N.  Katysheva,    D.  Chochol,    S.  Shugarov      

Sternberg  Astronomical  Institute  of  the  Moscow  State  University,  Russia  Institute  of  the  Slovak  Academy  of  Sciences,  Tatranská  Lomnica,  Slovakia  

 

Observing techniques, instrumentation and science for metre-class telescopes, September 23-26, 2013, Tatranská Lomnica, Slovakia

Cataclysmic variables (CVs) are highly evolved close binaries consisting of a red dwarf transferring a matter to his companion – a white dwarf. This matter creates an accretion disc around the white dwarf. CVs are subdivided into two large subclasses – dwarf novae (dNe) and nova-likes variables (NL). Dwarf novae usually live in quiescence but sometimes flare up. Their outbursts is caused by accumulation a matter in accretion disc and possibly a sudden increasing of mass rate. Outbursts of dNe last a few days. In turn, dNe are subdivided into some subclasses too. SU UMa-type stars is one of the subclasses of dNe. Short outburst and more prolongated superoutbursts, which have very special features on the light curves – superhumps with the periods

exceed the orbital periods on 1 – 3 %, are the prominent peculiarity of SU UMa class. Its orbital periods are below 2 h, on the average. In turn WZ Sge-type stars is a subclass of SU UMa and, therefore dNe. WZ Sge-systems have very short orbital periods (on an average, 80–90 min), large and rare superoutbursts (years or tens years), and very specific light curve features – “early”, “ordinary” and “last” superhumps. Some of them exhibit a complex post-superoutburst rises of brightness called rebrightening(s), or echo-outburst(s), whose nature is unclear by now.

Different types of Close binaries.

K. Petrov-Vodkin, Still life. From Mac Dougall’s

One of the most remarkable signatures of WZ Sge-type objects is a presence of “early” superhumps during the earliest stages of superoutbursts. “Early” superhumps have a period extremely close to the binary period and commonly show a double–humped profile, in contrast to the “ordinary” superhumps of SU UMa-type dwarf novae. “Early” superhumps have not been detected in other dNe. Osaki & Meyer (2002, A & Ap, 383, 574) suggested that a double peaked profile of “early” superhumps is a manifestation of the tidal 2:1 resonance in accretion disks of binary systems with extremely low mass ratios. “Early” superhumps can be

explained by a two-armed spiral pattern of tidal dissipation generated by the 2:1 resonance, first proposed by Lin & Papaloizou (1979, MN, 186, 799). “Ordinary” superhumps, with a single hump profile, can be explained by the thermal tidal instability model of an accretion disk (Osaki, 1989, PASJ, 41, 1005; Whitehurst, 1988, MN, 232, 35). The presence of the tidal 3:1 resonance in the disk (with the radius smaller than the 2:1 resonance radius) results in the formation of an eccentric outer ring undergoing apsidal precession with a period appreciably longer than the orbital one.

The beating of the orbital and precessional periods cause periodic variations, identified as superhumps. “Late” superhumps are proposed to originate in the precessing eccentric disk near the tidal truncation. The eccentric disk slowly expands during the decline of the superoutburst and finally reaches the tidal truncation, where the period is stabilized (Kato et al., 2008, PASJ, 60L, 23). The year before Y. Osaki, J. Smak, J. Cannizzo, F. Meyer and E. Meyer-Hofmeister discussed very much about the crucial points of the different theories on the conference in Warsaw. This discussion added a new impulse to a creation of new papers in 2013. But each of the theories has own pluses and minuses. Large observational and statistical bulk of information was collected in “Pdot-Survey” by Kato et al. (Kato et al., 2009, 2010, 2012, 2013 – PASJ, 61S, 395; 62, 1525; 64, 21; 65, 23).

Most of the WZ Sge-binaries have been discovered in a last few years. There about 60 WZ systems now. The sequence: “early” superhumps ~ “ordinary” superhumps ~ “late” superhumps was observed for all of them except

PNV J18422792+4837425. The periods of the WZ Sge-type stars are close to the so called period minimum of CVs which has to be near 78 min (observational) (Hellier, 2001, Cataclysmic Variable Stars…) and 65 – 70 min (theoretical) (Rappaport et al., 1982, ApJ, 254, 616, Kolb & Baraffe, 1999, MN, 298, L29). The periods of the AM CVn stars with ultracompact components and a hydrogen-deficient degenerate secondaries are below (from 5 till 65 min). Thus the WZ Sge systems are very interesting for investigation of the boardeline case of the star evolution.

