsuo T., KATO, Shoji, and FUKUE, Jun

Subjects
Condensed Matter::Quantum Gases, Active galactic nuclei, Astrophysics::High Energy Astrophysical Phenomena, Accretion disks, Compact stars, Trapped oscillations, Pulsations, Astrophysics::Earth and Planetary Astrophysics, Astrophysics::Galaxy Astrophysics
Abstract

Axially symmetric, isothermal pulsations of a geometrically thin disk rotating around a nonrotating relativistic object are investigated. The disk is assumed to be isothermal in the vertical direction and to have a constant vertical scale height. It is found that there exist global pulsation modes trapped in an inner region of the disk. Eigenfrequencies of the modes are in the range of 0 to kappa_{max}, Where kappa_{max} is the maximum value of an epicyclic frequency in the disk. For an oscillation to be trapped in the disk, the eigenfunctions must have at least one node in the vertical direction. The trapping is related to the fact that, in relativistic disks, the radial distribution of the epicyclic frequency has a maximum at a certain radius. The results suggest that the presence of global trapped oscillation modes is a general characteristic of thin relativistic disks. The oscillation period obtained is briefly compared with observations.

Published
1987

184. Structure around the Inner Edge of Geometrically Thin Accretion Disks

Author

KATO, Shoji, FUKUE, Jun, INAGAKI, Shogo, and OKAZAKI, Atsuo T.

Subjects
Black holes, Astrophysics::High Energy Astrophysical Phenomena, Accretion disks, Dynamical instability, Astrophysics::Earth and Planetary Astrophysics, Astrophysics::Galaxy Astrophysics
Abstract

In geometrically thin, nearly Keplerian disks, there is an inner edge inside which the gas falls toward the black hole by dynamical instability due to a relativistic effect. The disk structure around this edge is examined in detail when the geometry is Schwarzschildian and the vertically averaged kinematic viscosity is constant. A steady disk model is found in which the disk structure changes continuously at the edge from a nearly Keplerian structure to a thin non-Keplerian structure of the infalling gas. The structure of this transition region is demonstrated.

Published
1982

185. Emission-Line Profiles from Be-Star Envelopes with m= 1 Perturbation Patterns

Author

Okazaki, Atsuo T.

Abstract

We have studied the variations in the Balmer-line profiles from Be-star disks with eccentric (i.e., m= 1) perturbation patterns. In order to examine the effects of oscillations on the line profiles, we assumed that the nonlinear perturbation patterns are similar to the linear m= 1 eigenfunctions. In addition, we assumed that the source function is constant over the entire disk. Based on these assumptions, we computed the optically-thick line profiles for various values of the disk parameters. We found that the line-profile variability due to the m= 1 perturbation patterns agrees well with the observed V/R variability. We also found that the amplitude of the profile shift associated with the V/R variation is sensitive to the density gradient in the radial direction. In particular, disks in which the local density decreases as steeply as r−3exhibit remarkable variabilities like the observed V/R variations. These variabilities occur for a wide range of disk sizes and line optical depths.

Published
1996
Full Text
View/download PDF

187. Phase changes of the Be/X-ray binary X Persei

Author

Clark, J. S., Tarasov, A. E., Okazaki, A. T., Roche, P., Lyuty, V. M., Clark, J. S., Tarasov, A. E., Okazaki, A. T., Roche, P., and Lyuty, V. M.

Abstract

We present high resolution optical spectroscopy and V band photometry obtained during the period 1987-2001 for the Be star X Persei/HD 24534, the counterpart to the X-ray pulsar 4U 0352+30. We find that throughout this interval X Per is highly active, with significant photometric and spectroscopic variability. We identify one episode of complete disc loss during this period (1988 May-1989 June), characterised by significant ΔV=0.6 mag optical fading and the presence of purely photospheric Hα and He I 6678 Å lines. Two further episodes of pronounced optical fading which did not result in the complete dispersal of the circumstellar disc were also identified (1994 October-1995 October and 1999 November-present). The emission line profiles of both Hα and He I 6678 Å also show significant variability. Cyclic changes in the strength of the peaks in both emission lines are observed, with periods ranging from 0.6-2 yrs - we attribute these to the presence of a one armed density wave in the inner circumstellar disc. Additional structure at large projected velocities is also present in the He I line - suggesting the presence of a significant density enhancement in the disc near the stellar surface (the "double disc'' of Tarasov & Roche). The evolution of the outer edge of the Hα emitting region of the circumstellar disc is followed during disc formation, and is found to increase rather slowly. This observation, combined with the presence of the one armed density wave and the rate of disc formation and loss all provide strong evidence for the hypothesis that the circumstellar disc of X Per is a viscous decretion disc, with angular momentum being supplied by an as yet unknown physical mechanism near the stellar surface.

191. Constraining the absolute orientation of η Carinae's binary orbit: a 3D dynamical model for the broad [Fe iii] emission★.

Author

Madura, T. I., Gull, T. R., Owocki, S. P., Groh, J. H., Okazaki, A. T., and Russell, C. M. P.

Subjects
*BINARY stars, *STELLAR mass, *SPECTROGRAPHS, *HYDRODYNAMICS, *ASTRONOMICAL observations, *SIMULATION methods & models, *FIELD emission
Abstract

ABSTRACT We present a three-dimensional (3D) dynamical model for the broad [Fe iii] emission observed in η Carinae using the Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS). This model is based on full 3D smoothed particle hydrodynamics simulations of η Car's binary colliding winds. Radiative transfer codes are used to generate synthetic spectroimages of [Fe iii] emission-line structures at various observed orbital phases and STIS slit position angles (PAs). Through a parameter study that varies the orbital inclination i, the PA θ that the orbital plane projection of the line of sight makes with the apastron side of the semimajor axis and the PA on the sky of the orbital axis, we are able, for the first time, to tightly constrain the absolute 3D orientation of the binary orbit. To simultaneously reproduce the blueshifted emission arcs observed at orbital phase 0.976, STIS slit PA =+38° and the temporal variations in emission seen at negative slit PAs, the binary needs to have an i≈ 130° to 145°, θ≈−15° to +30° and an orbital axis projected on the sky at a PA ≈ 302° to 327° east of north. This represents a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula, in 3D. The companion star, ηB, thus orbits clockwise on the sky and is on the observer's side of the system at apastron. This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modelling to determine the stellar masses. [ABSTRACT FROM AUTHOR]

Published
2012
Full Text
View/download PDF

Catalog

Books, media, physical & digital resources
See catalog results