astro-informatics – ページ 4 – Makoto Uemura （植村誠）

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

2010-03-03 uemuram

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.

Dwarf Novae in the Shortest Orbital Period Regime. I A New Short Superhump Period Dwarf Nova, OT J055717+683226

2009-12-22 uemuram

M. Uemura, A. Arai, T. Kato, H. Maehara, D. Nogami, et al., “Dwarf Novae in the Shortest Orbital Period Regime. I A New Short Superhump Period Dwarf Nova, OT J055717+683226,” PASJ, vol. 62, p. 187-, 2010.

We report on the observation of a new dwarf nova (DN), OT J055717+683226 (OT: optical transient), during the period of its first-recorded superoutburst in 2006 December. Our observation shows that this object is an SU UMa-type dwarf nova having a very short superhump period of 76.67±0.03 min (0.05324±0.00002 d). The next superoutburst was observed in 2008 March. The recurrence time of superoutbursts (supercycle) is, hence, estimated to be ˜480 d. The supercycle (˜480 d) is much shorter than those (≳ 10 yr) of WZ Sge-type dwarf novae which are in the major of dwarf novae in the shortest orbital period regime (≲ 85 min). Using a hierarchical cluster analysis, we divided the dwarf nova in the shortest orbital period regime in seven groups. We found that objects, which have short supercycles, small outburst amplitudes, and large superhump period excesses compared with those of WZ Sge stars, form a small group. OT J055717+683226 probably belongs to this group.

Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars

2009-11-16 uemuram

M. Uemura, K. ~S. Kawabata, M. Sasada, Y. Ikejiri, K. Sakimoto, et al., “Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars,” PASJ, vol. 62, p. 69-, 2010.

We developed a method of separating a long-term trend from observed temporal variations of polarization in blazars using a Bayesian approach. The temporal variation of the polarization vector is apparently erratic in most blazars, while several objects have occasionally exhibited systematic variations, for example an increase of the polarization degree associated with a flare of the total flux. We assume that the observed polarization vector is a superposition of two distinct components: a long-term trend and a short-term variation component responsible for short flares. Our Bayesian model estimates a long-term trend that satisfies the condition that the total flux correlates with the polarized flux of the short-term component. We demonstrate that assumed long-term polarization components are successfully separated by the Bayesian model for artificial data. We applied this method to the three photopolarimetric data of OJ 287, S5 0716+714, and S2 0109+224. Simple and systematic long-term trends were obtained in OJ 287 and S2 0109+224, while no such trend was confirmed in S5 0716+714. We propose that the apparently erratic variations of polarization in OJ 287 and S2 0109+224 are due to the presence of the long-term polarization component. The behavior of polarization in S5 0716+714 during the period of observation implies the presence of a number of polarization components showing variations on a quite short time-scale.

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