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

WZ Sge-type dwarf novae are characterized by long recurrence times of outbursts (~10 yr) and short orbital periods (<~ 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, the estimation of the intrinsic population of WZ Sge stars is important for the study of the population and 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.Comment: 9 pages, 5 figures, accepted for publication in PAS

Spontaneously generated atomic entanglement in free space: reinforced by incoherent pumping

We study spontaneously generated entanglement (SGE) between two identical multilevel atoms in free space via vacuum-induced radiative coupling. We show that the SGE in two-atom systems may initially increase with time but eventually vanishes in the time scale determined by the excited state lifetime and radiative coupling strength between the two atoms. We demonstrate that a steady-state SGE can be established by incoherently pumping the excited states of the two-atom system. We have shown that an appropriate rate of incoherent pump can help producing optimal SGE. The multilevel systems offer us more chanel to establish entanglement. The system under consideration could be realized in a tight trap or atoms/ions doped in a solid substrate.Comment: have some difference with published version (please see PRA

The Stern-Gerlach Experiment Revisited

The Stern-Gerlach-Experiment (SGE) of 1922 is a seminal benchmark experiment of quantum physics providing evidence for several fundamental properties of quantum systems. Based on today's knowledge we illustrate the different benchmark results of the SGE for the development of modern quantum physics and chemistry. The SGE provided the first direct experimental evidence for angular momentum quantization in the quantum world and thus also for the existence of directional quantization of all angular momenta in the process of measurement. It measured for the first time a ground state property of an atom, it produced for the first time a `spin-polarized' atomic beam, it almost revealed the electron spin. The SGE was the first fully successful molecular beam experiment with high momentum-resolution by beam measurements in vacuum. This technique provided a new kinematic microscope with which inner atomic or nuclear properties could be investigated. The original SGE is described together with early attempts by Einstein, Ehrenfest, Heisenberg, and others to understand directional quantization in the SGE. Heisenberg's and Einstein's proposals of an improved multi-stage SGE are presented. The first realization of these proposals by Stern, Phipps, Frisch and Segr\`e is described. The set-up suggested by Einstein can be considered an anticipation of a Rabi-apparatus. Recent theoretical work is mentioned in which the directional quantization process and possible interference effects of the two different spin states are investigated. In full agreement with the results of the new quantum theory directional quantization appears as a general and universal feature of quantum measurements. One experimental example for such directional quantization in scattering processes is shown. Last not least, the early history of the `almost' discovery of the electron spin in the SGE is revisited.Comment: 50pp, 17 fig

Superhumps and Repetitive Rebrightenings of the WZ Sge-Type Dwarf Nova, EG Cancri

We report on time-resolved photometric observations of the WZ Sge-type dwarf nova, EG Cnc (Huruhata's variable, see also http://www.kusastro.kyoto-u.ac.jp/vsnet/DNe/egcnc.html), during its superoutburst in 1996-1997. EG Cnc, after the main superoutburst accompanied with development of superhumps typical of a WZ Sge-type dwarf nova, exhibited a series of six major rebrightenings. During these rebrightenings and the following long fading tail, EG Cnc persistently showed superhumps having a period equal to the superhump period observed during the main superoutburst. The persistent superhumps had a constant superhump flux with respect to the rebrightening phase. These findings suggest the superhumps observed during the rebrightening stage and the fading tail are a "remnant" of usual superhumps, and are not newly triggered by rebrightenings. By comparison with the 1977 outburst of this object and outbursts of other WZ Sge-type dwarf novae, we propose an activity sequence of WZ Sge-type superoutbursts, in which the current outburst of EG Cnc is placed between a single-rebrightening event and distinct outbursts separated by a dip. The post-superoutburst behavior of WZ Sge-type dwarf novae can be understood in the presence of considerable amount of remnant matter behind the cooling front in the outer accretion disk, even after the main superoutburst. We consider the premature quenching of the hot state due to the weak tidal effect under the extreme mass ratio of the WZ Sge-type binary is responsible for the origin of the remnant mass.Comment: 16 pages, 15 figures, to appear in PASJ (special issue on VSNET), for more information, see http://www.kusastro.kyoto-u.ac.jp/vsnet

Transient jets in V617 Sagittarii

Some of the luminous Compact Binary Supersoft X-Ray sources (CBSS) have shown indications of jets, also called satellites due to their appearance in the spectra. In V Sagittae (V Sge) stars, the galactic counterparts of the CBSS, such features have been reported only for WX Cen. If V Sge stars are indeed the analogs of CBSS, one may expect transient jet emission in other objects of this class. Spectroscopic observations of the V Sge star V617 Sgr have been made, both at high photometric state and at decline. We show that V617 Sgr presents Halpha satellites at high photometric state with velocities of +/-780 km/s. This feature confirms, once more, the CBSS nature of the V Sge stars, however the details of the spectral characteristics also suggest that the two groups of stars display some intrinsic spectroscopic differences, which are likely to be due to a selection effect related to chemical abundance.Comment: Four pages, accepted to be published as a Letter in A&