161 Acta Polytechnica CTU Proceedings 2(1): 161–164, 2015 161 doi: 10.14311/APP.2015.02.0161 Study of Photometric Variability of Selected SU UMa Dwarf Novae I. Voloshina1, V. Metlov1 1Sternberg Astronomical institute, Lomonosov Moscow State university, Moscow, Russia Corresponding author: vib@sai.msu.ru Abstract In this work we present time-resolved photometry of several poorly-studied dwarf novae during recent superoutbursts. Observations were made with a CCD, mounted on 50- and 60-cm telescopes of the Sternberg Astronomical Institute in Crimea in April - May and October 2012 and June - July 2013. Superhumps were detected in light curves of all the dwarf novae. The amplitudes and periods of detected light variations were calculated. Superhumps evolution was also followed up for all systems and classification is improved. Keywords: cataclysmic variables - dwarf novae - SU UMa stars - outbursts - photometry - light curves - periods. 1 Introduction SU UMa stars represent one of the three sub-types of dwarf novae. Besides the frequent normal outbursts, they could show outbursts of larger amplitudes and much longer duration than the normal ones,- super- outbursts. During superoutbursts they exhibit so- called superhumps,- an increase of system brightness on the small part of the orbital light curve, that re- peats with a period a few percents longer than the or- bital one. The amplitudes of superhumps are around - 0m.3. The orbital periods of SU UMa stars are about 80-180 min. There are two small subtypes of dwarf novae inside of this group: WZ Sge stars with short superhumps period and very long time of outburst re- currence and ER UMa stars with very short intervals between superoutbursts. It was found from observa- tions that evolution of the superhumps period consists of 3 stages: early evolutionary stage with a longer pe- riod of superhumps (stage A), middle stage with sys- tematically varying superhumps period (stage B) and the last stage with a shorter, stable period of super- humps (stage C). All stages are most distinct in WZ Sge systems. The superhumps, seen during the early stages of WZ Sge dwarf novae, are double-wave humps and their periods are close to the orbital ones (Kato, 2002), the ordinary superhumps are one-wave humps. They have the largest amplitude of periodicity and pe- riod slightly longer than the orbital period. According to the tidal-thermal model of Osaki (1996) superhumps arise as a result of accretion disk precession triggered by gravitational disturbances from the secondary compo- nent. The period of superhumps is beat period between the precession period Pprec and orbital period Porb. The present paper is aimed at investigating of su- perhumps phenomenon in selected SU UMa stars. 2 Program Stars The stars for our study were chosen from the VSNet list of dwarf novae undergoing superoutbursts during our observational sets. The criteria for our choice were, first of all, the brightness of an object limited by telescope power and relatively rare superoutbursts (and therefore poorly studied). They are: CSS 121004: 205146-035827 (J2051), V844 Her, PNV J19150199+0719471 (J1915), SDSS J150240.98+3334239 (J1502). 3 Observations The main part of our observations was obtained at the 60-cm telescope with CCD Apogee 47 (528 × 512 pxl, pixel size 12×12µm) in V,R and Rc bands and partly at the 50-cm telescope with a new CCD Apogee Alta-8300 (3326×2504 pxl, pixel size - 5.4 × 5.4µm) in V band in Crimea. The duration of observational sets varied from 3 to 6 h. The accuracy is 2 − 3% for R band, but is less for V band 4 − 5%. The reference stars were taken mostly from AAVSO list. If we used the local stan- dard star from the vicinity of a program star special high precision observations with UBV photometer at the 60-cm telescope were conducted during the nights with good weather conditions for the standard calibra- tion. CCD data were reduced using the MAXIM DL standard package, UBV data - with software developed by V. Lyuty. 