29,494 research outputs found
Reconstruction of the Structure of Accretion Disks in Dwarf Novae from the Multi-Band Light Curves of Early Superhumps
We propose a new method to reconstruct the structure of accretion disks in
dwarf novae using multi-band light curves of early superhumps. Our model
assumes that early superhumps are caused by the rotation effect of
non-axisymmetrically flaring disks. We have developed a Bayesian model for this
reconstruction, in which a smoother disk-structure tends to have a higher prior
probability. We analyzed simultaneous optical and near-infrared photometric
data of early superhumps of the dwarf nova, V455 And using this technique. The
reconstructed disk has two flaring parts in the outermost region of the disk.
These parts are responsible for the primary and secondary maxima of the light
curves. The height-to-radius ratio is h/r=0.20-0.25 in the outermost region. In
addition to the outermost flaring structures, flaring arm-like patterns can be
seen in an inner region of the reconstructed disk. The overall profile of the
reconstructed disk is reminiscent of the disk structure that is deformed by the
tidal effect. However, an inner arm-like pattern, which is responsible for the
secondary minimum in the light curve, cannot be reproduced only by the tidal
effect. It implies the presence of another mechanism that deforms the disk
structure. Alternatively, the temperature distribution of the disk could be
non-axisymmetric. We demonstrate that the disk structure with weaker arm-like
patterns is optimal in the model including the irradiation effect. However, the
strongly irradiated disk gives quite blue colors, which may conflict with the
observation. Our results suggest that the amplitude of early superhumps depends
mainly on the height of the outermost flaring regions of the disk. We predict
that early superhumps can be detected with an amplitude of >0.02 mag in about
90% of WZ Sge stars.Comment: 17 pages, 17 figures, accepted for publication in PAS
Crystal-field-induced magnetostrictions in the spin reorientation process of NdFeB-type compounds
Volume expansion associated with the spin reorientation
process of NdFeB-type compounds has been investigated in terms of
simple crystalline-electric-field (CEF) model. In this system,
is shown to be a direct measure of second order CEF energy. Calculated
anomalies in associated with the first-order magnetization
process of NdFeB are presented, which well reproduced the
observations.Comment: 2 pages, 2 figures, to appear in J. Magn. Magn. Mate
Effect of Ga irradiation on magnetic and magnetotransport properties in (Ga,Mn)As epilayers
We report on the magnetic and magnetotransport properties of ferromagnetic
semiconductor (Ga,Mn)As modified by Ga ion irradiation using focused ion
beam. A marked reduction in the conductivity and the Curie temperature is
induced after the irradiation. Furthermore, an enhanced negative
magnetoresistance (MR) and a change in the magnetization reversal process are
also demonstrated at 4 K. Raman scattering spectra indicate a decrease in the
concentration of hole carriers after the irradiation, and a possible origin of
the change in the magnetic properties is discussed
Ion Irradiation Control of Ferromagnetism in (Ga,Mn)As
We report on a promising approach to the artificial modification of
ferromagnetic properties in (Ga,Mn)As using a Ga focused ion beam (FIB)
technique. The ferromagnetic properties of (Ga,Mn)As such as magnetic
anisotropy and Curie temperature can be controlled using Ga ion
irradiation, originating from a change in hole concentration and the
corresponding systematic variation in exchange interaction between Mn spins.
This change in hole concentration is also verified using micro-Raman
spectroscopy. We envisage that this approach offers a means of modifying the
ferromagnetic properties of magnetic semiconductors on the micro- or nano-meter
scale.Comment: 4 pages, 4 figures, to appear in Jpn. J. Appl. Phys. (Part 2 Letters
Discovery of Extremely Large-Amplitude Quasi-Periodic Photometric Variability in WC9-Type Wolf-Rayet Binary, WR 104
We discovered that the Wolf-Rayet (WR)+OB star binary, WR 104, renowned for
its associated "dusty pinwheel nebula" recently spatially resolved with
infrared interferometry, exhibits strong quasi-periodic optical variations with
a full amplitude of 2.7 mag. Such a large-amplitude, continuous variation has
been unprecedented in a WR star. The optical quasi-period (~241 d) is in almost
perfect agreement with the interferometric period (243.5+/-3 d). The remarkable
agreement of the dominant period in optical variability with the orbital period
supports that the strongly varying dust obscuration is physically related to
the binary motion, rather than sporadic dust-forming episodes. Considering the
low orbital inclination (11+/-7 deg) and the nearly circular orbit inferred
from the interferometric observations, the strongly variable line-of-sight
extinction suggests that the highly structured extinction can be being formed
via an ejection of dust in the direction of the binary rotation axis. Another
viable explanation is that the three-dimensional structure of the shock front,
itself is the obscuring body. Depending on the geometry, the dusty shock front
near the conjunction phase of the binary can completely obscure the inner
WR-star wind and the OB star, which can explain the amplitude of optical fading
and the past observation of remarkable spectral variation.Comment: 5 pages, 4 figures, to appear in PASJ (Letters), using an alternative
style fil
Deep Transient Optical Fading in the WC9 Star WR 106
We discovered that the WR9-type star WR 106 (HDE 313643) underwent a deep
episodic fading in 2000. The depth of the fading (dV ~ 2.9 mag) surpassed those
of all known similar "eclipse-like" fadings in WR stars. This fading episode
was likely to be produced by a line-of-sight episodic dust formation rather
than a periodic enhancement of dust production in the WR-star wind during the
passage of the companion star though an elliptical orbit. The overall 2000
episode was composed of at least two distinct fadings. These individual fadings
seem to more support that the initial dust formation triggered a second dust
formation, or that the two independent dust formations occurred by the same
triggering mechanism rather than a stepwise dust formation. We also discuss on
phenomenological similarity of the present fading with the double fading of R
CrB observed in 1999-2000.Comment: 3 pages, 4 figures, to appear in Astron. Astrophys. (Letters
Recurrent Nova IM Normae
We detected the second historical outburst of the 1920 nova IM Nor. Accurate
astrometry of the outbursting object revealed the true quiescent counterpart
having a magnitude of R=17.0 mag and B=18.0 mag. We show that the quiescent
counterpart shows a noticeable variation. From the comparison of light curves
and spectroscopic signatures, we propose that IM Nor and CI Aql comprise a new
class of recurrent novae bearing some characteristics similar to those of
classical novae. We interpret that the noticeable quiescent variation can be a
result of either high orbital inclination, which may be also responsible for
the low quiescent brightness, or the presence of high/low states. If the second
possibility is confirmed by future observations, IM Nor becomes the first
recurrent nova showing state changes in quiescence. Such state changes may
provide a missing link between recurrent novae and supersoft X-ray sources.Comment: 4 pages, 3 figures, submitted to Astronomy and Astrophysics Letter
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