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Taichi KATO, Chatief KUNJAYA, Masami OKYUDO and Atsushi TAKAHASHI
(PASJ 1994)
Abstract
A faint dwarf nova, KV And, was observed for two nights during its long outburst in 1993 November. V-band CCD photometry revealed superhumps with a period of 0.07520 (+/- 0.00003) d or its alias 0.07427 (+/- 0.00003) d, and an amplitude of 0.28 mag. This discovery confirms KV And as being a new member of SU UMa-type dwarf novae, and makes it as a good candidate for being a TOAD (Tremendous Amplitude Dwarf Nova).
1. Introduction
KV Andromedae is a poorly studied faint dwarf nova discovered by Kurochkin (1977), who recorded four outbursts from the Sternberg plate collection. The object was reported to be barely seen in quiescence on the Palomar Sky Survey Plate; the range of the photographic variability was given as 14.6 -- 22.5: in the General Catalogue of Variable Stars.
Among dwarf novae (DNe), a subtype of cataclysmic variables (CVs), there exists a group having peculiar characteristics called TOADs (Tremendous Outburst Amplitude Dwarf Novae; Howell 1993) or WZ Sge-type stars (Bailey 1979; Downes 1990). The characterstics of TOADs can be summarized as follows:
1) By definition, the TOADs show exceptionally large outburst amplitudes (>6 mag) in contrast to 2 -- 5 mag in usual dwarf novae.
2) Outbursts are very infrequent. The typical recurrence time may be on the order of one year, or even decades.
3) Almost all well-studied objects are SU UMa-type dwarf novae (for a review of SU UMa stars, see Warner 1985), and almost all outbursts of the TOADs seem to be superoutbursts.
4) TOADs have been predominantly discovered at high galactic latitude.
From a theoretical point of view, TOADs may represent the terminal stage of the evolution of CVs, which are characterized by a short orbital period, and a low mass-accretion rate. The peculiar outburst pattern of TOADs may be naturally explained within the scheme of the disk-instability model with a very low mass-transfer rate and low viscosity in quiescent accretion disks (Osaki 1994).
According to the discovery report, KV And is a dwarf nova having one of the largest outburst amplitudes (~7.9 mag), and would be a good candidate for a TOAD. A determination of the physical parameters of this system has therefore been highly desired.
2. Observations
V-band CCD observations were carried out on 1993 November 16 and 22 using a 60 cm reflector and a Thomson TH7882 chip (576x384 pixels) at Ouda Station, Department of Astronomy, Kyoto University (OSDAK; for a description of the instruments see Ohtani et al. 1992) and on November 16 using a 60 cm reflector and an EEV P8603 chip (578$\times$416 pixels) at Nishi-Harima Astronomical Observatory (NHAO). The parameters of data acquisition for inidividual observing runs are summarized in table 1.
These frames were first corrected for standard de-biasing and flat fielding. The Ouda frames were then processed by a microcomputer-based automatic-aperture photometry package developed by one of the authors (T.K.). The Nishi-Harima frames were processed by the APPHOT task in the IRAF package (IRAF is distributed by the National Optical Astronomy Observatories, U.S.A.).
The differential magnitudes of the variables were determined using a local standard star (C$_1$: 02$^{\rm h}$ 17$^{\rm m}$ 19$^{\rm s}$.93 +40$^\circ$ 39' 50".9 (J2000.0), V=12.6). A comparison of the local standard star and three (Ouda) or four (Nishi-Harima) check stars ranging V=12.0 and V=14.1 has confirmed the constancy of the standards during a run, and gives the expected standard error in the differential magnitudes for the variable as 0.02 mag for a single frame for both observatories.
Although both observatories used filters which were designed to reproduce the Johnson V band, there remained a systematic difference in magnitudes of KV And. Since the differences between the standard and the check stars were found to be in good agreement between two sites, we have attributed the systematic difference of KV And as being an effect of the extremely blue color common to outbursting dwarf novae. Since we have no information of color variation during our observation, we have chosen to merge our observations simply by shifting the Nishi-Harima observations by a constant to match the more abundant Ouda data. The shift value has been determined so that the light curves of individual observatories are connected smoothly at their overlapping piont. For the following described analysis, a constant offset of 0.250 mag was subtracted from the Nishi-Harima data. We note that although this treatment may introduce a small bias to the resultant light curve, the effect is very small in a period analysis, since the wavelength dependence of superhump light curves are known to be small in the visible-light range.
