AAVSO NEWS FLASH No. 4
(vsnet-alert 318, Paula Szkody)
We will be having a multiwavelength campaign (with 3-4 X-ray and UV satellites) to study the disk and boundary layer changes throughout the supercycle of V1159 Ori that will begin Feb 8 and run through March 28. Optical magnitudes to delineate where we are in the cycle would be greatly appreciated (as well as any spectra with Balmer lines, especially Hbeta or Halpha). Any immediate attention to this object would also be very helpful to define what should be the start of the superoutburst phase. Further info can be obtained from Paula Szkody at: szkody@astro.washington.edu
(D. Nogami, T. Kato, S. Masuda, R. Hirata 1995 IBVS No. 4155)
This star was discovered to be variable by Wolf and Wolf (1906), called 36.1906, and then suggested by Kippenhahn (1953) to be a possible member of U Gem stars. In the New Catalogue of Suspected Variable Stars (Kukarkin et al. 1982), it was designated as NSV 02011 with a brightness range of mpg = 13.3 -- 14.2. The subsequent photographic observation by Natsvlishvili (1984) revealed that its magnitude varied from 12.5 at maximum to 16.6 at minimum, and it was finally named V1159 Ori of INS: type in the 68th namelist of variable stars (Kholopov et al. 1987).
Jablonski and Cielinski (1992) made spectroscopic and photometric observations and proposed its orbital period of 0.05890 +/- 0.00001 days, which suggests that V1159 Ori is a good candidate for an SU UMa star. Their observations, however, could not reveal superoutbursts or superhumps expected for this class of CVs. The peculiar nature of V1159 Ori was already suggested by their estimate of short outburst cycle length of 4.35 days together with its small outburst amplitude. VSOLJ (Variable Star Observers League in Japan) started CCD and visual observations, part of which was published by Kiyota (1993). These observations indicated presence of bright (reaching V~12.1) outbursts, some of which bore the charactersitics of superoutbursts.
The present outburst of V1159 Ori was first detected by the RoboScope, Indiana University Automated Photometric Telescope (Honeycutt and Turner 1992). It was independently discovered on Dec. 23 at V=12.9 by M. Yamada (VSOLJ; private communication) We made photometric observations on 1994 Dec. 23 and 24 using CCD camera (Thomson, TH7882CDA, 576x384 pixels with 23 micron square pixel size) attached to the Cassegrain focus of 0.6-m reflector with Johnson V-band filter at the Ouda Station, Kyoto University (Ohtani et al. 1992). The mode of 2x2 on-chip summantion was employed. A total of 225 frames were taken between 23.154 and 23.270 UT under clear sky, and 57 frames between 24.144 and 24.264 UT interrupted for 82 minutes by clouds. The exposure time was 30 seconds on Dec. 23 and varied between 60 and 180 seconds on Dec. 24.
We reduced the data using the personal-computer-based aperture photometry package developed by one of the authors (T.K.). This package automatically substracts bias-frames, applies flat fielding and enables us to estimate the instrmental magnitudes. The aperture size was 9" in radius. The sky level was determined from pixels whose distance from the indivisual objects are between 24" to 48".
Figure 1 shows the light curve of differntial magnitude between V1159 Ori and a comparison star on Dec. 23. The comparison star is "c" in Jablonsky and Cielinski (1992), whose V magnitude is given as 11.99 in thier Table 1. A decay of amplitude of humps is seen in the figure and the similar modulation was also observed in PG~0943+521 (Nogami et al. 1995). We analysed the light curve, using phase dispersion minimization (PDM) method (Stellingwerf 1978) implemented in IRAF package (IRAF is distributed by National Optical Astronomy Observatories, U. S. A.). Figure 2 shows the Theta diagram, whose abscissa is frequency (/day). The lowest minimum point in Theta corresponds to 0.06764 days. Robertson et al. (1995) independently detected the same hump features and reported that they repeated with a period of 0.067 +/- 0.001 days.
