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1
Content available remote On the Periods and Nature of Superhumps
EN
It is commonly accepted that the periods of superhumps can be satisfactorily explained within a model involving apsidal motion of the accretion disk provided the frequency of the apsidal motion in addition to the dynamical term includes also the pressure effects. Using a larger sample of systems with reliable mass ratios it is shown, however, that this view is not true and the model requires further modifications.
2
Content available remote Superhumps and their Evolution during Superoutbursts
EN
Light curves of superhumps and their evolution during superoutbursts are analyzed by decomposing them into their Fourier components, including the fundamental mode and the first three overtones. The amplitudes of the fundamental mode are found to decrease significantly during superoutburst while those of the overtones remain practically constant. The phases of maxima of the fundamental mode increase systematically during superoutburst while those of the overtones - systematically decrease. The combination of the two effects is responsible for the characteristic evolution of superhump light curves: the appearance and growth of the secondary humps and the spurious phase jumps in the (O-C) diagrams. Two intrepretations are possible. Either that instead of just one superhump period Psh there are four periods Pk which resemble - but are significantly different from - the fundamental mode and the first three overtones of Psh. Or - more likely - that those time-dependent phase shifts are genuine.
3
Content available remote One Thousand New Dwarf Novae from the OGLE Survey
EN
We present one of the largest collections of dwarf novae (DNe) containing 1091 objects that have been discovered in the long-term photometric data from the Optical Gravitational Lensing Experiment (OGLE) survey. They were found in the OGLE fields toward the Galactic bulge and the Magellanic Clouds. We analyze basic photometric properties of all systems and tentatively find a population of DNe from the Galactic bulge. We identify several dozen of WZ Sge-type DN candidates, including two with superhump periods longer than 0.09 d. Other interesting objects include SU UMa-type stars with "early" precursor outbursts or a Z Cam-type star showing outbursts during standstills. We also provide a list of DNe which will be observed during the K2 Campaign 9 microlensing experiment in 2016. Finally, we present the new OGLE-IV real-time data analysis system: CVOM, which has been designed to provide continuous real time photometric monitoring of selected CVs.
4
Content available remote On the Periods of Negative Superhumps and the Nature of Superhumps
EN
Osaki and Kato interpreted variations of the negative superhump periods, discovered by them in dwarf nova V1504 Cyg, as evidence in favor of the thermal-tidal instability model for superoutbursts. It is shown that their interpretation was incorrect. The observational evidence is recalled showing that superoutbursts are due to the enhanced mass transfer rate.
5
Content available remote On the Nature of Superhumps
EN
Further evidence is presented supporting the alternative interpretation of superhumps as being due to irradiation modulated periodically variable mass transfer rate. NZ Boo, HT Cas and PU UMa are added to the sample of high inclination dwarf novae showing - during their superoutbursts - modulation of the observed brightness of the disk with beat period. Simple model calculations confirm earlier hypothesis that this modulation is due to a non-axisymmetric structure of the outer parts of the disk, involving the azimuthal dependence of their geometrical thickness, rotating with the beat period. The modulation amplitude Amod is found to decrease during superoutbursts. In particular, it is found that during two superoutbursts of OY Car the rate of decline of the superhump amplitude dASH/dt was correlated with the rate of decline dAmod/dt. This leads to a simple explanation for the decreasing amplitudes of superhumps: it is a consequence of the decreasing modulation amplitude.
6
Content available remote On the Eclipsing Cataclysmic Variable Star HBHA 4705-03
EN
We present observations and analysis of a new eclipsing binary HBHA 4705-03. Using decomposition of the light curve into accretion disk and hot spot components, we estimated photometrically the mass ratio of the studied system to be q=0.62±0.07. Modeling of the system yields the following fundamental parameters: white dwarf mass M1=(0.8±0.2) Msun, secondary mass M2=(0.497±0.05) Msun, orbital radius a=1.418 Rsun, orbital inclination i=(81.°58±0.°5), accretion disk radius rd/a=0.366±0.002, and accretion rate M=(2.5±2)x1018 [g/s] (3×10-8 Msun/yr). Power spectrum analysis revealed a set of possible low-period Quasi Periodic Oscillations centered at the frequencies f1=0.00076 Hz, f2=0.00048 Hz and f3=0.00036 Hz. The system B-V=0.04 mag corresponds to a dwarf nova color during an outburst. The light curves of HBHA 4705-03 suggest that it is a nova-like variable star.
