SSimonsen, JAAVSO Volume 39, 2011 1
The Z CamPaign: Year 1
Mike Simonsen
AAVSO, 49 Bay State Road, Cambridge, MA, 02138; [email protected] November 29, 2010; accepted December 2, 2010
Abstract
The Cataclysmic Variable Section of the American Association ofVariable Star Observers (AAVSO) has initiated an observing campaign to studya subset of dwarf novae (DNe), known as Z Cam type (UGZ). We call this programthe Z CamPaign. Since there is no strong agreement between the variouspublished catalogues as to which few dozen DNe are actually Z Cam type systems,our primary goal is to accumulate enough data to construct detailed light curves,covering the entire range of variability, to determine unequivocally the 30 ZCamPaign subjects’ membership in the UGZ class of DNe. We discuss theorganization, science goals, and some early results of the Z CamPaign in detail.
1. Defining Z Cams
U Geminorum-type (UG) variables, also called dwarf novae, are close binarysystems in which a dwarf or subgiant K-M star fills the volume of its inner Rochelobe and loses mass to a white dwarf surrounded by an accretion disk. From timeto time the system goes into outburst, brightening rapidly by several magnitudes.After several days to a month, or more, it returns to its original state. These dwarfnova outbursts are believed to be caused by thermal instabilities in the disk. Gasaccumulates in the disk until it heats up and becomes viscous. This increasedviscosity causes it to migrate in toward the white dwarf, heating up even more,eventually causing an outburst (Warner 1995).Intervals between two consecutive outbursts for a given star may vary, butevery star is characterized by a characteristic mean value of these intervals. Thismean cycle corresponds to the mean amplitude of the outbursts. Generallyspeaking, the longer the cycle, the greater the amplitude of the outbursts.According to the characteristics of their light curves, UGs are further subdividedinto three types: SS Cyg, SU UMa, and Z Cam. SU UMa type UGs are not relevantto this discussion.SS Cygni-type variables (UGSS) increase in brightness by 2 to 6 magnitudesin V in 1 to 2 days and after several subsequent days return to their originalbrightness. The cycle times vary considerably, from 10 to several hundreds of days.Z Camelopardalis-type stars (UGZ) also show cyclic outbursts, but differ fromUGSS variables by the fact that sometimes after an outburst they do not returnto their quiescent magnitude. Instead they appear to get stuck, for months or evenyears, at a brightness of about one magnitude fainter than outburst maximum. imonsen, JAAVSO Volume 39, 20112 These episodes are known as standstills. Z Cam cycle times characteristicallyrange from 10 to 40 days, and their outburst amplitudes are from 2 to 5 magnitudesin V , but standstills are the defining characteristic of the Z Cam stars.
2. Standstills
If a dwarf nova has a high enough mass-transfer rate, it can resemble a dwarfnova continuously stuck in outburst. This is what nova-like variables are thoughtto be. One theory explaining Z Cam standstills is that the rate of mass transfer isapproximately equal to the critical rate that separates dwarf novae from the nova-like variables (Meyer and Meyer-Hofmeister 1983).Models can now explain why standstills are about a magnitude fainter thanoutburst maximum. The gas stream from the mass-losing star heats the disk, andbecause of this extra source of heat, the critical mass transfer rate at which astandstill occurs is about 40% less than the mass transfer rate during outburst(Stehle et al. et al.
3. Science Goals
The science goals of the Z CamPaign are:1. To determine convincingly which CVs are indeed UGZ and which areimposters. The plan is to analyze the light curves of all the candidateslooking for standstill episodes in their light curves. If there are standstills,we will accept them as UGZ. If there are no standstills we will remove themfrom the list of known Z Cams, and assign another type to them, if possible. imonsen, JAAVSO Volume 39, 2011 3
If the data are inconclusive, we will concentrate on obtaining adequate long-term data throughout the range of the variable to make a determination.2. To improve the overall data available on each of these stars and fill thegaps in the light curves. Since so little is known, even about the wellobserved Z Cam candidates, we will try to obtain as complete coverage aspossible, concentrating on V magnitude observations first, then extendingto other bandpasses.3. To determine if some UGZ actually do go into outburst from standstill, orif perhaps we have just missed the sudden drop to quiescence before thenext outburst, leading to the appearance of outburst from standstill behavior.4. To study and report any other serendipitous discoveries about “UGZ-ness” that come to light as a result of improved coverage.5. To publish the results in a peer-reviewed journal such as
The Journal ofthe AAVSO .
