(vsnet-alert 354, T. Kato)
During the monitoring of dwarf novae at Ouda Station , we noticed this morning (Mar. 10) HS Vir (PG 1341-079) is undergoing a bright outburst (V~13.7). A preliminary analysis of the subsequent time-resolved photometry clearly showed a typical superhump with an amplitude of 0.3 mag. Altouhgh this cataclysmic variable has been suspected to be an SU UMa-type dwarf nova due to its short orbital period (0.0836 day) determined by Ringwald (1993), the present observation first confirmed this suggestion. For a long-term behavior of HS Vir, see also IBVS No. 4193.
Taichi Kato and Ouda team
HS Vir (cf. alert 354) still continues to be bright at around V~13.7, with prominent superhumps with an amplitude of ~0.3 mag. A preliminary analysis of the light curve by the Phase Dispersion Minimization (PDM) method has yielded a superhump period of 0.0887 day, which is also supported by an analysis of times of superhump maxima. The period is by 6.1% longer than the suggested orbital period (Ringwald 1993). However, a shorter one-day alias (0.0812 day) solution can not be completely rejected due to the shortness of observing runs and the poor weather.
Taichi Kato and Ouda team
Outburst CCD image
We obtained a 6-hour photometric run of HS Vir under moderately fine conditions. Based on a preliminary analysis of the whole data set obtained at Ouda Station, we are now confident that the shorter one-day alias given in vsnet-alert 356 is the true superhump period. The best determined value is 0.08077 day.
A new light curve can be reached through the VSNET home page (www.kusastro.kyoto-u.ac.jp/vsnet/ "What's new") or by anonymous ftp (ftp.kusastro.kyoto-u.ac.jp, under pub/vsnet/DNe/HS_Vir, file sh2.gif).
Taichi Kato and Ouda team
The light curve on Mar. 13
(IBVS No. 4193)
HS Vir was discovered as an ultraviolet excess object PG 1341-079 and was subsequently confirmed by spectroscopy to be a cataclysmic variable by Green et al. (1982, 1986). They reported two magnitude variablity, but its nature remained little revealed. Some authors have classified this object as a nova-like star, others as a dwarf nova with a small amplitude.
Photographic photometry was undertaken by Osminkin (1985), who showed existence of relatively abundant short, faint outbursts, and presence of a bright (~12.8 mag) outburst. Unfortunately Osminkin's observations were so sparsely done that the materials were not enough to determine the outburst parameters or the dwarf nova subtype. Howell et al. (1990) tried to get the orbital period of this system by quiescent photometry, but the result remained inconclusive.
Another important observation was carried out by Ringwald (1993). He obtained a number of radial velocity mesurements, and derived the best estimate of the orbital period of 0.0836 day (with possible aliasing problems), which is just below the lower edge of the period gap. This suggests that HS Vir may be a candidate of SU UMa-type dwarf nova (for a review, see Warner 1985). One problem of this suggestion is that HS Vir has much smaller outburst amplitude (mostly 1 mag) which makes a clear contrast to usual SU UMa-type dwarf novae. However, recent discovery of a new subgroup of low outburst-amplitude SU UMa-type dwarf novae, named ER UMa stars or RZ LMi stars (Kato and Kunjaya 1995; Nogami et al. 1995; Robertson et al. 1995; Misselt et al. 1995; hereafter ER UMa stars) has called our attention to HS Vir with a similar outburst amplitude.
Observations were carried out using a CCD camera (Thomson TH 7882, 576 x 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 2 x 2 pixels to one pixel was adopted. An interference filter was used which had been designed to reproduce the Johnson V band. The exposure time was between 30 and 120 s. The frames were first corrected for standard de-biasing and flat fielding, and were then processed by a microcomputer-based automatic-aperture photometry package developed by one of the authors (T.K.). The differential magnitudes of the variable were determined using a local standard star [C1: 13h 43m 24s.84 -08o 10' 25".2 (J 2000.0), V=12.2: The position and magnitude were taken from the Guide Star Catalog]. The constacy of this comparison was checked agianst several stars in the same field (figure 1).
The resultant general light curve is given in Figure 2. The light curve first clearly established the dwarf-nova nature of HS Vir, with a very stable outburst cycle length of 8 days. The observed amplitudes of outbursts were 1.1--1.8 mag, which is clearly smaller than those of usual dwarf novae. Other important points are the shortness of outbursts (one or two days), and the rapid decline up to 1.1 mag/d from the outburst peak. The shortness and the rapid decay of outbursts are very characteristic to normal outbursts of SU UMa-type dwarf novae. The shortest known recurrence time of normal outbrusts in SU UMa stars has been 8 days in YZ Cnc, before the discovery of ER UMa stars. This value of HS Vir closely matches that of YZ Cnc, except outburst amplitudes, which are more similar to those of ER UMa stars. Although we have not yet detected superoutburst nor superhumps in this system, a search for these phenomena would surely brings the long confusing history of HS Vir to and end, and would prove the position of HS Vir among a group of low outburst-amplitude SU UMa-type dwarf novae, i.e. YZ Cnc, ER UMa stars and V503 Cyg (Harvey et al. 1995).
A record of the brightest outburst on JD 2443667 by Osminkin (1985), although severely undersampled, shows that this outburst lasted more than three days, and this outburst may be identified as a superoutburst. Confirmation of superoutbursts and superhumps, and determination of supercycle (recurrence time of superoutbursts) will be a next important step toward full understanding of this rather peculiar dwarf nova.
Part of this work is supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists (T.K).
Green R. F., Ferguson D. H., Libert J., Schmidt M. 1982, PASP 94, 560.
Green R. F., Schmidt M., Liebert J. 1986, ApJS 61, 305
Harvey D., Skillman D. R., Patterson J., Ringwald F. A. 1995, PASP in press
Howell S. B., Szkody P., Kreidl T. J., Mason K. O., Puchnarewicz E. M. 1990, PASP 102, 758
Kato T., Kunjaya C. 1995, PASJ 47, 163
Misselt K. A., Shafter A. W. 1995, AJ 109, 1757
Nogami D., Kato T., Masuda S., Hirata R. 1995, IBVS No. 4155
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
Osminkin E. Yu. 1985, Perem. Zvezdy 22, 261
Ringwald F. A. 1993, Ph. D. thesis, Dartmouth College
Robertson J. W., Honeycutt R. K., Turner G. W. 1995, PASP in press
Warner B. 1985, Interacting Binaries, ed P. P. Eggelton, J. E. Pringle (D. Reidel Publishing Company, Dordrecht) p367
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(V-magnitudes by Misselt in parentheses)
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