An example of “ordinary” superhumps in V466 And = a WZ Sge-system which bursted in 2009. A saw-tooth structure is general for plateau of superoutburst (Chochol et al., 2010, Contr. Astron. Obs. Skalnate Pleso, 40, 19).

In recent years we observed and investigated next WZ Sge- objects (in collaboration with CrAO, SAO RAS, MAO NAN, Japanese astronomers, VSNET etc): V466 And – a paper in CAOSP: Chochol et al., 2010, v. 40, 19. CT Tri − a paper in CAOSP: Chochol et al., 2009, v. 39, 43. OT J213806.5+261957.3 in Pegasus − a paper in CAOSP: Chochol et al., 2012, v. 42, 39. OT J18422792+4837425 in Draco − a paper in CEAB: Katysheva et al., 2013, v. 37, 35. SDSS J104411+211307 in Leo – a report in Japan (2010) EZ Lyn = SDSS J080434.2+510349.2 – several papers PR Her – a report in Poland, 2012 OT J102146.44+234926.3 – a paper in PASJ: Uemura et al., et al., 2008, v. 60, 227.

OT J174033.5+424756 in Hercules– a report by D. Chochol et al. in Palermo (2013) – a system with a period of 65 min!!! OT J19150199+0719471 in Aquila − a report by Golysheva, Shugarov et al. (this conference) Master J203749+552210.3 in Cygnus – the end of 2012 SBS 1108+57 = CSS120422:111127+571239 – the shortest orbital period of the class (55 min), 2012, May. It is a unique system (Kato et al., 2013, Littlefield et al., ApJ, 2013, 145, 145; Carter et al., Pavlenko et al. (in preparation)). V455 And – very interesting WZ Sge-star with pulsations, possible intermediate polar, superhumps… Several papers. V1363 Cyg – in a work, SDSS J1339+4847 – in a work, Master J203749+552210.3 in Cygnus – in a work. A part of our data were included in the “Pdot Survey of SU UMa stars…” (Kato et al., 2009, 2010, 2012, 2013)

Dwarf nova Pegasi 2010 (DNP2010 = J2138+26)

was discovered as a bright optical transient by Japanese amateurs. The object was of 10.8 mag oon May 6.77, on May 7.76 it reached 8.4 mag. Our early UBVRcIc CCD observations of the J2138+26 were taken in the Terskol Observatory (in Caucasus) with both the Zeiss-600 and MEADE 0.35m telescope. Further observations were obtained with the 0.5m reflector at the Stará Lesná AI SAS observatory and the Zeiss-600 and 1.25-m telescopes of SAI MSU. In this case we couldn’t observe the early stage of superoutburst and hadn’t “early” LCs. Crimea began our observations Laboratory of the Sternberg Astronomical Institute. We

The UBVRc LCs of the DNP2010 = J2138+26. The number of days after outburst are marked on the top axis. The superhumps' stages are designated by arrows. It is brighter in U-band during superoutburst and in minimum.

The nightly LCs with “ordinary” superhumps are presented. (P sh = 0. 055106 d = 79.35 min).

The nightly LCs with “late” superhumps. Psh = 0.05490 d

The nightly LCs with “late” superhumps in the last stage of outburst and in quiescence.

The analysis of our data revealed the presence of “ordinary” superhumps with the mean period of 0. 055106 d and “late” superhumps with the period of 0.05490 d. The orbital period of the DNP2010 was calculated 0.0542±0.005d and estimated the mass of the red dwarf as 0.090±0.01 Msun. The period change of superhumps is in agreement with other WZ Sge-type stars. Superhump period increase: Pdot = dP/dt /P = +6.2*10^(-5) during the stage B. The superoutburst of the DNP2010 finished at the end of September 2010. In October and November 2010 night-to-night brightness variations as well as a flickering with an amplitude 0.2 mag on a time scale of minutes were detected. The colour temperature of the hot component during the superoutburst increased from 10 000 to 24 000 K, temperature declined to 16 000 K in day 132 AO. (about two-colour diagrams – below).

CT Tri = Drawf nova in Triangulus 2008

The UBV (RI)c photometry of the dwarf nova Tri 2008 = CT Tri = OT J023839.1+355648 was obtained during its superoutburst started on October 25, 2008 (14.12–21.7 mag). The object can be classified as a WZ Sge-type dwarf nova. The “early” superhumps with the period 0.0531 days and “ordinary” superhumps with the period 0.0537 days were discovered by Shugarov et al. (2008, IBVS, 5862).The period of “early” superhumps 0.05307 days = 76.42 min was detected in the first 7 days of the superoutburst. The period of “ordinary” superhumps 0.053663 days, detected in days 8–23 of the superoutburst, was the shortest one among WZ Sge-type objects. After one month, the dwarf nova returned to its pre-outburst state.

