カテゴリー: dwarf nova

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.

uem10bayes

 

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

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.

uem10cluster

 

Optical and Near-Infrared Photometric Observation during the Superoutburst of the WZ Sge-Type Dwarf Nova, V455 Andromedae

R. Matsui, M. Uemura, A. Arai, M. Sasada, T. Ohsugi, et al., “Optical and Near-Infrared Photometric Observation during the Superoutburst of the WZ Sge-Type Dwarf Nova, V455 Andromedae,” PASJ, vol. 61, p. 1081, 2009.

We report on optical and infrared photometric observations of a WZ Sge-type dwarf nova, V455 And during a superoutburst in 2007. These observations were performed with the KANATA (V, J, and Ks bands) and MITSuME (g’, RC, and IC bands) telescopes. Our 6-band simultaneous observations allowed us to investigate the temporal variation of the temperature and the size of the emitting region associated with the superoutburst and short-term modulations, such as early and ordinary superhumps. A hot (>11000 K) accretion disk suddenly disappeared when the superoutburst finished, while blackbody emission, probably from the disk, still remained dominant in the optical region with a moderately high temperature (˜8000 K). This indicates that a substantial amount of gas was stored in the disk even after the outburst. This remnant matter may be a sign of an expected mass-reservoir, which can trigger echo outbursts observed in several WZ Sge stars. The color variation associated with superhumps indicates that viscous heating in a superhump source stopped on the way to the superhump maximum, and a subsequent expansion of a low-temperature region made the maximum. The color variation of early superhumps was totally different from that of superhumps: the object was bluest at the early superhump minimum. The temperature of the early superhump light source was lower than that of an underlying component, indicating that the early superhump light source was a vertically expanded low-temperature region at the outermost part of the disk.

mat09v455

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