7
Content available remote Stream Overflow in Z Cha and OY Car during Quiescence
EN
Eclipses of the hot spot in Z Cha and OY Car observed by many authors during their quiescence are re-analyzed. Distances of the spot from the center of the disk rs are determined from phases of ingress and egress. In the case of several eclipses of Z Cha and nearly all eclipses of OY Car it is found that rs(egress)s(ingress). This is interpreted as being due to the effects of stream overflow and implies that those values of rs are not representative for the radius of the disk rd. The rs(ingress)-dt relations (where dt is the time since last outburst) are improved when points affected by the stream overflow are omitted.
EN
Contribution from the irradiated secondary component is detected in the light curves of five dwarf novae observed during superoutbursts. Their superhump light curves show that irradiation is modulated with the superhump phase. This strengthens the new interpretation of superhumps as being due to the irradiation controlled mass transfer rate resulting in modulated dissipation of the kinetic energy of the stream.
9
Content available remote The Cluster AgeS Experiment (CASE). Variable Stars in the Globular Cluster M55
EN
We report time-series photometry for 55 variable stars located in the central part of the globular cluster M55. The sample includes 28 newly identified objects of which 13 are eclipsing binaries. Three of these are detached systems located in the turn-off region on the cluster color-magnitude diagram. Two of them are proper motion (PM) members of M55 and are excellent candidates for a detailed follow-up study aimed at a determination of the cluster age and distance. Other detached binaries are located along the unevolved part of the cluster main sequence. Most of the variable stars are cluster blue straggler stars. This group includes 35 SX Phe stars, two contact binaries, and one semi-detached binary. V60 is a low mass, short period Algol with the less massive and cooler component filling its Roche lobe. The more massive component is an SX Phe variable star. The orbital period of V60 increases at a rate of dP/P=3.0×10-9. In addition to numerous variable blue stragglers we also report the detection of two red stragglers showing periodic variability. Both of these are PM members of M55. We note and discuss the observed paucity of contact binaries among unevolved main sequence stars in M55 and NGC 6752. This apparent paucity supports an evolution model in which the formation of contact binaries is triggered by stellar evolution at the main-sequence turn off.
10
Content available remote Superhumps and their Amplitudes
EN
Superhump amplitudes observed in dwarf novae during their superoutbursts depend on orbital inclination: the maximum amplitudes in systems with low inclinations are A0≈0.25 mag, while at higher inclinations they increase from A0≈0.3 mag to A0≈0.6 mag. The mean maximum superhump amplitudes normalized to the average luminosity of the disk are: n>=0.34±0.02 mag in low inclination systems and only n>=0.17±0.01 mag in high inclination systems. This shows that at high inclinations the superhump light source is partly obscured by the disk edge and implies that it is located close to the disk surface but extends sufficiently high above that surface to avoid full obscuration. Superhump amplitudes in high inclination systems show modulation with beat phase (φb), interpreted as being due to azimuth-dependent obscuration effects in a non-axisymmetric disk. In addition they show modulation with 2φb which implies that the orientation of the superhump light source is correlated with the direction of the stream. The dependence of superhump amplitudes on orbital inclination and their modulation with beat phase eliminate the tidal-resonance model for superhumps. Instead they support an alternative interpretation of superhumps as being due to periodically modulated dissipation of the kinetic energy of the stream. Superhump amplitudes in permanent superhumpers are =0.12 mag, i.e., much smaller than the maximum amplitudes observed during superoutbursts.
EN
The mass transfer rate in SS Cyg at quiescence, estimated from the observed luminosity of the hot spot, is log Mtr=16.8±0.3. This is safely below the critical mass transfer rates of log Mcrit=18.1 (corresponding to log Tcrit0=3.88) or log Mcrit=17.2 (corresponding to the "revised" value of log Tcrit0=3.65). The mass transfer rate during outbursts is strongly enhanced.
12
Content available remote 2009 Superoutburst of Dwarf Nova 1RXS J053234.9+624755
EN
We present photometric observations of the dwarf nova 1RXS J053234.9+624755. We performed a detailed analysis of the superoutburst that occurred in August 2009. We found the superhump period to be Psh=0.057122(14) days. Based on the O-C diagram we conclude that Psh increased during the plateau at the rate of dPsh/dt=(9.24±1.4)×10-5. Both the O-C analysis and evolution of the superhumps light curve favor the model in which superhumps originate in a variable source located in the vicinity of the hot spot. In addition, the evolution of the light curve suggests that the superhump light source approaches the disk plane as the superoutburst declines. Detailed analysis of the superoutburst plateau phase enabled us to detect a signal which we interpret as apsidal motion of the accretion disk. We detected additional modulations during the final stage of the superoutburst characterized by periods of 104 s and 188 s which we tentatively interpret as quasi periodic oscillations. Estimations of A0 and An are in agreement with the dependence between the amplitude of superhumps and the orbital inclination.