The UBV (RI)c light curves of CT Tri during its superoutburst.

The phase diagram of “early” superhumps of dN Trianguli = CT Tri (a period 76.42±0.50 min).

A sudden increase of activity of the object during quiescence was detected on January 11, 2009, when the brightness of the object increased in the Rc-passband for about 1 mag. In January the V − Rc color was 0.3±0.15 mag, compared to V − Rc = –0.01 mag in the superoutburst brightness maximum. It should be noted that the several ”minioutbursts” were detected a few months after the superoutburst of the star SDSS J080434.20+510349.2 (a new WZ Sge-type system) (Zharikov et al., 2008, A&A, 486, 505). They repeated after 16–17 days (Pavlenko & Malanushenko, 2009, KPCB, 25, 48). We can suppose that such behavior is typical for quiescence of some WZ Sge-type stars.

Dwarf Nova Leonis 2010 = CSS100217:104411+211307 = OT J104411.4+211307

was discovered on February 16, 2010 (Drake et al. 2009, ApJ, 696, 870). The object reached 12.5 mag at maximum. At quiescence it was ~19 mag. We began to observe on February 17, 2010 and carried out the observations at 3 observatories – in Slovakia (Stará Lesná), in Ukraine (Crimean Laboratory of the SAI) and in Russia (Mt. Terskol). The 4700 observational frames in UBVRcIc-bands were obtained. Most of them in V-band (3600 frames) and Rc-band (670 frames). The overall light curves in UBVRc–bands are plotted on the next figure. One rebrightening was detected during the March 15–17, 2010.

m

Double wave of “early” superhumps. JD max=2455251.4943+0.06053*E

A classic saw-form of “ordinary” superhumps during some nights. On the right – phased LCs.

The mean phase light curves of “early” and “ordinary” superhumps in particular nights and their evolution are presented on the next figures.

The phased LCs of “late” superhumps during some nights.

The evolution of superhumps is clearly seen with disappearance of the double-wave and change of its amplitude. The peak of main hump drifts. The “ordinary” superhumps are more stable than the “early” superhumps.

The stage of “late” superhumps lasted from JD 2454258 (March, 2) till …264 (March, 8). Double wave with an unstable amplitude were detected. The average value of the superhumps period began to decrease.

There is no information for any previous outburst of this object. The typical change of superhump period with time. Superhump period excess: ε =1-Psh/Pearly = 0.020 is typical for SU UMa and WZ Sge objects. Color-indices are typical for cataclysmic variables, too. Large amplitude rebrightening (~ 3 magnitude).

And now about WZ Sge stars with more complex behavior

OT J18422792+4837425 – a WZ Sge system in Draco

PNV J18422792+4837425 was discovered by H. Nishimura on the 5th of September, 2011 as a

new transient with U = 11.8 mag. �  In the early days of the superoutburst the “early” superhumps

with a period of 0.0591(4) days were seen. But later on they had either very small amplitude (<0.01 mag), or disappeared.

�  Up to September 24, 2011 narrow superhumps appeared and then vanished.

�  During the decline stage of superoutburst there were no visible “ordinary” superhumps. This behaviour is very unusual for WZ Sge-type systems.

Our observations: 1. Stará Lesná, Slovakia – 50 cm, Zeiss-600 telescopes 2. Mt. Koshka, Crimea – Zeiss-600, Zeiss-1000 3. Nauchny, Crimea – Zeiss-600, AZT-11 (125 cm), K-38 (38 cm) 4. Mt. Terskol, Russia – 35 cm, Zeiss-600 telescopes 5. Apache Point Obs., USA – 50 cm telescope ARCSAT 6. Nyzhny Arkhyz, Russia – 35 cm, Zeiss-1000 telescope We got more than 11 000 CCD-frames in UBVRcRjIcIj-bands.

. UBVRI during the 1st rebrightening

The total duration of the 1st rebrightening was about 20 days, and the plateau phase ~ 14 days. It is uncommon. The second rebrightening was narrow and lasted ~ 3-4 days. The shape of the upper part of rebrightening is very similar to the plateau of superoutburst. Kato et al. (2012) suppose that it was not the echo-outburst but the sequent outburst. We detected the second rebrightening which was missed by another observers.

The nightly R-light curves during the plateau of superoutburst (left – beginning, JD 2455811, 12, 13; middle – 25, 26, 30) and rebrightening (right – 41, 42, 43).

