"PR Her - a new cataclysmic variable of a WZ Sge-type".

N.A. Katysheva1, S.Y. Shugarov1,2

1) Sternberg State Astronomical Institute, Universitetskij Prosp. 13, 119992, Moscow, Russia 2) Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranská Lomnica, The Slovak Republic

WZ Sge-type dwarf novae

The WZ Sge-type objects are the most extreme subgroup of SU UMa-type DNe with a long (several years) superoutbursts recurrence time. Their orbital periods are the shortest ones observed for SU UMa-type stars. Some of them exhibit a complex post-superoutburst rise of brightness called rebrightening(s), rarely seen in other SU UMa-type DNe. One of the most remarkable signatures of WZ Sge-type objects is a presence of ”early superhumps” during the earliest stages of superoutbursts. This feature is also referred to as orbital superhumps or outburst orbital humps. “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 are the most discriminative feature of the WZ Sge-type objects, and have not been detected in other dNe (see Kato et al., 2009, 2010,.

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).

ReferencesC. Hoffmeister, 1951, Astron. Nachr., Erg. 12, H1, 14 .

D. V Bisikalo,., A.A. Boyarchuk, P.V. Kaygorodov, O.A. Kuznetsov, T. Matsuda, 2005, AIP Conf. Proc. 797, 295

T. Kato, 2002, Publ. Astron. Soc. Japan, 54, L11.T. Kato, H. Maehara, M.Uemura, M.,et al., 2010, PASJ, 62,

1525T. Kato, H. Maehara, I, Miller, et al. 2012a, PASJ, 64, 21T. Kato et al., 2012 b, PASJ, (in press)T. Kato, A. Imada, et al., 2009, Publ. Astron. Soc. Japan, 61S,

395Y. Osaki, 1989, Publ. Astron. Soc. Japan, 41, 1005Y. Osaki, F..Meyer, 2002, Astron. Astrophys. 383, 574R. Whitehurst, 1988, Mon. Not. R. Astron. Soc. 232, 35

Fig. 1. The overall d(BVR) light curves (LCs) of PR Her from November, 22 till December. 10.

Fig. 5. The nightly light curves (dBVRc and R-bands).

Fig. 2. The periodograms for the days 3 – 5 after outburst (AO) – time of “early superhumps” (top) and for “ordinary superhumps” (bottom). The significant peaks are marked by arrows . The value of the period in days is showed.

We folded the data of different segments of time series of PR Her with the period of “early superhumps” 0.0542 d and “ordinary superhumps” 0.05497 d. The results are shown in Fig. 3 and 4, accordingly . The “early superhumps” are presented in the form of a double wave for the period.

Corresponding ephemeris are: max = 2455894.3252 + 0.0542 *E (early) and max = 2455899.2045 + 0.05497*E (ordinary)

The CCD observations of PR Her

Our observations of PR Her were taken at the Crimean laboratory of SAI (the CCD-camera Apogee-47a mounted in the 0.6m Zeiss-reflector) and at the Stara Lesna observatory of AI SAV (CCD-camera ST-10 mounted in the 0.5m reflector). We got ….. of UBBVRcRjI – CCD frames from November 24 to December 10, 2011.

Kato et al. (2012b) noted that “the due to the unfavorable location, the object soon became hard to access in the low evening sky”. But they found the period of “early superhumps” 0.05422 d and “ordinary superhumps” and qualified PR Her as a WZ Sge-type dwarf nova.

In Fig.1 we present the nightly LCs in BVRc-bands during the plateau of superoutburst (Nov 24 – Dec 10, 2011). The brightness of the PR Her declined by approximately 2 magnitudes in 16 days after the beginning of our observations.

A Fourier analysis of BVRc data after removal of the declining trends revealed the presence of “”early” and “ordinary” superhumps. Unfortunately, it was impossible to observe the end of superoutburst. The periodograms fot the beginning and plateau stage of superoutburst are plotted in Fig. 2.

Early, ordinary and late superhumps

The “early superhumps” are a remarkable feature of WZ Sge-type stars. They appear near the maximum magnitude of superoutburst and have periods almost identical to the orbital one. Osaki and Meyer (2002) suggested that a double peaked profile of “early superhumps” is manifestation of the tidal 2:1 resonance in accretion disks of binary systems with extremely low mass ratios. They can be explained by a two-armed spiral pattern of tidal dissipation generated by the 2:1 resonance. According to Kato (2002), the expansion beyound the 3:1 resonance radius can be responsible for the appearance of “early superhumps”. The common, or “ordinary superhumps” with a single peak profile, appear during the plateau of superoutburst of WZ Sge-type stars. Their periods are a few percent longer than the orbital period. The ordinary superhumps can be explained by the thermal tidal instability model of an accretion disk (Osaki, 1989; Whitehurst, 1988). 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. This model is supported by numerical simulations (Bisikalo et al., 2005).

"Accretion flow instabilities: 30 years of the thermal-viscous disc instability model“, Warsaw, Poland, 4-7 September 2012

Fig. 4. The typical one-hump phase light curve of the “ordinary superhumps”. (delta BVR and mean, folded with the period of 0.05497 d).

Fig. 3. The typical two-hump phase light curve of the “early superhumps”. (delta BVR and mean, folded with the period of 0.05419 d).