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Z CamPaign Update February 2015

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SXN's picture
Z CamPaign Update February 2015

First, thank you to all the CCD and visual observers who have contributed to this campaign since 2009. With your help we have met all the original science goals, published several papers, unearthed a poorly understood group of anomalous Z Cams and whittled down the list of Z Cam suspects. 

I'm happy to report there is still plenty to do.  

All the bona fide Z Cams should be observed every night. They still have surprises in store for us and now that we have classified them unambiguously we need to maintain a historical record of their unique and interesting behavior. There are plenty of bright Z Cams that visual observers can follow throughout their range of variability. CCD observers should concentrate on Z Cams that get fainter than 15.0V.

The Z Cam List website lists all the bona fide and suspected Z Cams.

The full Z CamPaign update can be read here.

Mike Simonsen (SXN)

lmk's picture
Ambiguous classification methodology?


They still have surprises in store for us and now that we have classified them unambiguously we need to maintain a historical record of their unique and interesting behavior.


Dear Mike S., I want to thank you for your time and effort you have put into this Z Cam project. It is certainly a worthy and scientifically useful endeavor, in principle.  However, as I mentioned to you before in private communications, I find some issues with the methodology you employ in classifying these stars.

Apparently, you rely principally on visual inspection of the light curves to look for signs of standstill behavior. This is fine, if the light curves have dense coverage and the standstill periods stand out clearly and unambiguously by inspection. We all could agree that Z Cam itself is undoubtedly a star which exhibits clear periods of standstills.

But, I am less confident of the classification of several others you have listed as "bonafide Z Cams", and several others, I simply do not see any clear evidence of standstill behavior at all!

At this point, I do not want to go into the details on each and every "bonafide" one you have listed, but I do wish to just point out an example, where no such standstill behavior can be seen:

HP Nor: Looking at the longer term lightcurve, I do not see any clear sign of standstill (at least in the appearance of Z Cam's lightcurve as a template). All I see is a typical oscillatory behavior, with seasonal gaps, and variations in quiescent and outburst brightness. When the campaign on this star really kicks in the last two years, this behavior is quite evident as a pretty smooth oscillation with varying amplitude. There is no sign of a steady period of standstill.

So, unfortunately, I find some concerns with this and several other stars listed as "bonafide" Z Cams. If there is scant or ambiguous sign of standstill, how can you say they are Z Cams with such certainty? Many of the "bonafides" should definitely be dropped from that section of the list!

Sorry Mike, I just don't see what you think you are seeing... :(

Mike LMK



stellakafka's picture
Interesting science!

Hello all,

Ah, science in the making! Working on CVs can be a tricky business, and classifying them is part of the “problem”. CV classes are primarily phenomenological, based on one or a couple of the object’s apparent behavior (reflected on light curves). Classifications reflect descriptions of different physical processes happening in those systems; as we all know, many CVs exhibit multiple behaviors at the same time so they belong in more than one category, demonstrating how complex they really are. There is a reason why they are called “cataclysmic” and “variable”!

Sometimes it is difficult to classify objects with the limited data in our hands, especially since we can’t know all their possible behaviors during their multi-million year lifetimes. Some systems have sparse coverage, so we need to use our best judgment while placing them in a class bin. This is where a “by eye” estimate, as much as it may be subjective, is still justifiable and a good starting point. New data (or digging in the archive) could indicate that those classifications are invalid.  Part of what we do is to keep questioning what those objects are and what is the physics behind their brightness variations; as long as we learn something about those physical processes, we all win. And, of course, we need to keep observing them – more data and better coverage always help.

Keep up the good work. Keep questioning. And, please, continue observing!

Best wishes - clear skies,


lmk's picture
Lets take a look at a few Z Cams that may not be?

Thanks Stella for your input into this interesting issue!  Just to be a little bit more specific, from a scientific point of view, here are the stars which Mike Simonsen lists on his webpage, and in his paper, as being "bonafide" Z Cam stars, but which upon my inspection of their lightcurves, seem fairly ambiguous as to type:

BX Pup, Leo5, VW Vul, HX Peg, HP Nor.

I just list them here, so maybe some others can take a look, and see if they agree with Mike Simonsen's classification or not? To me, these seem pretty sparsely observed, or too irregular in amplitudes and timings of outbursts and periods, to be so definitely classified as Z Cam?


Mike LMK


WGR's picture
Z Cams and Definitions

Hello All

I think we have a problem of definitions here.  I looked up the GCVS definition of a Z Cam and its:  "

UGZ Z Camelopardalis-type stars. These also 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 mag in V.


This definition is not specific as to the range in magnitude of a standstill.  One would have to imply that since the outburst cycles are 2-5 mags, the standstills would be somewhat less than that, but its not defined and would vary in my mind with the magnitude of the outburst range.  Bigger range should allow bigger range of standstill.  The definition is also silent as to the duration of standstills and to repetetive standstills.  

With this as a basis, I looked at the light curves for the 5 stars questioned by Mike Linnolt, BX Pup, Leo5, VW Vul, HX Peg, HP Nor.

Using the Light Curve Generator, and adjusting the range, I found at least one standstill for these stars at JD2456760, 2455600, 2456150, 2455880 and before 2456400.  BX Pup,& HX Peg, had standstill durations of 10-20 days, while  Leo5, VW Vul had standstill durations of 100 days.  HP Nor appeared to be in standstill prior to 2456400 for more than several thousand days thanks to the persistant observations of our Visual Observers, then goes into outburst.  

I conclude that all 5 meet the definition according to the GCVS.  Most do not have repetetive standstills.  Perhaps Stella can comment on the need to expand the definition of Z Cams and speak on the silent issues.  I can also see that these definitions need to be flexible, otherwise we end up with way too many types.  On the other hand, we don't want to lump several types together.  We should also remember that (I think these Z Cam classifications were published in jAAVSO and other papers, that were peer reviewed by some of the best names in the field, and they found no issue with the classification).  