The periodogram for the rebrightening stage. The peak at 0.07275 day is clearly seen. VSNET gives the period from 0.07 to 0.079 d.

It was established that a new dwarf nova in Draco certainly belongs to the WZ Sge type. We have no information for any previous outbursts of our object. We found the arguments for the second rebrightetning. The mean period of the “ordinary” superhumps P = 0.07275 d. It was very difficult to find the typical change of the superhump period with time because of unsufficiency of clear-cut humps. We also determined a period in quiescence 0.0724 d which can be close to the orbital one (104 min). It will be observed that the system had strong UV-excess even at minimum of brightness between rebrightenings. It should also be noted that superhump period is long because the most of WZ Sge stars have the superhump periods < 0.06 d.

EZ Lyn = SDSS J080434.20+510349.2

(also a report by Sergey Zharikov this morning)  

EZ Lyn = SDSS J080434.20+510349.2 (hereafter SDSS J0804) was first discovered as a CV and was considered as a potential dwarf nova with an underlying white dwarf (WD) in quiescence prior to the 2006 outburst (Szkody et al. 2006) and it was first found in outburst by Pavlenko et al. (2007, ASP Conf. Ser., v. 372, 511). Its 0.060-d superhump period and 0.059005-d (= 84.97 min) orbital period suggested SDSS J0804 as a period bouncer, passed the period minimum (Zharikov et al., 2008, A&A, 486, 505). Pavlenko (2009, JPhCS, 172, 2071) first discovered the non-radial pulsations of the WD that appeared 8 months after the 2006 outburst and lasted for 2 years.

             

Among the accreting pulsators that have experienced a dwarf novae outburst, SDSS J0804 has the most dramatic history of events in a short time scale. During 2006–2010 interval J0804 underwent two outbursts in 2006 and 2010, accompanied by 11 and 6 rebrightenings consequently (Pavlenko et al. 2007, ASPC, 372, 511; Kato et al. 2011, PASJ, Pavlenko et al., MemSAI, 83, 520). Additionally a series of mini-outbursts have been observed in December, 2006 – January, 2007 (Zharikov et al. 2008, A&A, 486, 505). This CV gives a unique opportunity to study the evolution of the WD pulsations under such rare conditions.

B-V

J0804 experienced the first superoutburst in 2006 and the next one in 2010. It is very unusual that the number of echo-outbursts (rebrightenings) were different. Another WZ Sge stars with large number of rebrightenings , like WZ Sge, usually have the same number. The inspection of Sonneberg and Odessa photo plates was done. Only one outburst was found on Odessa plate (12.5 mag) in 1979.

B-V

Two outburst light curves of SDSS J0804 in 2006 and 2010 yrs.

The J0804 outburst during the March 04, 2006–March 04, 2007. The rebrightenings are given in the cut-off. The different stages of outburst are marked by letters: The main outburst A, rapid decline B, rebrightenings C, next rapid decline D, slow return to quiescence E.

. A group of echo-outburst (rebrightenings) in 2006.

. “Early” superhumps in SDSS J0804 (outburst of 2010). Upper: PDM analysis. Lower: Phase-averaged profile (from Kato et al., 2012, PASJ, 64, 21). P = 0.5897 d

. “Ordinary” superhumps in SDSS J0804 (outburst of 2010). Upper: PDM analysis. Lower: Phase-averaged profile (from Kato et al., 2012, PASJ, 64, 21). P = 0.05958 d

.

The post-rebrightening phase (after BJD 2455494.0) after subtracting the global variations of rebrightenings. Phase-averaged profile, P = 0.05901 d corresponds to the orbital period .(from Kato et al., 2012, PASJ, 64, 21).

V455 And (= HS2331+3905) was discovered by Gaensicke et al. from the Hamburg Quasar Survey. Araujo-Betancor et al. (2005, A & A, 430, 629) analyzed their data described the main characteristics of this. They defined HS2331 as a cataclysmic variable with very wide spectra of the periods – orbital (81.08 min) and superhump ones (83.38 min), non-radial pulsations of white dwarf (5 – 6 min) and a coherent signal at 1.12 min. The spectroscopic period was about of 3.5 hour. They proposed a possible superoutburst of HS2331+3905 as WZ Sge-type binary. And superoutburst happened on September 5, 2007.