13
Content available remote Enhanced Mass Transfer in U Geminorum during its Outbursts and Superoutburst
EN
The mass transfer rate in U Gem at quiescence, estimated to be M≈1.3-2.0×1016 g/s, is used to calculate the amount of mass ΔMtr transfered to the disk during quiescence. Light curves of U Gem are used to estimate the amounts of mass ΔMaccr accreted during its three types of outbursts. In the case of wide outbursts and the 1985 superoutburst ΔMaccr are much larger than ΔMtr, indicating significant enhancement in the mass transfer by a factor of f≈20-50. There is no evidence for comparable enhancement during narrow outbursts.
EN
We report the identification of a new cataclysmic variable (denoted as CV2) and a probable microlensing event in the field of the globular cluster M22. Two outbursts were observed for CV2. During one of them superhumps with Psh=0.08875 d were present in the light curve. CV2 has an X-ray counterpart detected by XMM-Newton. A very likely microlensing event at a radius of 2.'3 from the cluster center was detected. It had an amplitude of Δ V=0.75 mag and a characteristic time of 15.9 days. Based on model considerations we show that the most likely configuration has the source in the Galactic bulge with the lens in the cluster. Two outbursts were observed for the already known dwarf nova CV1.
15
Content available remote The 1985 Superoutburst of U Geminorum. Detection of Superhumps
EN
Superhumps are detected in the AAVSO light curve of the 1985 superoutbursts of U Gem. They appeared not later than 2-3 days after reaching maximum and disappeared not later than about 4 days before the final decline. The superhump period was Psh≈0.20 d and increased at a rate of dP/dt≈2×10-4. The corresponding superhump period excess was ε=0.130±0.014. The full amplitude of the superhumps was 2A≈0.3 mag. During the last ten days of the superoutburst additional periodic variations were also present. Their period was ≈0.18 d and their full amplitude grew from 2A≈0.2 mag to ≈0.5 mag. U Gem, together with the permanent superhumper TV Col (Retter et al. 2003), form a challenge to the theory which is unable to explain superoutburst and superhumps in systems with long orbital periods and mass ratios q>qcrit=1/3. Another challenge to the theory comes from a comparison of the theoretical ε-q relation resulting from numerical simulations (Murray 2000) with its observational counterpart: for q>0.15 the model values of ε are systematically - by a factor of 2 - too large.
16
Content available remote On the Enhanced Mass Outflow during Dwarf Nova Outbursts
EN
During dwarf nova outbursts the secondary component is irradiated by the boundary layer and the white dwarf. The problem is whether the hotter material from irradiated regions can reach the vicinity of the inner Lagrangian point L1, producing an enhanced mass outflow. Osaki and Meyer (2003, 2004) presented a model in which the pressure gradient in the direction towards L1 is always exactly balanced by the Coriolis force and, consequently, does not produce any acceleration and motion in that direction. Instead the material is predicted to flow in the perpendicular direction along the shadow boundary. It is shown that this model, as based on an arbitrary assumption, is incorrect. My model (Smak 2004) produced flow trajectories directed (due to the pressure gradient) toward the equator, but deflected (due to the Coriolis force) with respect to the direction toward L1. In systems with short orbital periods the effects of the Coriolis force were not large enough, however, to prevent the material from reaching L1. Criticism of this model by Osaki and Meyer (2004), concerning incomplete treatment of the equation of continuity, is shown to result primarily from misunderstandings.
EN
Model calculations describing the flow of material heated by irradiation into the shadow zone around L1 are described. In dwarf novae with very short periods (P≈1-2 hrs) the resulting enhancement in the mass outflow rate is very large: M/M0≈10-30. In systems with periods just above the period gap (P≈4 hrs) the enhancement is smaller: M/M0≈2-6. In systems with longer periods (P≈6 hrs), however, it turns out that the flow does not reach the vicinity of L1 and, consequently, the enhancement is negligibly small. This means that irradiation induced enhanced mass outflow plays important role in SU UMa stars but is negligible in Z Cam stars.
EN
Observational data for five dwarf novae of the SU UMa type provide several strong arguments in favor of a significant enhancement in the mass transfer rate during their outbursts and superoutbursts. The mass transfer rates during normal outbursts are estimated from the amount of accreted mass assuming the theoretical mass transfer rate at quiescence, based on models involving gravitational waves. The resulting rates Menh≈0.6-1.8×1017 g/s imply enhancement by a factor of 15-46, which is consistent with observational evidence for VW Hyi and with theoretical predictions. The mass transfer rates during superoutbursts are enhanced to Menh≈3-6×1017 g/s. The corresponding enhancement ratios range from ≈20-60 (when the mass transfer rates at quiescence are obtained under assumption of no enhancement during normal outbursts) up to ≈100 (resulting from the assumption of the theoretical mass transfer rate during quiescence).
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