Z CamPaign Update December 2013

Let me begin by thanking all the observers who have contributed data to the Z CamPaign up to now. Your dedication and persistence has paid off admirably, and none of the results I am about to share with you would have been possible otherwise. We passed the four year mark in September 2013. As a direct result of this observing campaign there have been eight papers published, accepted or submitted for publication.

The first paper describing the project and its goals, came out in 2011- The Z CamPaign- Simonsen, M., 2011, JAVSO, 39, 66.

That was followed by the announcement that WW Ceti is in fact a bona fide Z Cam star. The First Historic Standstill of WW Ceti- Simonsen, M. Stubbings, R., 2011, JAVSO, 39, 73.

Next came the discovery paper describing Leo5 as a Z Cam. Leo5 is a Z Cam Star - Wils, P.; Krajci, T.; Simonsen, M., 2011JAVSO, 39, 77.

In 2012, Fred Ringwald produced two papers based on AAVSO (Z CamPaign) data on ES Dra- Ringwald, F. A.; Velasco, Kenia, 2012, NewA, 17, 108, and V849 Her - Ringwald, F. A. et al, 2012, NewA, 17, 570.

Accepted for the December 2013 issue of PASP, and released on arXiv, A Study of the Unusual Z Cam Systems IW Andromedae and V513 Cassiopeia– Paula Szkody, Meagan Albright, Albert P. Linnell, et al http://arxiv.org/abs/1311.1557.

The big paper, 26 pages with 14 co-authors, summarizing the results of the first four years of research, will be out soon. Z Cams in the 21st Century – accepted JAAVSO, Simonsen et al, November 2013.

And this month I submitted another paper, ST Chamaeleontis and BP Coronae Australis: Two Southern Dwarf Novae Confirmed as Z Cam Stars, Simonsen, M., Hambsch, J., and Stubbings, R.

To summarize the findings to date, we now know of 21 bona fide Z Cam systems. We have eliminated 29 impostors, stars that at one time or another were listed as or suspected of being Z Cam stars in the literature.

Our work is far from over, which is a good thing, because this has been a fun and interesting project for many of you, as well as for me. I will be on a crusade in 2014 (once Z Cams in the 21st Century is published) to get all the catalogs and other sources of CV information to stop listing these impostors as Z Cams or suspects. If you find a source that is spreading misinformation, please let me know, and we'll drive them crazy until they stop!

There are still things to be learned from continued monitoring of the bona fide Z Cam stars. We're finding some unusual and unexpected behaviors. For one- outbursts from standstills- which turned out to be more common that any of us thought at the beginning of this campaign in 2009. Many Z Cam stars also exhibit deep VY Scl-type fading episodes. In recent years Michael Shara has found ancient nova shells around two Z Cams (Z Cam and AT Cnc) bolstering the hibernating novae theory. I've become more convinced than ever that Z Cams represent a relatively short period in the life cycle of CVs as they evolve. Now it appears that CVs can and do evolve from one type to another. We can't have too much data on these rare star systems. What's more, we still have a list of 20 suspects that will require more intensive long-term monitoring to determine once and for all their variability type. I expect to be writing summaries and papers from this campaign for several years to come.

So, before we look forward, I'd like to look back on the four years of the campaign so far and highlight all the fantastic work you have done. Most of these light curves are from September 1, 2009 to December 20, 2013, the period coinciding with the Z CamPaign to date. (You can click on the curves to view enlarged images)

First, the prototype and star of the show, Z Camelopardalis herself. In the four years since we began, Z Cam has had two protracted standstills, and a period of inactivity with a relatively faint minimum. The coverage has been excellent throughout this time. This is a beautiful light curve and you should all be proud of this effort.

Another spectacular light curve showing standstills, outbursts from standstills, and a deep fade in 2011 belongs to AH Herculis. This is a fantastic and interesting star worth observing every clear night forever. You never know what it will do next and it is always doing something!

AT Cancri had a nice standstill in the first year of the campaign and another one in 2011. Since then it has been acting like a typical dwarf nova. I don't think I can predict the next standstill, but with very solid coverage like this I doubt we will miss it unless it happens while the star is behind the Sun.

AY Piscium has been in standstill more or less continuously since the beginning of the 2012 observing season. AY Psc is one of the few eclipsing Z Cams with a period of 5.22 hours.

Here is a light curve showing the first historical standstill in the AAVSO data for BP Coronae Australis. Rod Stubbings and Josch Hambsch have supplied convincing proof of this star's Z Cam-ness and our paper has been submitted to the JAAVSO. We could stand to have more observers covering this southern Z Cam from now on. It is proving to be another interesting system and probably has more secrets to reveal in the future.

Over the last four years we have not collected any data showing BX Puppis in a standstill, but I'm going to go out on a limb here and predict that we will see one in the next two years. The reason I believe this is the current trend of brighter minima and fainter maxima over the last two years. I've seen this kind of plucked string vibration damping down into a standstill in many Z Cam light curves. I think Z Cams know in advance that they are going into standstill and sometimes give us subtle clues. The last two observing seasons' minima are at least 0.5 magnitudes brighter than before and outbursts are only rising to the mid 14's instead of 14.0 or brighter. A standstill should occur in the 15th magnitude range.

