(vsnet-alert 332, Caffey James)
While testing the feasibility of starting a dwarf novae monitoring program at Southwest Missouri State University, we observed BZ UMa just a few hours ago (near 9 h UT, Feb. 18) and found it to be approx. 3 magnitudes above the listed minimum of v=17.8 in Downes and Shara (1993). This is an estimate based on the comparison stars given in the latest Februrary PASP article by Misselt. We have not yet reduced the data. We were observing with a good Photometrics CCD on a 16-inch.
Since we are newcomers to the study of dwarf novae, we have no good idea of how significant this may be. We only noticed that no period is listed for this star, so we surmise that its behavior is not well known. We hope someone finds this information useful! We were clouded out before we could finish observing it (got B and V only), and it looks like we will be clouded out for the next several days.
Claia Bryja Jim Caffey
There is a very good article by Jurcevic et al. (1994, PASP, 106, 481) treating of the long term behavior of BZ UMa. The present reported value may be within its "usual" quiescent range of 15-16 mag (or maybe slightly above its upper limit depending on the exact value). The extreme value of 17.8 mag was only once recorded by Kaluzny (1986, IAU Circ. No. 4287), which might still need confirmation. I also think there is a good chance that BZ UMa may be experiencing its early rise to a outburst. Close follow-up observations would reveal the truth.
In 1994 October (just the moment when the star was in outburst) we had a discussion concerning the "peculiarity" of BZ UMa, and what the word "peculiar" means in CVs. I have just dug out the corresponding articles from the VSNET log where they have been buried. Since they are too lengthy to be posted here, I will soon make them available through the VSNET Home Page (http://www.kusastro.kyoto-u.ac.jp/vsnet/).
Well, we've now fully reduced last night's data on BZ UMa, and found
V = 15.88 +- 0.07 B-V = -0.02 +- 0.12
The rough estimate that we made on-site was based only on peak counts, and turned out to be about a magnitude brighter than the truth. Sorry about that. This is still well above the minimum of 17.8, but as we *now* know from Jarcevic et al. (thank you Dr. Kato for the reference!) the normal range for this star is near to V=15. If anything, it's a bit below normal right now, though it seems to vary by up to a magnitude on timescales of a day or two. There's certainly no need for any special alert. (Of course, having said that, it will probably superoutburst two days from now! 8-)
Sorry for the false alarm. It wouldn't have happened if we'd had better references from the start.
Claia Bryja email@example.com Jim Caffey firstname.lastname@example.org
Patrick Schmeer reports an outburst of BZ UMa. I have just confirmed the observation. Details........
Oct 07.076 UT <13.9 P. Schmeer 20cm SCT Oct 07.966 11.4 Schmeer Oct 07.979 11.3 G. Poyner 22cm
My records show the last outburst was recorded on Oct 24 1993 at 11.5 (J. Pietz). This is quite a long gap. Has anybody got details of one in between?
It would be highly desirable to obtain time-resolved photometry of this dwarf nova to search for superhumps. BZ UMa has an orbital period of 97.8 minutes (below the period gap), and has outbursts only rarely, perhaps once per year or less (Jurcevic J. S., Honeycutt R. K., Schlegel E. M., Webbink R. F., 1994, PASP, 106, 481; Ringwald F. A., Thorstensen J. R., Hamwey R. M., 1994, MNRAS, in press). Its rare outbursts all seem to be of extreme amplitude and duration, and may well be superoutbursts: BZ UMa is probably a low-luminosity SU UMa star, and may only have superoutbursts. It is therefore likely to have superhumps.
Finding charts are given by Downes R. A., Shara M. M., 1993, PASP, 105, 127 and by Bruch A., Fischer F.-J., Wilmsen U., 1987, A&AS, 70, 481.
Coordinates are: 08 49 52.5 +57 00 04 (Epoch 1950.0) 08 53 44.1 +57 48 41 (Epoch 2000.0)
It is currently up late, after midnight at most Northern Hemisphere sites. I have telescope time next week at Kitt Peak; a multi-longitude study could make resolving period aliases easier.
