Doppler tomography by total variation minimization

M. Uemura, T. Kato, D. Nogami, and R. Mennickent, “Doppler tomography by total variation minimization,” PASJ, vol. 67, p. 22, 2015

We have developed a new model of Doppler tomography using total variation minimization (DTTVM). This method can reconstruct localized and nonaxisymmetric profiles with sharp edges in the Doppler map. This characteristic is emphasized in the case where input data are small in number. We apply this model to natural data for the dwarf nova WZ Sge in superoutburst and TU Men in quiescence. We confirm that DTTVM can reproduce the observed spectra with high precision. Compared with the models based on the maximum entropy method, our new model can provide Doppler maps that little depend on the hyperparameter and on the presence of the absorption core. We also introduce a cross-validation method of estimating reasonable values of a hyperparameter in the model from the data themselves.



Height Mapping of Dwarf Nova Accretion Disks

M. Uemura, T. Kato, T. Ohshima, and H. Maehara, “Reconstruction of the Structure of Accretion Disks in Dwarf Novae from the Multi-Band Light Curves of Early Superhumps,” PASJ, vol. 64, p. 92, 2012.

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.

Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum

M. Uemura, T. Kato, D. Nogami, and T. Ohsugi, “Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum,” PASJ, vol. 62, p. 613-, 2010.

WZ Sge-type dwarf novae are characterized by long recurrence times of outbursts (˜10 yr) and short orbital periods (lsim 85 min). A significant part of WZ Sge stars may remain undiscovered because of low outburst activity. Recently, the observed orbital period distribution of cataclysmic variables (CVs) has changed partly because outbursts of new WZ Sge stars have been discovered routinely. Hence, estimations of the intrinsic population of WZ Sge stars are important to study the population and the evolution of CVs. In this paper, we present a Bayesian approach to estimate the intrinsic period distribution of dwarf novae from observed samples. In this Bayesian model, we assumed a simple relationship between the recurrence time and the orbital period, which is consistent with observations of WZ Sge stars and other dwarf novae. As a result, the minimum orbital period was estimated to be ˜70 min. The population of WZ Sge stars exhibited a spike-like feature at the shortest period regime in the orbital period distribution. These features are consistent with the orbital period distribution previously predicted by population synthesis studies. We propose that WZ Sge stars and CVs with a low mass-transfer rate are excellent candidates for the missing population predicted by the evolution theory of CVs.



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