The difference in the photometric and spectroscopic periods confirmed the superhump nature of these humps. The outburst observed by Jablonsky and Cielinsky (1992) did not show superhump-like modulation and corresponds to normal outburst of usual SU UMa-stars. Thus, V1159 Ori was unambiguously identified to be an SU UMa-type dwarf nova with a superhump period of 0.06764 +/- 0.0001 days. The present discovery of superhumps in V1159 Ori, together with its extreme shortness of its outburst recurrence time, its long duty cycle (Jablonski and Cielinsky 1992, Kiyota 1993), and its extremely short supercycle (recurrence time of superoutbursts) (Robertson et al. 1995), has established a new subgroup of peculiar dwarf novae whose prototype is PG 0943+521 (Kato and Kunjaya 1995, Misselt and Shafter 1995, Osaki 1995).
The fact that the supercycle of V1159 Ori (44.5 days, Robertson et al. 1995) is almost equal to that of PG 0943+521 (43 days) is possibly not accidental. According to Osaki (1995), the shortest supercycle predicted by disk-instability theory under a given strength of the tidal torques is about 40 days and the supercycle is insensitive to the mass transfer rate from the secondary around this minimum value. However, as first pointed out in the case of PG 0943+521 by Kato and Kunjaya (1995) and later confirmed by numerical simulation by Osaki (1995), a dwarf nova with minimum supercycle length should have much (~ten times) larger mass transfer rate than those of usual SU UMa-type dwarf novae, near the borderline between nova-like stars and SU UMa-type dwarf novae. For a dwarf nova with an orbital period below the period gap, it is generally believed that its mass transfer is powered by the gravitational-wave radiation, that is, its rate is mainly dependent on the orbital period. The shortness of the orbital period of V1159 Ori (0.05890 days, Jablonsky and Cielinsky 1992) and the superhump period of PG 0943+521 (0.06549 days, Kato and Kunjaya 1995) implies small mass transfer rates in the present scheme of CVs, which are clearly in conflict with such large mass-transfer rate expected from their outburst behavior.
The present discovery of "another" PG 0943+521 star suggests that this type of CVs may not considered to be unique but occupy a larger population in CVs than ever expected. In this case, the present picture of the mechanism of mass transfer and the evolution of CVs below the period gap would unavoidably be modified.
The authors are grateful to the VSOLJ (Variable Stars Observers League in Japan) members for supplying us of visual and CCD estimates, and especially M. Yamada for notifying us of the outburst. Thanks also to Y. Osaki, K. A. Misselt and R. K. Honeycutt for sending us their preprints. We acknowledge R. K. Honeycutt for openning the real-time light curve by RoboScope to public through the World-Wide Web. This research has been partly supported by Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists (T.K.).
References (published status at the time of early 1995)
Honeycutt, R. K., and Turner, G. W. 1992,in Robotic Telescopes in the 1990's, ed. A. Filippenko,(San Francisco, ASP), 277
Jablonsky, F. J., and Cielinsky, D. 1992, Astron. Astrophys., 259, 198
Kato, T., and Kunjaya. K. 1995, PASJ, in press
Kholopov, P. N., Samus', N. N., Kazalovets, E. V., and Kireeva, N. N. 1987, IBVS No. 3058
Kippenhahn, R. 1953, Astron. Nachr., 281, H4,153
Kiyota, S. 1993 in "Variable Stars" (in Japanese) Japan Variable Star Study Association, Vol. 23
Kukarkin, B. V., et al. 1982, in "New Catalogue of Suspected Variable Stars", ed. P. N. Kholopov (Nauka Publishing House, Moscow)
Misselt, K. A., and Shafter, A. W. 1995, Astron. J., in press
Natsvlishvili, R. Sh. 1984, IBVS No. 2565
Nogami, D., Kato, T., Masuda, S., and Hirata, R. 1995, in preparation
Ohtani, H., Uesugi, A., Tomita, Y., Yoshida, M., Kosugi, G., Noumaru, J., Araya, S., Ohta, K. 1992, Memoirs of the Faculty of Science, Kyoto University, Series A of Physics, Astrophysics, Geophysics and Chemistry, 38, 167
Osaki, Y. 1995, PASJ, in press
Robertson, J. W., Honeycutt, R. K., and Turner, G. W. 1995, PASP, submitted
Stellingwerf, R. F. 1978, ApJ, 224, 953.
Wolf, M., and Wolf, G. 1906, Astron. Nachr., 171, 77
Complete material with figures
The 1995 Feb. superoutburst of V1159 Ori; a transition from normally outbursting phase to a superoutburst is clearly visible. ( Ouda Station )
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