EM Cygni has been active in the last four years. There was a standstill in the summer of 2010 followed by a fade and relatively quiet period. Then in the beginning of 2013 it went into standstill again. This is another eclipsing system with a period just shy of 7 hours.

ES Draconis was in standstill from the beginning of the campaign in 2009 until the fall of 2012. It returned to normal dwarf novae activity for about a year and then got stuck at 15.0V again in the fall of 2013 where it remains at this time. Unlike most Z Cams, this star seems to spend more time stuck in standstill than bouncing back and forth from maximum to minimum.

HL Canis Majoris has not rewarded our efforts with a standstill in a long time. It is possible we have missed one or more due to the brief observing season. This one is definitely championed by the visual observers, since acquiring CCD images this close to Sirius presents a real challenge for photometry. Please continue monitoring this star visually and with CCDs.

HP Normae is a bona fide member of the Z Cam class, but it has been observed almost exclusively by visual observers in the past, most recently by Rod Stubbings. In the last year, Josch Hambsch has taken this star into his program and begun supplying us with nightly snapshot CCD observations. We can now see more clearly the full range of activity. Standstills do not occur frequently with this Z Cam, the best examples being in 1995 and 2003, so it could be a while before we see one again. Please keep monitoring this star for Z Cam activity, and if you are in the southern hemisphere, consider adding this relatively bright CV to your program.

HX Pegasi is one of the Z Cams known to go into outburst from standstill. However, HX Peg has only had one brief standstill since we began the campaign (November - December, 2011). I'm keeping a watchful eye on this one.

IW Andromedae does not reveal her secrets when inspecting her light curve at this resolution. You must look closer to see the mysterious behavior of this enigmatic variable star.

The main thing, and I cannot stress this enough, is that we would never have known about any of this unusual activity if it had not been for the dedication and determination of the CCD observers taking part in the Z CamPaign. Here is the VSTAR light curve from April 2012 to March 2013. It is truly bizarre- standstills, outbursts from standstill, short and long outbursts - keep observing this one, if only for the entertainment value!

Leo5 was in standstill for almost two years shortly after we began the Z CamPaign. It also had an outburst from standstill in 2011. Discovered in CRTS data and brought to my attention by Patrick Wils, this is a new member of the Z Cam class and is also an eclipsing system.

After making us wait for three years, RX Andromedae finally had standstills in fall of 2012 and early in 2013. This Z Cam is also known to have deep fading episodes, reminiscent of VY Scl type variables.

Like IW And, ST Chamaeleontis' true identity as a Z Cam was hidden from us in the existing visual data. Only after Josch Hambsch began supplying nightly snapshots did it become clear we had another IW And-like Z Cam on our hands. It's hard to see at this resolution.

Below is the VSTAR light curve of ST Cha, up close and personal, showing the IW And-like behavior in the CCD data from March to July of 2013. ST Chamaeleontis and BP Coronae Australis: Two Southern Dwarf Novae Confirmed as Z Cam Stars, Simonsen, Hambsch and Stubbings, submitted to JAAVSO 2013.

Although a bona fide member of the class, SY Cancri rarely exhibits standstills. In fact, there is only one in the entire AAVSO record. When it goes into standstill again it will be an historic occasion.

After making us wait for more than three years, TZ Persei finally went into standstill in November 2012. The standstill lasted until June 2013. Since then TZ Per has not returned to a normal outburst cycle. It's behavior has been somewhat erratic.

The erratic behavior of TZ Per just before and after the 2012-2013 standstill can be seen clearly in this VSTAR light curve.

UZ Serpentis is another Z Cam that doesn't have standstills very often. The most obvious examples were in 1989 and 1995.

For the duration of the Z CamPaign it has behaved much like a typical dwarf nova.

V513 Cassiopeiae is another IW And-like variable whose unusual behavior might never have been known if the Z CamPaign CCD observers hadn't covered this star's nightly gyrations so exquisitely. So far it has shown us two distinctly different types of activity.

The VSTAR light curve of V513 Cas from September 2009 to March 2012 shows the familiar IW And-like features. Frequent outbursts, followed by dips to minimum followed by rapid rises and standstills followed by outbursts.

Then in March 2012 it stopped falling to minimum and remained more or less in a standstill (punctuated by occasional outbursts) until November 2013, when it finally fell to quiescence once more.

VW Vulpeculae underwent a fading episode early in the campaign, reaching 17.05V in July 2010. Then in September 2012 it went into standstill until the end of October 2012. It has been rather ordinary since then, exhibiting a rather SS Cygni type outburst cycle.

As I wrote earlier, there is still work to do.

It is still valuable to continue submitting data for the known Z Cam stars on a nightly basis. There are only 21 known Z Cams, making them a rare and special breed of CV.

There are still 19 stars whose variable classifications are not secure. I strongly suspect at least two of these, and maybe more, are also Z Cams. The current list of suspects can be found on the Z Cam List website.

Taichi Kato has written that he suspects CSS J220031.2+033429 may be a Z Cam variable. This one may end up on the list of suspects before long. It is another faint target that will prove to be a challenge, with standstills in the 17.5-18.0 magnitude range.

Once more, thank you to all the observers, visual and CCD, supporting this campaign with your hard earned data. Please keep up the good fight in 2014 and beyond.

Mike Simonsen

December 21, 2013