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MUY's picture

I have a lot of 'larger amplitude' (6 - 7 mag) dwarf novae on my program. A few are marked UGWZ but the most are simply marked UGSU. The ones with an amplitude of around 7 magnitudes should be UGWZ? Don't they? 

The line between large amplitude UGSU and typical UGWZ is not always easy to decide...



pukemaru's picture

Hi Eddy
I can't answer your question but I can make an observation. There have been a lot more UGWZs identified over recent years. And as we get more data, we may find more of those UGSUs are actually UGWZs.
Kind regards
Stephen [HSP]


lmk's picture
Arbitrary lines of categorization


The line between large amplitude UGSU and typical UGWZ is not always easy to decide...


Well, maybe the UGSU and UGWZ are just two extremes of a continuum? Just a range of amplitudes and outburst periods, but the same underlying mechanism? If there is nothing fundamentally different about the process of their outbursts, then you can pick any arbitrary amplitude you want to, split them into these two categories.

I might add, the same issue appears in categorizing the UGZ as well. There are some with very obvious standstills, but others have very short or irregular amplitudes/times of "standstill" that its quite uncertain they really are UGZ!

So, it seems, unless you have some unique property to definitively identify a specific type of variable, you will end up with this subjective "judgment call" where people can honestly disagree about its category.

Mike LMK


pukemaru's picture
What exactly is a UGWZ?

I just received this mesage form Taichio Kato:
"ASASSN-15bp: new WZ Sge-type dwarf nova We have received observations from Ian Miller, Josch Hambsch, Kiyota-san, Itoh-san, Okayama U. of Sci. team, Osaka Kyoiku U. team and Enrique de Miguel. The data clearly showed double-wave early superhumps. The period is 0.05583(5) d and the full amplitude is 0.02 mag.  The variation was particularly well demonstrate in Itoh-san's long run."
So my question is: how do we define a UGWZ? All I know is that they are a subclass of UGSU, there is a long  gap (what does that mean?) between outbursts and outburts are of a wide range (how wide). Also apparently orbital periods are from 0.05-0.08d.
We have a working definition of UGZs now we need the same for UGWZs. Or maybe people believe it is well defined already? Or like Mike says, maybe they are just at opposite ends of a continuum?
Stephen [HSP]


SXN's picture
UGWZ are a sub-class of UGSU

The definitions we use for VSX do as good a job of explaining the differences as I've seen or read anywhere.

UGSS (SS Cygni-type variables) increase in brightness by 2-6 mag. in V in 1-2 days and in several subsequent days return to their original brightnesses. The values of the cycle times (period between outbursts) vary widely from 10 days to several thousand. 

UGSU (SU Ursae Majoris-type variables) are characterized by the presence of two types of outbursts called “normal” and “super-outbursts”. Normal, short outbursts are similar to those of UGSS stars, while super-outbursts are brighter by 2 mag., are more than five times longer (wider), and occur several times less frequently. During super-outbursts the light curves show superposed periodic oscillations (super-humps), their periods being close to the orbital ones and amplitudes being about 0.2-0.3 mag. in V. Orbital periods are shorter than 0.1 days; companions are of dM spectral type.

UGWZ (WZ Sagittae type) are a subclass of UGSU dwarf novae where the interval between super-outbursts is unusually long (due to a very low mass-transfer rate), typically measured in decades, while normal outbursts are few and far between. They show re-brightenings or "echo outburst". Orbital periods range from 0.05 to 0.08 days.

One excellent paper on CV evolution is The Evolution of Cataclysmic Variables as Revealed by their Donor Stars by Christian Knigge (University of Southampton), Isabelle Baraffe (University of Exeter) and Joseph Patterson (Columbia University).

You can get it from astro-ph here-

Mike Simonsen

spet's picture
Hi Eddy UGWZ stars have

Hi Eddy

UGWZ stars have amplitudes of 7 to 8 magnitudes. For example BW Scl in 2011 was 8 magnitudes, GW Lib in 2007 8 magnitudes, and WZ Sge 7 magnitudes. They only exhibit superoutbursts and not normal outbursts. UGSU stars exhibit both with a small amplitude. UGWZ stars superoutbursts repeat over a period of decades. UGWZ have 3 different types of superhumps starting with early superhumps which are double humped and decrease in amplitude over each day, followed by normal superhumps starting at a high amplitude and decreasing each day after that. This occurs for around 3 weeks decreasing in overall magnitude by around 0.1 and 0.2 magnitudes per day. This part of the lightcurve is called the plateau. There is a sharp dip in brightness by several magnitdues where the late superhumps start and are 0.5 out of phase with the previous superhumps. Some stars but not all exhibit rebrightenings  when the large dip occurs. The star then slowly decreases into quiescence over a few weeks and stays in quiescence for decades.

UGSU stars have amplitudes of 2 to 5 magnitudes and reoccur over a number of months.

The different types of CVs sit on a spectrum depending on their age or step in evolution. We like to group things and so some objectsof different groups may lie close in the spectrum. As CVs evolve the binary star separation decreases and orbital period increases. Stars below 'the period gap' are the dwarf novae and as they evolve further the secondary star mass diminishes and the gas transfer rate decreases resulting in less frequent outbursts. These stars become harder to detect due to their infrequent outbursts and low luminosity in quiescence. Eventually the secondary star becomes degenerate and behaves like a white dwarf and these stars have the minimum theoretical period of around 65 minutes. The UGWZ stars have periods close to this and are therefore very evolved. Theory predicts that CVs at the minimum period will increase in period. These are termed 'period bouncers'. This is quite a simplistic view as there are other differences in these stars for example magnetic fild strengths and primary stars being neutron stars or even blackholes..


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