Visual magnitude estimates by P. Schmeer, Bischmisheim, Germany (203-mm SCT, AAVSO seq.): BZ URSAE MAJORIS (UGSU:) Oct. 7.076 UT, (13.9 ; 7.967, 11.4 ; 7.976, 11.3 ; 8.064, 11.4 ; 8.096 UT, 11.4 ; 8.176, 11.2
The most recent superoutburst was detected by me on 1990 July 9.94 UT at mv= 12.2: and was confirmed by Gary Poyner on July 10.96 UT at mv= 12.4 (cf. IAUC 5051). BZ UMa is not well placed in July, and the true maximum was probably missed.
Dr. Fred Ringwald wrote:
| Its rare outbursts all seem to be of extreme amplitude and duration,
| and may well be superoutbursts: BZ UMa is probably a low-luminosity
| SU UMa star, and may only have superoutbursts.
It may not be the case for BZ UMa, at least this object is not a very ordinary SU UMa star. This star shows infrequent bright normal outburst; a representative one in 1992. The following data are taken from IAUCs.
1992 10/23.963UT, [13.7(Poyner); 26.052, 10.7 (Poyner); 26.979, 11.7 (P.Schmeer, Germany); 27.810, 12.4(W.Worraker, Didcot); 28.721, 13.2 (Worraker).
We have additional two sets of CCD photometry of similar short outbursts. From these records, we may conclude that the normal outbursts of this dwarf nova are more abundant than superoutbursts.
We have no further information on 1992 outburst, but if the identification of a superoutburst by P. Schmeer is correct, superoutbursts of this star may be _fainter_ than the normal outbursts. Such behavior is sometimes observed in TOADs (Tremendous Outburst Amplitude Dwarf Novae), but the amplitude of BZ UMa is less than five magnitudes (typical quiescent magnitude V=15.5). How can one explain all these facts?
In addition, BZ UMa showed a long period without detectable outbursts (Dr. Mattei of may pick it up from the AAVSO records ..), and at least one occasion when the system went dwon to B=17.8 probably during this period (the exact time and reference I don't remember). Changing mass-trnasfer rate?
Anyway we are looking forward to watching what happens during the current (already bright) outburst. The Ouda team may observe after the runs of IP Peg and TT Ari, but the unfavorable sky location (and very unfortunate cloudy sky!) may not allow us an enough time coverage.
Taichi Kato and Ouda team (Kyoto University)
There seems to be a lot of misinformation about BZ UMa, some of which I have propagated myself. First, it turns out that while any outbursts are rare, BZ UMa does have normal outbursts as well as superoutbursts: see the long-term lightcurve of Jurcevic et al. 1994, PASP, 106, 481.
Second, while there were no confirmed outbursts between 1974 and 1990, the AAVSO lightcurve for this period that I have (Mattei J. A., 1989, private communication) is sparsely sampled. Many normal or even superoutbursts could have been missed.
It is also unclear that superoutbursts do not get as bright as normal outbursts, since so few outbursts of any type have been observed. Again, this may be just sparse sampling: more monitoring is definitely needed.
Last, BZ UMa may sometimes get as faint as B = 17.8, but this has been seen only once (Kaluzny J., 1986, IAU Circ. No. 4287). Confirmation is needed, and will require a CCD.
Still, if this is a superoutburst, it should be searched for superhumps. So please continue observing it!
Dear Dr. Fred Ringwald,
I agree with you in that the star has been too sparsely sampled to draw a definite conclusion.
I have picked up the paper by Jurcevic et al. and examined the light curve. The authors state that all the three observed outbursts have durations shorter than ten days, and we can fairly firmly conclude that these outbursts are all "normal" (short) outbursts, and not (at least typical) superoutbursts. The bightest one reached at least V=10.2, which already outranged the catalogued limit. If one assumes the superoutbursts are brighter than the normal outbursts by 0.5 or 1 magnitude in this system (analogy with other SU UMa systems), one should conclude that superutbursts have historically escaped detection. I am eager to know how P. Schmeer has identified the 1990 outburst (there was a confusion in my previous mail with the 1992 outburst) as a superoutburst, because we know only two visual positive observations. If that outburst turns out to be a normal one, this star may not have shown any superoutbursts at all. In this case one may expect a very bright superoutbursts, probably brighter than V=10; otherwise this system may be classified as a peculiar DN-like CVs below the period gap, like EX Hya or V4140 Sgr. Don't you think existence of high excitation emission lines like He II, C III/N III may somewhat contradict with the "low luminosity SU UMa-type" picture?
