UGZ or not UGZ?

By Mike Simonsen SXN

The old adage "you can’t believe everything you read" was never truer than for cataclysmic variables of the type UGZ. Named after the prototype Z Camelopardalis, the most distinguishing property of UGZ type CVs are the "standstills" they occasionally undergo in their cycles. This is described in the definition from the General Catalog of Variable Stars (GCVS):

"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."

Do all UGZs exhibit standstills? Are objects that don’t exhibit standstills therefore, not UGZs? It would seem fairly straightforward from the GCVS definition, but things are seldom as simple as they seem in the world of variable stars.

TW Tri- (0130+31) Outbursting to 13^th magnitude a little less often than the 40 days defined in GCVS, the case for TW Tri’s UGZ-ness is less clear. There are no obvious standstills in the AAVSO light curves, possibly because the magnitude at which it might rest before returning to quiescence (15^th mag?) is too faint for most visual observers to record. Another very similar star is VW Vul (2053+25). Outbursting as bright as 13.1 at times and having a minimum in the 16^th magnitude range, short standstills occur in the mid-14’s and can be difficult to follow visually.

KT Per- (0130+50) Listed as UGZ+ZZ in the GCVS, this suffix to the classification is explained in GCVS as "ZZ Ceti variables. These are nonradially pulsating white dwarfs that change their brightnesses with periods from 30 s to 25 min and amplitudes from 0.001 to 0.2 mag in V. They usually show several close period values. Flares of 1 mag are sometimes observed; however, these may be explained by the presence of close UV Ceti companions." Another explanation for dwarf nova oscillations (DNOs) and quasi-periodic oscillations (QPOs) observed in cataclysmic variable stars is proposed by Brian Warner in 2004PASP..116..115W - Publ. Astron. Soc. Pac., 116, 115-132 (2004) - February 2004, Rapid oscillations in cataclysmic variables.

The interpretation of these modulations is that they are "magnetically channeled accretion from the inner accretion disk for DNOs" and "magnetically excited traveling waves in the disk for QPOs".

TT Ari- (0201+14) GCVS lists this star as UGZ. Downes online CV catalog lists the type as vy/dq: In other words, they are not sure if it is a VY Scl type, which exhibit sudden fades, or a magnetic variable, specifically an intermediate polar, whose accretion disk is interrupted by the presence of a strong magnetic field.

Most other references describe it as nova-like (NL).

As far back as 1979 this star is referred to as a NL object. An early IBVS, from 1979, IBVS 1622, TT Ari; describes fast photoelectric photometry on this NL.

More recently, IBVS 5664, December 2005, describes the recent fading of this star. Again TT Ari is described as a NL variable, in spite of the VY Scl-like fading.

TT Ari: Out from the Positive Superhump State

This fading episode is apparent in the ASAS light curve for this star.

It is also visible in any recent light curve from AAVSO data. No obvious outbursts are shown in the long-term light curve of TT Ari, certainly not every 40 days or less, so why is this one listed as UGZ in GCVS?

TZ Per- (0206+57A) Plot a light curve for this star going back 500 days and you’ll see a standstill episode centered on magnitude 13.5, beginning around JD 2453375. It appears to last at least 100 days, but the end of the standstill is ambiguous due to the seasonal gap in the curve. This is pretty typical for TZ Per, making it a great star for visual observers to monitor. Varying between 12.0 and 15.6, it is visible more often than not.

CN Ori- (0547-05) This is a very active star, with a range of 11.0-16.2, outbursting about every other week. But the light curve looks very much like a UGSS to me. Try as I may, I don’t see evidence of standstills in the data. Perhaps, as with TW Tri, the standstills occur below the threshold of most visual observers. But I would expect to see gaps in the outburst frequency if this were happening, and I just don’t see it.

What magnitude does it park at when in standstill? Are the standstills short-lived, or do they exist at all? This star also seems to be at or beyond the amplitude limit described in GCVS. Is this a UGZ?

Two more variables fit this mold SV CMi (0725+06) and AB Dra (1953+77). They are both active stars and interesting to follow visually, but they appear to be more UGSS-like than UGZ. AB Dra in particular, is so active I can’t believe it has time to go into standstill between its frequent outbursts.

Z Cam- (0814+73) This is the prototype of this class, and a great star for visual monitoring. It ranges from 10.0-14.5V and will sometimes get stuck on the way down to minimum at or around 11.5. The last standstill was relatively short, and not very stable. However, the one before that lasted almost a year!

AT Cnc- (0822+25) AT Cnc is included as UGZ in Downes et al, but not GCVS. This unusual variable has been spending more time in standstill than in outburst or quiescence in recent times. It is therefore not surprising to find a great number of papers on AT Cnc in standstill, including this recent paper co-authored by AAVSO’s Elizabeth Waagen.