4. Coordinating the campaign
The campaign is coordinated through the Cataclysmic Variable Section of theAAVSO. There is a special campaign page online explaining the details to thoseinterested in observing these stars: https://sites.google.com/site/aavsocvsection/z-campaign. The list of campaign stars can be downloaded in several formats fromthis web page. The star list is divided into four sub-categories, based on the typeand magnitude range.The first group of stars are confirmed UGZ suitable for continued observationby visual observers throughout their cycles. These are generally the brightestdwarf novae in the campaign and have well sampled light curves, some going backas far as the 1940s. We strongly urge visual observers to continue monitoringthese stars for their expected outbursts and standstills as well as unexpectedbehavior.The second group of stars are unconfirmed UGZ stars that visual observersshould continue to monitor for outbursts and standstills if or when they mayoccur.The third group of stars are unconfirmed UGZs that both visual and CCDobservers are encouraged to monitor for outbursts, but the standstills are likelyto only be visible to CCD observers due to their relative faintness (15th or 16thmagnitude). We encourage CCD observers to concentrate on these stars whenthey are known to be in outburst in particular, so they can monitor the fade frommaximum looking for a standstill.The last group are stars best suited to CCD observers for monitoring for imonsen, JAAVSO Volume 39, 20114 outbursts and standstill behavior. These stars are too faint, even at maximum, formost visual observers to waste valuable time and resources on.We also take special note of those UGZ that appear to go into outburst fromstandstill. When one of these stars enters a standstill we will be asking forintensive coverage until the star either goes into quiescence or outburst.Activity is tracked in near real time as observations come in from AAVSOMyNewsFlash, BAAVSS-Alert, CVnet-Outburst, VSObs-share, and VSNET-outburst email notifications on the Activity at a Glance portion of the section homepage: https://sites.google.com/site/aavsocvsection/Home.
5. Early results
The Z CamPaign was launched on September 25, 2009. From increasedcoverage of some stars and a thorough analysis of the AAVSO light curves wehave positively confirmed ten UGZ systems: RX And, TZ Per, Z Cam, AT Cnc,SY Cnc, AH Her, UZ Ser, EM Cyg, VW Vul, and HX Peg. Most of these were knownor suspected UGZ.We have also been able to identify several potential Z Cam imposters. Thereis no evidence of standstills in their AAVSO light curves going back decades, inmost cases. Included in this group are: TW Tri, KT Per, BI Ori, CN Ori, SV CMi,and AB Dra. Some of these stars have been erroneously classified as UGZ fordecades in major variable star catalogues.V344 Ori and V391 Lyr have uncharacteristic long outburst cycles of hundredsof days, and V1363 Cyg is an unusual, unique star, but its light variations are nottypical of a UGZ. None of these is a Z Cam. FY Vul has an outburst cycle between30 and 50 days, but also shows some quasi-periodic variation on shorter timescales, perhaps 15 to 20 days. The amplitude of variation is rather small for a UGZtype dwarf nova. It has been suggested that this star and V1101 Aql may representa previously unrecognized group of low-amplitude dwarf novae (Kato et al.
6. Conclusion
Depending on which catalogue you reference, there are only 30 to 40 Z Camdwarf novae. If a significant percentage of suspected Z Cams eventually provesnot to be Z Cam, the remaining few represent a fairly rare and unique class of starsworthy of further investigation. Z Cam stars are rather ignored for the most partby amateur and professional alike. This leaves the door to discovery open forthose patient and persistent enough to devote time and energy to long-term imonsen, JAAVSO Volume 39, 2011 5 monitoring of this unique class of cataclysmic variable. We plan to continue thiscampaign through 2011, modifying the targets list as new information becomesavailable.
7. Acknowledgements
We acknowledge with thanks the variable star observations from the AAVSOInternational Database contributed by observers worldwide and used in thisresearch. Several individual observers have been of particular help to thiscampaign: Gary Poyner, Richard Sabo, George Sjoberg, Tim Crawford, KennethMenzies, David Boyd, Bart Staels, Ken Mogul, Keith Graham, and Bill Goff.
