(vsnet-obs 2440, J. Pietz)
CY UMa 1996-03-21.871 13.0 -------------------------------------------- Jochen Pietz jpietz@astro.uni-bonn.de http://aibn55.astro.uni-bonn.de:8000/~jpietz
(vsnet-obs 2441, T. Vanmunster)
Dear Colleagues,
I have been able to confirm the outburst of CY UMa, reported by Jochen Pietz, as follows :
1996 Mar 21.999 UT, 13.0 (0.35-m refl., seq: AAVSO);
The previous outburst of CY UMa was reported on Jul 20, 1995 (T. Vanmunster) [see CVC 46], when the object reached mag 13.9. This was a very short outburst. In less than 2 days the object faded below mag 14.4 [see CVC 47].
Since this object is no longer on any alert programme, we will not issue a CVC. Readers interested in the superhump behaviour of CY UMa should read a recent paper by D. Harvey and J. Patterson (Center for Backyard Astrophysics, Columbia University, USA), which appeared in PASP, 107, November 1995. The authors report a superhump period P=0.07210 +- 0.00003 d. The orbital period of CY UMa (0.06798 +- 8x10-5 d) is discussed by I.G. Martinez-Pais and J. Casares in MNRAS, 275, 1995.
Please monitor the present outburst to at least determine if this is a normal outburst or superoutburst.
Best regards,
Tonny Vanmunster
light curve in 1995, from observations reported to vsnet-obs
VSOLJ light curve of the 1988 outburst - first ever visually observed outburst
(vsnet-obs 2443, T. Kato)
Some readers might be interested in the following, which is extracted from my article published in IBVS No. 4236. The paper by Thorstensen et al. referred here was already also published. The orbital period determined by Thorstensen et al. is subtly different from that by D. Harvey and J. Patterson; I think the value by Thorstensen et al. might have been better determined, owing to their longer baseline.
Since there have been only very few observations of late superhumps except those in VW Hyi, systematic observations of post-superoutburst behavior would be fruitful. If the present one really turns out to be a superoutburst, it may provide a good chance owing to its good seasonal condition.
Regards,
Taichi Kato
CY UMa was discovered as a dwarf nova by Goranskij (1977). Little attention had been paid until regular visual monitoring by VSOLJ members detected a long outburst in Jan. 1988 (Kato et al. 1988). From the analysis of visual and photographic light curve during this outburst, they detected a possible superhump period of 0.0593 day, and concluded that CY UMa belongs to SU UMa-type dwarf novae. Since this detection of superhumps was suspected to be severely affected by limits of accuracy of visual and photographic observations, the author has been trying to confirm superhumps of CY UMa by CCD photometry. The observations reported here were done during two long outbursts in Dec., 1991 -- Jan., 1992 and in Mar., 1993.
The observations were carried out using a CCD camera (Thomson TH~7882, 576 $\times$ 384 pixels) attached to the Cassegrain focus of the 60 cm reflector (focal length=4.8 m) at Ouda Station, Kyoto University (Ohtani et al. 1992).
To reduce the readout noise and dead time, an on-chip summation of 3$\times$3 pixels to one pixel was adopted. An interference filter was used which had been designed to reproduce the Johnson {\it V} band. The exposure time was between 20 and 120 s depending on the observing condition. The frames were first corrected for standard de-biasing and flat fielding, and were then processed by a personal-computer-based aperture photometry package developed by the author. The differential magnitudes of the variable were determined against a local standard star GSC 3446.344 (10$^{\rm h}$ 57$^{\rm m}$ 05$^{\rm s}$.38 +49$^\circ$ 37$'$ 30$''$.4 (J2000.0), {\it V}=12.9: The position and magnitude were taken from the Guide Star Catalog). The constancy of this comparison was checked against several stars in the same field.
The first outburst was covered from the terminal stage to its return to near quiescence (Figure 1). The light curve on Dec. 30 (Figure 2) clearly shows superhumps with an amplitude of 0.18 mag. A period analysis of observations on Dec. 29 -- 30 using the Phase Dispersion Minimization (PDM) method (Stellingwerf 1978), after heliocentric correction and removing a linear trend of decline, has yielded a superhump period of 0.0714 $\pm$ 0.0005 day. This observation clearly confirmed the SU UMa-type nature of CY UMa. Additional observations were performed on four nights from Jan. 1 through Jan. 4 just after the star returned to near quiescent brightness. A period analysis gives a theta diagram (Figure 3), which suggests persistence of the superhump period near P=0.0723 day and possible periodicity near 0.0678 day. Although irregular variation and relatively low signal-to-noise ratio have made these periods less confident, one may attribute the variability of the first period to late superhumps and the second possible orbital humps [however, one should note that the latter period may be affected by the one-day alias of the first period].
The second outburst was followed on one night, Mar. 5, 1993. Doubly humped superhumps were clearly detected (Figure 4). A period analysis with the same procedure as in the first outburst has yielded a superhump period of 0.0721 $\pm$ 0.0003 day. A small difference of superhump periods obtained during two different superoutbursts may reflect intrinsic period variation of superhumps. From these observations we may safely conclude that the superhump period of CY UMa is 0.0719 $\pm$ 0.0005 day, which was later independently confirmed during the 1995 superoutburst by Harvey, Patterson (1995), who gave a period of 0.0724 $\pm$ 0.0005 day. The star was recently investigated by two groups by radial velocity study; Mart\'inez-Pais, Casares (1995) gave an orbital period of 0.06795 $\pm$ 0.00008 day, and Thorstensen et al. (1995) 0.06957 $\pm$ 0.00004 day. The value of fractional superhump excess ($(P_{\rm SH}-P_{\rm orb})/P_{\rm orb}$) obtained by our superhump period seems to support the latter period, but near coincidence of the former period with one observed after the first outburst would require additional confirmation of the true orbital period.
The author is grateful to P. Schmeer for notifying us of the outbursts, and J. R. Thorstensen and J. Patterson for providing their preprints. Part of this work is supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.
References
Goranskij V. P. 1977, Astron. Tsirk. No. 942
Harvey S., Patterson J. 1995, PASP, submitted
Kato T., Fujino S., Iida M., Makiguchi N., Koshiro M. 1988, VSOLJ Variable Star Bulletin 5, 18
Mart\'inez-Pais I. G., Casares J. 1995, MNRAS 275, 699
Ohtani H., Uesugi A., Tomita Y., Yoshida M., Kosugi G., Noumaru J., Araya S., Ohta K. et al. 1992, Memoirs of the Faculty of Science, Kyoto University, Series A of Physics, Astrophysics, Geophysics and Chemistry 38, 167
Stellingwerf R. F. 1978, ApJ 224, 953
Thorstensen J. R., Patterson J. O., Shambrook A., Thomas G. 1995, in preparation
[Note added in proof: the discrepancy of the orbital periods between above two groups now seems to be solved: the period of Martinez-Pais and Casares can be interpreted to be a 3-day alias of the Thorstensen's period. see Harvey and Patterson 1996]
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