Good Critique

This is an excellent example on how to critique and comment on a paper.

Perhaps most importantly the reviewer had done some homework so his comments were informed and on target.  He spoke to the science and the issue(s).  He was mindful that the paper had been peer reviewed.  He delivered his conclusions calmly and on-point.

He didn't find it necessary to be in-your-face, to challenge the author or to make demands of the author for this or that.

Thanks Gary.

Jim Jones


lmk's picture
Beware sampling errors!


HP Nor appeared to be in standstill prior to 2456400 for more than several thousand days thanks to the persistant observations of our Visual Observers, then goes into outburst.  


Gary, HP Nor is probably the worst example of trying to assign a Z Cam classification!  I really haven't the time right now to address all your comments on the other questionable ones I mentioned, but this star just does not show standstills. You would really have to stretch the definition to include the small changes in amplitude to do so, and then almost any CV could be called a Z Cam!

The evidence of standstill before JD 2456400 appears non-existent. Keep in mind, that appears to be the point where the campaign kicked in, and you can see a large increase in CCD observations after that date. Just as in some prior times (around JD 2454000), once more observations come in, the range increases. So, the issue is statistical undersampling error. The lower your sampling rate, the lower the range will appear to be. This is analogous to A/D conversion, the Nyquist-Shannon theorem.

It is incorrect to judge and claim a standstill occurs if you have widely varying amounts of data points in different intervals. GIGO!

We need a rigorous analysis of light curves, from a proper statistical standpoint, before any conclusion can be drawn about standstills.

Mike LMK


lmk's picture
And whats with AB Dra?

Perusing the Z Cam list a bit more, I notice that the star AB Dra, which Mike Simonsen has listed as definitely NOT a Z Cam, appears to have been in standstill the past few months. Of course, this is based only on a rough visual inspection of the light curve, and assuming that there are no undersampling issues (missed extreme values).

The more I research this whole issue of classification of Z Cam's, the less interested I am becoming in further pursuing such an ambiguous endeavor. (I think to everyone's relief as well :( My migraines and blood pressure is talking to me...

Mike LMK


LMA's picture
Z Cam

Mike LMK wrote

...........   (I think to everyone's relief as well :(  .......


Damien LMA

WGR's picture
HP Nor

Hello Michael;

I agree with you that HP Nor is probably the most problematic when it comes to classification as a Z Cam of the 5 stars that you mentioned.  I went back and found an even more compelling case in the data.  BTW:  the data goes back about 10000 days, nearly 30 years and there are 8532 observations there as of yesterday.  Its a tribute to the Visual Observers.  

I found a very nice Standstill from JD2449400 to 2450010.  There were 653 observations over a period of 610 days, for a cadence of 1.07 observations per day.  Not what I would call sparcely observed for the purpose of classification of light curves of this type.  The std deviation of these data were 0.27 mags one sigma.  

Contrast this with the Outburst behavior from JD2447000 to 2449400, just prior to this standstill.  In this section, there were 2569 observations over a period of 2400 days, for a cadence of 1.07 observations per day.  Once again, not what I would call sparcely observed for this purpose.  The std deviation of this outburst section was 0.45 mags, nearly twice that of the standstill, which suggests a difference from the standstill section. 

I also call your attention to the Outburst behavior from JD2450010 to 2456000, just after the standstill.  In this section, there were 3823 observations over a period of 990 days, for a cadence of 3.8 observations per day.  Once again, not sparcely observed for this purpose.  The standard Deviation of this outburst section following the standstill was 0.68 magnitudes.  This is nearly three times the std dev of the Standstill section, which again suggests a difference.  

I must say that looking at the observer initials, was a walk down memory lane.  Some of the biggest names in the business were among the regular observers of this object--Tom Cragg, Albert Jones, Danie Overbeek, and Rob Stubbings.  These guys knew how to observer and I think we can trust their data.  I had the good fortune to meet most all of them at AAVSO meetings over the past years.  In addition to these 4, there were 9 other observers in these sections.  

So I find compelling evidence to keep HP Nor as a Z Cam, granted it was 19 years ago.    I think this data speaks for itself.   None of the recent CCD data enters any of these 3 sections.  

As for AB Dra, good pickup Michael, it looks like a 53 day standstill, and the last data point suggests it may be coming out of standstill.  Its worth some followup.  



Gary Walker


Carl Knight
Carl Knight's picture
Defining "standstill"...

In line with Stella's response, prior to reading her remarks I went off to try and get a good definition of the term "standstill" in the given context.

Stehle, R., King, A. & Rudge, C. (2000), The standstill luminosity in Z Cam systems, Mon. Not. R. Astron. Soc. 000, 1–3 (2000) has:

"Z Cam systems are a subset of the dwarf novae which occasionally show standstills, i.e. epochs where the luminosity is constant for several days, at a value intermediate between the outburst and quiescent states (e.g. Warner, 1995, Oppenheimer, Kenyon & Mattei 1998, Section 3.4)."

which is kind of vague and open to interpretation - please offer correction if there is a more robust definition.

Mike (S), can you comment on the criteria you use further? I looked at the sites linked to in your post and that criteria is missing or I've missed it somehow.

  1. How many days of constant luminosity do you consider a standstill?
  2. What is "constant luminosity"? What delta is acceptable that we might still consider a standstill.
  3. Are there proportional criteria you consider? e.g. proportion of rise time to max?

I totally appreciate the edge case problems in any taxonomic system! Nature gets all fuzzy around the edges of our attempts to classify things.

- Carl (KCD)

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