V455 And = HS2331+3905

V455 And = HS2331+3905

V455 And = HS2331+3905

The nightly and mean orbital phase light curves (R-band). The ephemeris: JDmin = 2451812.67765 + 0.05630921*E. The 13th, 15th and 18th of November 2007

PR Her is a very poor studied cataclysmic variable. At first PR Her was discovered as a dwarf nova (S 4247) by Hoffmeister (1951) with a photographic range of 14.0– 17.5. In 1999 A. Henden identified the object as a V =21-mag blue star (vsnet-chat 1800). PR Herculis bursted on November 21, 2011, reached V=12.84 and was classified as a new WZ Sge-type system (Kato et al., 2012, PASJ, v. ). Our observations of PR Her were taken at the Crimean laboratory of SAI (Zeiss-600) and at the Stara Lesna observatory of AI SAS (the 0.5m reflector).

PR Her

The existance of “early superhumps” (P = 0.05422 d) and “ordinary superhumps” qualified PR Her as a WZ Sge-type dwarf nova. The brightness of the PR Her declined by approximately 2 magnitudes in 16 days after the beginning of our observations. Unfortunately, it was impossible to observe the end of superoutburst.

The overall d(BVR) light curves (LCs) of PR Her from November, 22 till December. 10, 2011.

The typical two-hump phase light curve of the “early” superhumps. (delta BVR and mean, folded with the period of 0.05419 d = 78.05 min).

The typical phase light curve of the “ordinary” superhumps. (delta BVR and mean, folded with the period of 0.05497 d).

for “ordinary” superhumps (right). The significant peaks are marked by arrows .

The periodograms for the days 3–5 after outburst (AO) – time of “early” superhumps (left) and

CSS130418: 174033.5+414756 in Hercules

Discovery: CRTS (Catalina Real-Time Transient Survey), time: 2013, April 18.47, brightness: 14.04, ASAS-SN (Prieto et al., 2013, ATel 4999) time: 2013, April 19.53, V = 12.7. CRTS circular: The transient had an outburst in 2007: 7.5 mag and lasted two months. Previous outburst: GALEX UV source detected in the GALEX DR5 catalog, with FUV = 13.62 and NUV = 14.17 mag on 05/27/2007. Our observations: UBV(RI)C CCD observations: 0.6-m telescope - Stará Lesná, 0.4-m, 0.6-m, 1-m telescopes - Crimea, 1-m telescope – SAO.

CCD photometry during the superoutburst

“Early” superhumps, JDmax = 2456402.076 + 0.045026E

Period 64.84±0.01 min (~Porb) is under the observed period minimum ~ 78 min for normal hydrogen rich CVs.

“Ordinary” superhumps, JDmax = 2456402.076 + 0.0452269E

J174033.5+414756 – Results: •  New sub-period-minimum CV with the Porb = 64.84±0.01 minutes. •  Fractional superhump excess ε = [Psh /Porb] – 1 = 0.0111(2) determined from photometry. •  Mass ratio (Patterson’s formula) q = 0.063 ±0.001 •  For the average white dwarf mass M1 = 0.83 Msun (Zorotovic et al. 2011) this implies a mass of the donor M2 = 0.052 Msun. •  Helium rich object. •  Evolution – mass transfer in WZ Sge system near the end of the main sequence lifetime of the donor star. Evolved donor has been stripped of most of their hydrogen by mass-transfer. V485 Cen (Augusteijn et al. 1996), Porb = 57.6 min, CSS 120422 (Littlefield et al., 2013, AJ, 145, 145; Carter et al., 2013, MN, 431, 372; Pavlenko et al., in preparation), Porb = 55.4 min.

And at last a few words about SBS 1108+574: •  The outburst of last year SU UMa star SBS 1108+574 = CSS 120422:111127+571239. • Catalina Sky Survey announced about outburst of SBS 1108+574 on 2012, April 22. • Light curve is very similar the WZ Sge-type stars. • But Kato et al., 2013, Carter et al., 2013, Littlefield et al., 2013 classified it as SU UMa or strong evolved dNe. • Spectroscopy showed a helium rich spectrum. • The orbital period is 0.03845 d = 55.3 min • The superhumps period was about 0.03911 d • It’s a possible progenitor of AM CVn stars.

SBS 1108+574. The average LCs of outburst in 2012 (from Pavlenko et al., in preparation)

And two-color diagrams of some above-mentioned systems.

Two-colour diagrams U-B,B-V and B-V, V-R. The evolution track of the J174033.5+414756 during superoutburst. The numbers – days after outburst.

The two-colour diagram U-B,B-V for OT J2138+2619

The two-colour diagram for CT Tri.

The two-colour diagram for OT J104411.4+211307

The two-colour diagram for J1842+4837

The small telescopes are very useful for the observations of superoutburst of WZ Sge-stars.

Thanks !

Supe-humps amplitude, O – C and dV vs Cycle number for dN Pegas