The Z CamPaign

UGZs are defined in the General Catalog of Variable Stars as dwarf novae that “show cyclic outbursts, differing from UGSS variables by the fact that sometimes after an outburst they do not return to the original brightness, but during several cycles retain a magnitude between maximum and minimum. The values of cycles are from 10 to 40 days, while light amplitudes are from 2 to 5 magnitudes in V.”

So it’s all about the standstills, those episodes where the star gets stuck at a mid-point between maximum and minimum. If it doesn’t exhibit standstills it isn’t a Z Cam star.

Typical standstill of Z Camelopardalis

So UGZ can be classified by their light curves alone. Orbital period is not a factor in classification, even though they all tend to be on the long side of the period gap, 3 hours to 10 hours orbital period.
There is no strong agreement between the various CV catalogs as to which few dozen or so stars are actually Z Cam type systems. There are a handful of bright objects that have been densely covered by amateurs throughout their range that are obviously UGZ from their light curves. They show the typical Z Cam-like standstills, have short outburst cycles and amplitudes around 3 or 4 magnitudes.

There are also some bright systems listed as UGZ, like AB Draconis, that have the short cycle and small amplitude, don’t show obvious standstills, and yet it seems everyone agrees they are UGZ.

AB Draconis- where are the standstills?

There are many more CVs that have some of the characteristics of UGZ, but it is not at all apparent from the existing data that they show standstill behavior because the range at which you would expect to see this, somewhere mid-point between maximum and minimum brightness, is too faint for visual observes to have accumulated useful data over the years. All we really know from the data is the average outburst cycle and approximate amplitude. There is no detail in the middle where the real story lies.

Depending on which catalog you use, there are only 30 to 40 Z Cam dwarf novae. If any significant percentage of the number of Z Cams eventually proves not to be Z Cam, the remaining few represent a fairly rare and unique class of stars worthy of further investigation.

Other well-quoted characteristics are that “standstills are always initiated by an outburst,” and “standstills always end with a decline to quiescence” (Hellier, 2001). This may be convenient because it fits the expected behavior, if the models are correct, but there are at least three Z Cam stars that appear to go into outburst from standstill, HX Peg, AH Her and AT Cnc. If this is in fact true, it throws a real monkey wrench into current CV theory.

Another interesting idea is that these Z Cams may be part of a population of “hibernating novae.” According to theory, classical novae systems can evolve into hibernating novae when the secondary star underfills its Roche lobe and mass transfer ceases, possibly centuries after eruption, causing the binary to go into hibernation.

Mass loss during the nova event (or events) results in an increase in orbital separation. The secondary, induced by irradiation of the red dwarf's surface by the white dwarf, continues mass transfer onto the white dwarf. Through this continuous mass loss, the secondary star eventually underfills its Roche lobe and mass transfer ceases.

The shell of ionized gas around Z Cam detected by the Galaxy Evolution Explorer
 is explained as the remnant of a full-blown classical nova explosion.

Credit: NASA/JPL-Caltech/M. Seibert(OCIW)/T. Pyle(SSC)/R. Hurt(SSC)

The binary systems that are most likely to go through hibernation after a nova outburst suffer the largest reduction in mass transfer and increase in separation. In particular, systems with a higher mass ratio are more likely to be induced into hibernation.

The Z CamPaign
The list of stars in the Z CamPaign can be found here.
Stars highlighted in yellow are stars that are confirmed UGZ suitable for continued observation by visual observers throughout their cycles. We strongly urge visual observers to continue monitoring these stars for their expected outbursts and standstills.
Stars highlighted in green are stars that visual observers should continue to monitor for outbursts and standstills if or when they may occur.
Stars with no highlights are stars which both visual and CCD observers are encouraged to monitor for outbursts, but the standstills are likely to only be visible to CCD observers due to their relative faintness (15th or 16th magnitude).
Stars highlighted in blue are best suited to CCD observers for monitoring for outbursts and standstill behavior.
Stars highlighted in red are those which appear to go into outburst from standstill. When one of these stars enters a standstill we will be asking for intensive coverage until the star either goes into quiescence or outburst.
We will devote a special place on the home page for notifications and reminders of current Z Cam and suspected Z Cam activity, the Z Cam Corner.
We also plan to build a website devoted to Z Cam and suspected Z Cam stars, with pages for individual stars, finder charts, data tables and links to relevant literature, along the lines of The Big List of SW Sextantis Stars (D. W. Hoard) and Intermediate Polar Home Page (Koji Mukai).

Science Goals
1. To determine convincingly which CVs are indeed UGZ and which are imposters.
2. To improve the overall data available on each of these stars and fill the gaps in the light curves.
3. To determine if some UGZ actually do go into outburst from standstill, or if perhaps we have just missed the sudden drop to quiescence before the next outburst, leading to the appearance of outburst from standstill behavior.
4. To make 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 of the AAVSO.
Z Cam stars are not the sexy, super-humping members of the CV family. In fact, they are rather ignored for the most part by amateur and professional alike. Perhaps because it is easier to make a classification of a UGSU from only a few nights observations of superhumps, or because the reason for standstills to occur is not well-understood. This leaves the door open for discovery to those patient and persistent enough to devote time and energy to observing this unique class of cataclysmic variable. We hope you will join us in this endeavor.

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