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 Nd2_2Fe14_{14}B-type compounds

Volume expansion $\Delta V / V$ associated with the spin reorientation process of Nd$_2$Fe$_{14}$B-type compounds has been investigated in terms of simple crystalline-electric-field (CEF) model. In this system, $\Delta V / V$ is shown to be a direct measure of second order CEF energy. Calculated anomalies in $\Delta V / V$ associated with the first-order magnetization process of Nd$_2$Fe$_{14}$B 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