Study of Photometric Variability of Selected SU UMa Dwarf Novae

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 50and 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.


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,-superoutbursts.During superoutbursts they exhibit socalled superhumps,-an increase of system brightness on the small part of the orbital light curve, that repeats with a period a few percents longer than the orbital one.The amplitudes of superhumps are around -0 m .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 recurrence and ER UMa stars with very short intervals between superoutbursts.It was found from observations that evolution of the superhumps period consists of 3 stages: early evolutionary stage with a longer period of superhumps (stage A), middle stage with systematically varying superhumps period (stage B) and the last stage with a shorter, stable period of superhumps (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 period 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 component.The period of superhumps is beat period between the precession period P prec and orbital period P orb .
The present paper is aimed at investigating of superhumps phenomenon in selected SU UMa stars.

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).

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 R c 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 standard star from the vicinity of a program star special high precision observations with U BV photometer at the 60-cm telescope were conducted during the nights with good weather conditions for the standard calibration.CCD data were reduced using the MAXIM DL standard package, U BV data -with software developed by V. Lyuty.

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 (∼ 18 m ) 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.
Observations provided by Dr. Maehara on October 8 (vsnet-outburst 14685) demonstrate superhumps with an amplitude of 0 m .2 and with a short period of 0 d .056 in the light curves of this object.We started our observations later, on October 10 because of the bad sky conditions before this date.At this moment the brightness of J2051 was 14 m .9.The object started to fade rapidly on October 20 with rate 0.37m • d −1 and became too faint for observations on October 22 (∼ 18 m ).
Our observations of dwarf nova J2051 in V band are shown in Figure 3.
Study of Photometric Variability of Selected SU UMa Dwarf Novae

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 every 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.P dot for stage B was positive, 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 variations suggests that the object more resemble ordinary SU UMa-dwarf novae than WZ Sge-stars.The numerous 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 observations, June 4.No superhumps were detected in the light curve of J1915 on this particular night.Subsequent observations 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 amplitude of present outburst, values of parameters (like , P dot 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 calculated for each star as an example are shown in Figures 5-8.

Figure 1 :
Figure 1: V844 Her light curves obtained at the beginning of the outburst.Decreasing of superhumps which is clearly seen on the light curve at the bottom indicates that a new stage is approaching.

Figure 2 :
Figure 2: Light curves of high inclination system J1502 showing both superhumps and eclipses.

Figure 3 :
Figure 3: Stage B superhumps in the light curves of dwarf nova J2051.
June 1, 2013 E.de Miguel (vsnet-outburst 15466) reported discovery of an outburst of this transient with magnitude ∼ 10 m .8.The next day he reported about detection of early superhumps in the light curve of this object with a possible period of 0 d .0641(2)(probably stage A).On June 9 the double-peaked superhumps with an average amplitude of 0 m .1 and P sh ∼ 0 d .0569,were detected by several observers.Spectroscopic observations 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.During our observations the brightness of J1915 dropped from 11 m .47 at the beginning to 18 m (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.

Figure 4 :
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; dpost-fading stage superhumps.Scale is the same for all panels.

Figure 5 :
Figure 5: One of the V844 Her power spectra as an example.

Figure 6 :
Figure 6: The power spectrum of J1502, calculated for all 6 nights of our observations.