3. Detection of Superhumps and its Implications
The present outburst of KV And was first detected by M. Iida (1993, private communication) using a unfiltered ST-6 CCD camera attached to a 16 cm reflector. He estimated the variable at 14.3 mag on 1993 November 15.432 UT. He also reported that the variable was fainter than 17.5 mag on 1993 November 9.492 UT. This outburst was subsequently confirmed by CCD observations of the authors. The variable showed a gradual linear decline at a rate of 0.13 mag/d, which is clearly visible in the CCD light curve (figure 1). This rate of outburst decline is typical for a superoutburst of SU UMa-type dwarf nova.
The {\it V}-band light curve on 1993 November 16 constructed from Ouda and Nishi-Harima observations, is shown in figure 2. Periodic humps with an amplitude of 0.28 mag (slightly variable) can be clearly seen. The humps show a steeper rise and a gradual decline, as in other SU UMa-type stars early in their development of superhumps. No secondary humps were detectable at the superhump phase between 0.4 and 0.6, which are sometimes seen at a later stage of a superoutburst in some SU UMa stars.
A period analysis was first applied to the continuous set of the November 16 data using the phase dispersion minimization (PDM) method (Stellingwerf 1978) implemented in the IRAF package. The best estimate of the period was 0.0747 (+/- 0.0010) d. The error of the period was estimated from the residuals of least-squres fit to the times of the maxima, minima, and the mid-rising points.
Secondly, we applied the PDM to all available data on November 16 and 22, after removing the trends of steady decline, and yielded better estimates of the period. The theta-diagram is shown in figure 3, which shows numerous minima resulting from aliasing due to a large (6 d) gap between two sets of observations. The lowest minimum is seen at a frequency of 13.30/d which corresponds to a period of 0.07520 d. The adjacent minimum is at a frequency of 13.46/d, corresponding to a period of 0.07427 d. These two most likely candidate periods are within the error estimate from the Novevmber 16 data, and are equally acceptable. The error of these periods was estimated to be 0.00003 d just as was done for the November 16 data.
The variable is thus confirmed to be a new member of SU UMa-type dwarf novae with well-determined superhump periods. The accurate position of KV And has been determined as 02$^{\rm h}$ 17$^{\rm m}$ 13$^{\rm s}$.84 +40$^\circ$ 41' 30".0 (J2000.0) using four GSC stars in the same field. This astrometry during an outburst confirms the quiescent identification by Downes and Shara (1993). KV And is very faintly visible on their photographic finding chart. Although we should wait for additional well-calibrated photometry in quiescence to suggest an accurate outburst amplitude of this object, the available materials have strengthened the idea that KV And is a TOAD, and the hypothesis that all TOADs are actually SU UMa-type dwarf novae. Further monitoring for outbursts of this object is therefore encouraged in order to determine its outburst frequency and number ratio of super- and normal outbursts, both of which would also be good indicators for discriminating TOADs from other dwarf novae (see section 1).
The superhump period of KV And (1.79 +/- 0.03 hr) is rather typical for SU UMa-type dwarf novae (Ritter, Kolb 1993). Among known SU UMa systems with similar superhump periods, KV And resembles HT Cas and V1251 Cyg in terms of a large outburst amplitude. Both of them are known to show infrequent outbursts, and would also be good candidates for TOADs. Despite the fact that TOADs have been predominantly discovered in the high galatic latitude region, relatively abundant similar systems, such as HT Cas and V1251 Cyg, at the low galactic latitudes would encourage a more extensive search for galactic-plane counterparts of TOADs in order to clarify the population and evolutionary status of TOADs among caraclysmic variables.
The authors are grateful to Makoto Iida (VSOLJ) for notifying us of the outburst, and providing the astrometric program. Part of this was is supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.
References
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