References
Cataclysmic Variable Stars , Springer-Praxis, Chichester, U.K.Kato, T., Nogami, D., and Baba, H. 1999,
Inf. Bull. Var. Stars , No. 4766, 1.Meyer, F., and Meyer-Hofmeister, E. 1983,
Astron. Astrophys. , , 29.Oppenheimer, B. D., Kenyon, S. J., and Mattei, J. A. 1998, Astron. J. , , 1175.Stehle, R., King, A., and Rudge, C. 2001, Mon. Not. Roy. Astron. Soc. , , 584.Szkody, P., and Mattei, J. A. 1984, Publ. Astron. Soc. Pacific , , 988.Warner, B. 1995, Cataclysmic Variables Stars , Cambridge Univ. Press, Cambridge. imonsen, JAAVSO Volume 39, 20116
V513 Cas 00 18 14.90 +66 18 14.0 Cas UGZ: — 15.5–<17.2 pIW And 01 01 08.90 +43 23 26.0 And UGZ — 14.2–17.4 pRX And 01 04 35.50 +41 17 58.0 And UGZ (14) 10.3–14 VTW Tri 01 36 37.00 +32 00 40.0 Tri UGZ: (28) 13.3–17.0 pKT Per 01 37 08.50 +50 57 20.0 Per UGZ+ZZ (26) 11.5–15.39 VTZ Per 02 13 50.90 +58 22 53.0 Per UGZ (17) 12–15.6 VPY Per 02 50 00.10 +37 39 23.0 Per UGZ — 13.8–16.5 pBI Ori 05 23 51.80 +01 00 30.0 Ori UGZ (20.5) 13.2–16.7 pCN Ori 05 52 07.80 –05 25 01.0 Ori UGZ (15.85) 11–16.2 VV344 Ori 06 15 19.00 +15 31 00.0 Ori UGZ: — 14.2–17.5: pWZ CMa 07 18 49.20 –27 07 43.0 CMa UGZ: (27.1) 14.5–<16.0 pSV CMi 07 31 08.40 +05 58 49.0 CMi UGZ (16:) 13.0–16.3 pBX Pup 07 54 15.60 –24 19 36.0 Pup UGZ (18) 13.76–16 VZ Cam 08 25 13.20 +73 06 39.0 Cam UGZ (22) 10–14.5 VAT Cnc 08 28 36.90 +25 20 03.0 Cnc UGZ (14) 12.3–14.6 pSY Cnc 09 01 03.32 +17 53 56.0 Cnc UGZ — 10.6–14.0 pAH Her 16 44 10.00 +25 15 02.0 Her UGZ (19.8) 10.9–14.7 pUZ Ser 18 11 24.90 –14 55 34.0 Ser UGZ (26.4) 12.0–16.7 pV391 Lyr 18 21 12.00 +38 47 44.0 Lyr UGZ: (100:) 14.0–17.0 pHS 1857+7127 18 57 20.40 +71 31 19.2 Dra UGZ+E — 13.9–17.2V868 Cyg 19 29 04.40 +28 54 26.0 Cyg UGZ: (20.38) 14.3–<17.8 pV1505 Cyg 19 29 49.00 +28 32 54.0 Cyg UGZ: — 15.2–<17.5 pEM Cyg 19 38 40.10 +30 30 28.0 Cyg UGZ+E — 11.9–14.4 pFY Vul 19 41 40.00 +21 45 59.0 Vul UGZ:/NL — 13.4–15.33 BAB Dra 19 49 06.40 +77 44 23.0 Dra UGZ (13.4) 11–15.3 VV1363 Cyg 20 06 11.60 +33 42 38.0 Cyg UGZ:/UGSU: — 13.0–<17.6 pVW Vul 20 57 45.10 +25 30 26.0 Vul UGZ (30) 13.1–16.27 BV1404 Cyg 21 57 16.40 +52 12 00.0 Cyg UGZ: (19.15) 15.7–<17.7 pMN Lac 22 23 04.60 +52 40 58.0 Lac UGZ: — 15.1–<18.0 pHX Peg 23 40 23.70 +12 37 42.0 Peg UGZ — 12.9–16.62 V