I still think this system is peculiar.
Cloudy night in Japan! Wish to know how this star behaves tonight.
| Don't you
| think existence of high excitation emission lines like He II, C III/N III
| may somewhat contradict with the "low luminosity SU UMa-type" picture?
No, because the continuum is very weak from the low mass transfer rate, so all lines are strong. The equivalent width of H alpha is over 200 Angstroms! He II and CIII/N III are still much weaker than H beta, not like in a magnetic CV or in an old nova, where it can be comparable or stronger. But the smoking gun is that in high-S/N red spectra, I detected the M5.5 +/- 0.5 secondary star (Ringwald F. A., 1993, Ph.D. thesis, Dartmouth College; also Ringwald F. A., Thorstensen J. R., & Hamwey R. M., 1994, MNRAS, in press). At an orbital period of 97.8 minutes, an M5.5 dwarf would fit inside the Roche lobe, so that with an M5.5 dwarf at the observed dilution of 4.5%, this implies an absolute magnitude in R of 10.7 (+0.7/-0.9).
It still needs photometry, too, both monitoring and time-resolved.
| I still think this system is peculiar.
Since when was there any such thing as a normal cataclysmic variable? ;-)
BZ UMa Observations (source: The Astronomer) 1990-7 9: 12.2, 10: 12.4, 11: 12.7, 13: 13.9, 18: 14.5 1991-4 12: 12.0, 13: 13.8, 14: 14.5 1991-10 21: 12.0 1992-10 26: 10.7, 27: 12.4-13.2, 28: 15.0 (FID.afoev) 1993-4 29: 10.6, 30: 11.9 1993-10 24: 11.5, 25-26: 12.4
Dr. Fred Ringwald wrote:
| this implies an absolute magnitude in R of 10.7 (+0.7/-0.9)
Really a low luminosity system!
I don't have spectroscopic atlases of low-luminosity CVs at hand, so I would like to search later how the line strengths of He II etc. hehave in lower luminosity systems. To my poor memory, He II line is not this strong in extreme TOADs with lowest Mdot.
|| I still think this system is peculiar. | Since when was there any such thing as a normal cataclysmic variable?
A wide (too simplified) belief that "SU UMa-type DNe are one-parameter (Porb) systems" urged me.
Visual magnitude estimates by P. Schmeer, Bischmisheim, Germany (203-mm SCT, AAVSO seq.): BZ URSAE MAJORIS (UGSU:) Oct. 7.076 UT, (13.9 ; 7.967, 11.4 ; 7.976, 11.3 ; 8.064, 11.4 ; 8.096 UT, 11.4 ; 8.176, 11.2 ; 8.806, 12.0:; 9.103, 11.8 ; 9.136 UT, 11.8 ; 10.074, 13.5 ; 10.121, 13.8 ; 10.17 , 13.6 ; 11.164 UT, (13.9 ; 12.072, (13.7
Once again a normal outburst!
In 1993 two normal outbursts were observed.
The first one was detected by me on Apr. 29.890 UT at mv= 11.4 and independently by Tonny Vanmunster on Apr. 29.972 UT at mv= 10.6.
The second one was detected by Jochen Pietz on Oct. 24.953 UT at mv= 11.5 and independently by me on Oct. 26.072 UT at mv= 12.4.
Observations of the 1990 (super)outburst: July 9.94 UT, 12.2: (Schmeer); 10.96 UT, 12.4 (Poyner); 11.90 UT, 12.8: (Schmeer); (from IAUC 5051) 13 13.9 18 14.5 (from TA via J. Pietz)The rate of decline for this outburst was 0.2-0.3 magnitudes per day, much less than for the 7 normal outbursts (1-2 magnitudes per day) that have been recorded since 1974. I was not able to observe BZ UMa between 1990 July 1 and 8 due to clouds or haze, so the true maximum may have been missed.
Light curve contructed from vsnet-obs observations. A short outburst in July 1995 is indicated. Note red "v" marks represent upper limits.
Taken at Ouda Station. Magnitudes are relative to the comparison star.
(V mag in parentheses by Misselt)
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