Ronald Downes is an astronomer at the Space Telescope Science Insitute, and has spent his entire career (in one way or another) on HST. His thesis was a survey for CVs (he expected to find ~200, and found 1!), and has continued doing discovery-type work ever since. Besides CVs, he has worked on quasars, supernova remnants, and more recently carbon stars. Ron's observing has been done at University of California and National observatories, as well as with HST.
CVnet: What is the story behind how and why the original scientific paper, Downes and Shara 1993, PASP 105, 127; Downes, Webbink, and Shara 1997, PASP 109, 345, turned into an online catalog and atlas?
*Ron: There were two main reasons for the decision to go on-line.
First, publishing >100 page papers is very time-consuming (for example, getting all the formatting correct) and expensive. But more importantly, new objects and updated information were appearing so fast that the catalog was woefully out-of-date almost the day it was published. Also, the original aim of the catalog was to have one place were people could go for basic information on CVs, and since we did not reproduce all the finding charts with each edition, I was concerned that the catalog was getting too fragmented.
CVnet: The Downes et al, Living Edition of the Catalog and Atlas of Cataclysmic Variables is a continuous work in progress. Who are the others, the 'et al', involved in updating, researching and revising the catalog?
*Ron: My original collaborator was Mike Shara, who provided a double-check on the catalog entries, but more importantly, got me access to the HST Guide Star Catalog images, which allowed us to provide updated coordinates and the finding charts. Ron Webbink was the next to join, providing updated coordinates, revised/new identifications, and proper motions for many objects. Finally, Hilmar Duerbeck, who provided identifications for novae (most but not all from his published nova catalog), and Hans Ritter and Ulrich Kolb, who provide the period information (based on their separate period catalog), joined the team. I would be remiss if I did not identify the technical staff here at STScI who made the web site possible - Anne Gonnella and Steve Hulbert - and who help me with maintainance and enhancements - Mike Wiggs.
CVnet: What is the 'standard' for inclusion in the atlas? There seem to be many unconfirmed or unknown types and stars. Is this meant to be a clearing house for all 'possible' CVs or is there a higher standard?
*Ron: The standard for inclusion is either publication in a refereed journal, or a definitive report on an object (either from a newsgroup like cvnet or from a direct email sent to me). The catalog is indeed meant to be a clearinghouse for all objects confirmed or proposed to be CVs, although only those objects proposed in the published literature make it into the catalog.
CVnet: If someone discovers a potential CV or determines a typology from observations of superhumps or the lack of superhumps during an outburst, how is the type determination made for inclusion in the catalog? Do results of observations or discoveries from online data such as ASAS need to be published in some "official" form to be included in the atlas?
*Ron: As I mentioned above, information gets included in the catalog either from published results, or from definitive reports. As long as I hear about it, it can (potentially) go into the catalog.
CVnet: Along the same lines, why are there non-CVs in the catalog? Is this to eliminate or elucidate errors in other published works?
*Ron: The catalog includes all objects that are either CVs or were at one time classified as CVs. The non-CVs were left in so that people who only knew of the CV classification would not waste time observing the object. I would also note that the works that originally proposed the CV classifications are not necessarily in error, as the information they had may have been consistent with the objects being CVs. However, it is new information that has allowed us to refine the classification.
CVnet: There are types and sub-types of CVs recognized by different factions in the study of CVs. For example, UGSU sub-type ER, UGSU(ER), is recognized as a distinct sub-group by some, but not officially recognized in the GCVS or many other places. Another one is SW Sex stars. This is a group of stars that has its own websites and devotees, yet it is not recognized as a sub-set or type of CV by either the GCVS or Downes et al. Why? What is the Downes et al criteria for inclusion as a sub-set or type?
*Ron: I started with the GCVS classifications, and at the time of the first edition, added what I thought were established sub-types. The GCVS is still the official keeper of variable star classifications, so I would only accept an new sub-type (not in the GCVS) if I thought the CV community as a whole accepted it. For example, I added the VY Scl sub-type for NLs.
CVnet: Looking at the bigger picture, how do you wish or hope the online catalog will benefit astronomy in general? Are there uses or potential uses of the catalog that have not yet been realized?
*Ron: I am hoping that the catalog serves the CV community, both professional and amateur, to aid in obtaining data on CVs. I have already seen papers come out from people who are trying to obtain modern spectra for the many objects without spectroscopic data, and those who are trying to confirm the CV nature of little-studied objects. And others who are trying to recover the quiescent counterparts to objects where there is no certain identication. I hope that having the catalog available can facilitate these studies, which will allow us to get a better picture of what CVs are, and how they work.
CVnet: What can CVnet observers and participants do to contribute to the future and potential of the online CV catalog and atlas?
*Ron: Just continue making your observations and reporting them. I subscribe to cvnet-discussion, and always welcome direct emails on new discoveries or updated identications/classifications.
CVnet: You've been great to discuss this with us. Will you stay in touch, and let us know about future developments on the website?
*Ron: I certainly will!Catalog and Atlas of Cataclysmic Variables Search Downes et al
Dr. Boris Gaensicke is an assistant professor in the Department of Physics at the University of Warwick, England. His main research interests are the evolution of compact binaries, in particular cataclysmic variables, observational population studies of cataclysmic variables, stellar atmospheres, and ultraviolet astronomy. He is a regular contributor to CVnet discussion, and supports amateurs in any way he can in the field of CV study!