2005PASP..117..931S - Publ. Astron. Soc. Pac., 117, 931-937 (2005) - September 2005

A recurrence time versus orbital period relation for the Z Camelopardalis stars.

It was surprising to find this paper describing superhumps in AT Cnc in standstill. Apparently, superhumps do not preclude inclusion in the UGZ classification!

2004A&A...419.1035K - Astron. Astrophys., 419, 1035-1044 (2004) - June(I) 2004

Detection of superhumps in the Z Camelopardalis-type dwarf nova AT Cnc at standstill.

More papers on AT Cnc in standstill seem to make it clear that this variable is a UGZ.

Unusual Slow Fading of Standstill in AT Cnc

1999PASJ...51..115N - Publ. Astron. Soc. Jap., 51, 115-125 (1999)

Spectroscopic and photometric observations of a Z Cam-type dwarf nova, AT Cancri, in standstill.

SY Cnc- (0855+18) Varying between 10.6 and 14.0, this active UGZ is always visible in modest telescopes. Lying so near the ecliptic presents problems as the moon passes through Cancer each month, and occasionally a bright planet, like Jupiter, will plant itself right in the field, making observations interesting and a bit difficult.

AH Her- (1640+25) This is another fairly typical UGZ. The range from outburst to quiescence is 10.6-14.7, with occasional standstills around 12.5-13.0 after outbursts.

More recently, this star was included as one of 16 examples of UGZ.

2005PASP..117..931S - Publ. Astron. Soc. Pac., 117, 931-937 (2005) - September 2005

A recurrence time versus orbital period relation for the Z Camelopardalis stars.

It seems to be general knowledge amongst many that this star is a UGZ, but it is not included as such in GCVS or Downes et al.

EM Cyg- (1934+30) This is a very active UGZ in a beautiful star field in Cygnus. It ranges from 12.5-14.4, with short standstills in the low 12^th mag range. This is also the only known case of an eclipsing UGZ, so the accurate type is UGZ+E.

V1285 Cyg- (1941+35) Listed in GCVS as a suspected UGZ, this star is actually a SR as described in the paper 1987A&A...185..203B - Astron. Astrophys., 185, 203-205 (1987)

The reclassification of the supposed dwarf nova V1285 Cygni as a semiregular variable.

EV Aqr- (2101+00) Listed as UGZ in both GCVS and Downes et al, here is another case where both catalogs have it wrong. EV Aqr is obviously a SR with a period of roughly 124 days, ranging from 11.2-14.0V. This can be seen clearly in the ASAS data for this star.

HX PEG- (2335+12) Although not classified as UGZ in either GCVS or Downes’ catalog, HX Peg exhibits obvious standstills as well as the outbursts and quiescent periods that generally describe UGZ-like behavior. It is variously described as UGZ in other sources (Honeycutt et al. 1998 and the aforementioned 2005PASP..117..931S - Publ. Astron. Soc. Pac., 117, 931-937 (2005), but not the two catalogs we have examined.

Another puzzling fact seems to contradict the accepted normal behavior of Z Cam variables. It is generally accepted that standstills are triggered by outbursts, and that standstills always end with a fade to quiescence. This is stated plainly in Cataclysmic Variable Stars, How and Why They Vary by Coel Hellier (pp. 73-74). However, both AT Cnc and HX Peg have been known to go into outburst from a standstill. Does this fact make them non-UGZ, does it make them a special sub-group of UGZ or does this throw a wrench into the current models of enhanced mass-transfer sustained by enhanced irradiation? How do these systems ramp back up to outburst levels after entering a standstill? I don’t know, but clearly the observational evidence does not always agree with the theories.

So, where does this leave the visual observer interested in Z Cam type variables? I’d say it puts you in great demand. Obviously, there is much to learn about these CVs and there are plenty of examples bright enough to follow and active enough to keep your interest for years to come. Unfortunately, there is no one definitive list of UGZ type CVs from which to pick your targets. Don’t waste your precious observing time on EV Aqr and V1285 Cyg if you want to observe CVs. Do keep an open mind and don’t accept everything you read as gospel.

Your observations may help shine a light on a previously suspected UGZ, or help to classify a new type of variable, or just add to the important data and general knowledge of CVs. It is your data that researchers use to create models of CVs and try to untangle the mysteries of their behaviors. And it is the unpredictable nature of CVs that keeps observers observing them night after night, year after year. Who knows, maybe tonight AT Cnc will go into outburst from a standstill. We’ll never know if you don’t get out there and make the observations.

Good luck and clear skies to you all.