161 http://dx.doi.org/10.14311/APP.2015.02.0161 I. Voloshina, V. Metlov 3.1 V844 Her The dwarf nova V844 Her underwent a superoutburst in May, 2012 (vsnet-alert 14525). This object showed frequent outbursts during 2009–2011, but was relatively inactive next year. The superoutburst that we followed up occurred 370 days after the last superoutburst of 2011. Our first observation of V844 Her was conducted on April 22 when this nova was in quiescence (∼ 18m) but restarted later after receiving information about the outburst and lasted practically until the end of the outburst when V844 Her returned to its pre-outburst brightness. Figure 1 show the evolution of superhumps in V844 Her. 6052.40 6052.45 6052.50 6052.55 13.1 12.9 12.7 6053.40 6053.45 6053.50 6053.55 13.2 13.0 12.8 6054.40 6054.45 6054.50 6054.55 13.4 13.2 13.0 JD 2450000+ 04.05.2012 05.05.2012 06.05.2012 V V V Figure 1: V844 Her light curves obtained at the begin- ning of the outburst. Decreasing of superhumps which is clearly seen on the light curve at the bottom indicates that a new stage is approaching. 3.2 SDSS J150240.98+3334239 The discovery of outburst of this dwarf nova was re- ported by VSNet on April 2, 2012. Its optical magni- tude ranges from 13m.7 to 19m.6 during quiescent and outburst phases, respectively. USNO photometry re- veals a deep (2.5 mag) eclipse occurring at a period of 84.24 minutes (Szkody et al., 2006) which evidence that it is a high inclination system. Our observations of J1502 cover 6 nights from April 23 to June 2, 2012. Ob- servational light curves for three of them are presented in Figure 2. They show deep eclipses besides super- humps. Our analysis was restricted by consideration of superhumps phenomenon in this system. Figure 2: Light curves of high inclination system J1502 showing both superhumps and eclipses. 3.3 CSS 121004: 205146-035827 The object was discovered by CRTS on October 4, 2012. Observations provided by Dr. Maehara on October 8 (vsnet-outburst 14685) demonstrate superhumps with Figure 3: Stage B superhumps in the light curves of dwarf nova J2051. an amplitude of 0m.2 and with a short period of 0d.056 in the light curves of this object. We started our ob- servations later, on October 10 because of the bad sky conditions before this date. At this moment the bright- ness of J2051 was 14m.9. The object started to fade rapidly on October 20 with rate 0.37m · d−1 and be- came too faint for observations on October 22 (∼ 18m). Our observations of dwarf nova J2051 in V band are shown in Figure 3. 162 Study of Photometric Variability of Selected SU UMa Dwarf Novae 3.4 PNV J19150199+0719471 On June 1, 2013 E.de Miguel (vsnet-outburst 15466) reported discovery of an outburst of this transient with magnitude ∼ 10m.8. The next day he reported about detection of early superhumps in the light curve of this object with a possible period of 0d.0641(2) (probably stage A). On June 9 the double-peaked superhumps with an average amplitude of 0m.1 and Psh ∼ 0d.0569, were detected by several observers. Spectroscopic ob- servations provided by Echevarria et al.(2013) suggested the high inclination and low massive component,- brown dwarf. Our observations of this nova started on June 4, 2013 and lasted until the middle of July 2013. Dur- ing our observations the brightness of J1915 dropped from 11m.47 at the beginning to 18m (V) at the end of the superoutburst. The light curves of J1915 obtained at the different moments of 2013 superoutburst as an example are shown in Figure 4. 6449.3 6449.4 6449.5 11.8 11.7 11.6 11.5 11.4 R c a 6457.3 6457.4 6457.5 12.5 12.4 12.3 12.2 R c b 6467.3 6467.4 6467.5 13.0 12.9 12.8 12.7 R c c 6482.3 6482.4 6482.5 16.0 15.9 15.8 15.7 R c JD2450000 + Figure 4: Evolution of superhumps in dwarf nova J1915: a - beginning of stage A superhumps; b - stage B superhumps; c - transition from B to C stage; d - post-fading stage superhumps. Scale is the same for all panels. 4 Results For data analysis and search of periodicities we used a special code period98 (Sperl, 1998) and period04 (Lenz & Breger, 2004). The analysis was firstly done for ev- ery observational night and later the mean values for different stages of each star were calculated. For V844 Her although stages A-C were observed, the period of stage A superhumps was not determined, due to limited observations during this stage. Pdot for stage B was pos- itive, as seen in other superoutbursts of this object. In the light curves of dwarf nova J2051 the stages B and C can be distinguished. The superhumps period varia- tions suggests that the object more resemble ordinary SU UMa-dwarf novae than WZ Sge-stars. The numer- ous observations obtained for J1915 (about 6000 images at all) permitted us to follow up superhumps evolution in detail beginning from the first night of our observa- tions, June 4. No superhumps were detected in the light curve of J1915 on this particular night. Subsequent ob- servations provided on the next night, June 5, recorded a periodic variation in the light curve of J1915. We suggest that it could be beginning of the stage A. The superhumps period for different stages was determined. The absence of recorded outburst in the past, large am- plitude of present outburst, values of parameters (like �,Pdot and other) and the pattern of superhumps period variation are typical for WZ Sge-stars. So we conclude that the recently discovered dwarf nova J1915 belongs to the WZ Sge-subtype. Some of the power spectra cal- culated for each star as an example are shown in Figures 5–8. Figure 5: One of the V844 Her power spectra as an example. Figure 6: The power spectrum of J1502, calculated for all 6 nights of our observations. 163 I. Voloshina, V. Metlov Figure 7: The power spectrum of J2051. Figure 8: The power spectrum of J1915. The periods which were determined in frame of this study are collected in Table 1. The orbital periods were taken from the literature. Table 1: Determined periods. Object Psh(stage B) Psh(stage C) Porb V844 Her 0.05590 ± 2 0.05587 ± 3 0.054643 J2051 0.05715 ± 5 0.05680 ± 6 — J1502 0.05891 ± 2 — 0.06944 J1915 0.05833 ± 2 0.05800 ± 3∗ 0.06164 * For J1915 the period in the second column repre- sent the post-fading superhumps period. 5 Summary The main results of our study of SU UMa dwarf nova photometric variability can be summarized as follows: • The time-resolved photometry in V and R bands was obtained for several SU UMa-type dwarf no- vae whose superoutbursts were observed during the 2012–2013 season: – for V844 Her - 10 nights, ∼ 1400 images in V and Rc bands; – for CSS 120004: 205146-03582 - 8 nights, ∼ 1000 images in V and R bands; – for PNVJ19150199+0719471 - 13 nights, about 6000 images both in V and Rc bands; – for SDSS J150240.98+3334239 - 6 nights, ∼ 750 images in V band. • The individual light curves of all program stars were constructed on the base of these numerous observations. Superhumps were detected in the light curves all of them and their evolution dur- ing the outbursts was followed up. • The superhumps periods and amplitudes were de- termined from our data and mean superhumps pe- riods were obtained for each nova (see Table 1). • The amplitude of outburst, phase-averaged pro- file of superhumps and superhumps period varia- tions for CSS 121004: 205146-035827 suggest that this object more resemble ordinary SU UMa-stars than WZ Sge-dwarf nova. • On the base of its outburst properties, the re- cently discovered NV J19150199+0719471 was classified as a dwarf nova of WZ Sge subtype. The obtained results are preliminary and reflect the work in progress. They may be slightly improved by using more sophisticated methods and thorough subse- quent analysis. Acknowledgement I. Voloshina gratefully acknowledges professor F. Giovannelli for the invitation to take part in this con- ference as well as his kind hospitality in Palermo. The authors thank A. Bykov for helping with observations in Crimea in 2012. This work was supported in part by the Program of State Support of Russian Federation Leading Scientific Schools through grant 2374.2012.2. References [1] Lenz, P., Breger, M.: 2005, CoAst 146, 53L. [2] Sperl, M.: 1998, CoAst 111, 1. [3] Szkody, P., et al.: 2006, Astron. J. 131, 973. doi:10.1086/499308 [4] Osaki, Y.: 1996, PASP 108, 39. doi:10.1086/133689 [5] . Kato, T., et al.: 2012, PASJ 64, 21. [6] Kato et al.: 2013, PASJ 65, 23. (Survey IV) 164 http://dx.doi.org/10.1086/499308 http://dx.doi.org/10.1086/133689 Introduction Program Stars Observations V844 Her SDSS J150240.98+3334239 CSS 121004: 205146-035827 PNV J19150199+0719471 Results Summary