CVnet: Over the past couple of years, your CV's from the HQS (Hamburg Quasar Survey) have become an important source of objects for amateurs to monitor. How did you become involved with this, and where do you see this line of research extending in the future? Also do you have any plans to update or even extend your web list?
Boris: As a PhD student, most of my research was focused on determining the properties of white dwarfs in cataclysmic variables. It soon became clear that the observed characteristics of these accreting white dwarfs, such as their temperatures, masses, rotation rates or chemical abundances are very tightly linked to ways that CV’s evolve. Reading up on this topic, I realised that while there were a number of very detailed theoretical models, the amount of observations to quantitatively test our understanding of CV evolution was rather meagre, and the comparisons between the available data and the models showed a number of strong conflicts. One reason for this discrepancy are selection effects: while there were already hundreds of CVs known in the mid-90's, they formed a very mixed bag of objects discovered by all kinds of techniques. For a real test of the theoretical models, we will need a large and well-defined sample of systems where we understand the selection effects. Looking around, the best there was at the time were the ~35 CVs from the Palomar Green Survey, many of which John Thorstensen and Fred Ringwald hunted down. I was aware of a number of studies on single white dwarfs coming out of the Hamburg Quasar Survey, and I realised that that survey might be very useful for CV work as well. So I got into touch with Hans Hagen and Dieter Engels, two of the master minds behind the HQS, and asked them simply "can I dig in your waste bin for zero-red shift emission line objects?". Their answer was "yes". Hans and Dieter were very helpful in helping me understanding how to use their survey data, and how to select CV candidates for follow-up studies.
Yes, I will keep on updating the HQS CV web page, and adding some additional objects. In fact, that page really needs a major overhaul, but as always, there are not enough hours in a day to get things done in a timely manner. The current state of the project is that we have identified about 50 new CVs, and there are still about a dozen of those that need some more data and analysis before being published.
Along the lines of what I said above, I think that we have seen over the last years an enormous growth in a relatively new type of CV research: "observational population studies", which quite simply means trying to (1) establish a sample of CVs that were selected in a homogenous and well-understood way, and (2) to determine the detailed properties of the individual systems in this sample. Just to illustrate the growth in this field, consider that the total number of CVs in the PG survey (new discoveries and previously known) was ~35. Covering the same sky area, but going ~1.5mag deeper, the HQS contains ~100 CVs. And SDSS, though still smaller in sky area compared to PG and HQS, contains already ~250 CVs, a number likely to grow to >300 by the end of the SDSS operations. It will take many years to establish the details on each of these systems (and monitoring their long-term behaviour is part of this endeavour), but this sample has the potential to finally unlock our understanding of how CVs evolve.
CVnet: I sense an emergence of interest in magnetic CV's amongst amateurs these days, to which you have played no small part in stimulating that interest. How do you see the amateurs role in monitoring magnetic systems, apart from monitoring a change of state from high to low and vice versa? The same question might apply to SW Sex stars and NL's.
Boris: I think that detailed monitoring of magnetic CVs and novalike variables is a very important task in itself. We know so little about the long-term behaviour of these systems, with e.g. really only one very well studied polar: AM Herculis. In Polars and novalikes, we have the great opportunity to study the white dwarf and the donor star in great detail during their low states, when the accretion activity decreases or stops altogether. However, catching these low states is as ambitious as finding dwarf novae in outburst, so regular monitoring is essential!
Besides regular monitoring, many amateurs are now well-equipped to obtain time-resolved photometry of these systems to complement e.g. satellite observations of magnetic CVs. Another task where the community can get involved is to keep track of ephemerides, e.g. the white dwarf spin periods of IPs, or the orbital periods in novalike variables or Polars. Such "time-keeping" tasks are, just as the monitoring of the system's brightness, long-term tasks that will pay off only after a couple of years, maybe even decades. But they are very worthwhile, and just can not be done by professionals because of the way that their observatories operate.
CVnet: Staying with the magnetic CV theme, why are high magnetic field systems apparently less active than lower field Polars?
Boris: A possible reason is that the strong magnetic field of the white dwarf reduces the average value of the accretion rate in these systems, and hence they spend much more time in low states than "normal" low and intermediate-field Polars. As a consequence, they are harder to find, as they are, for most of the time, inconspicuous faint binaries with no X-ray emission and no strong variability, such as AR UMa. They just have odd colours, and one might expect that high field Polars should turn up in SDSS. However, not too many new ones have been found so far, and thus it remains a bit of a puzzle why they are apparently quite rare.
It is thought that for CVs above the gap, "magnetic wind braking" is responsible for removing angular momentum from the system. It’s the same process that is slowing down the rotation of the Sun, i.e. wind flowing off the star, carrying away angular momentum. In CVs, because the spin period of the secondary star is tidally locked to the orbital period, this process extracts angular momentum from the binary orbit, bringing the two stars closer together.
Now comes the point where the magnetic field of the WDs in Polars enters the equation. Some people believe that in the case of a strong field on the WD, its magnetic field lines reach out into the secondary star, effectively suppressing the "magnetic wind braking" by some fraction - and the consequence would be that the strongly magnetic systems have on average lower mass transfer rates than the non or weakly magnetic ones.
We don't really understand the details of what is happening at the surface of the secondary star, but as L1 is the point closest to the WD, and has the lowest gravity on the secondary star, it is quite likely that the effect of the WD magnetic field is felt most strongly in L1 - and in high-field polars, this could result in more frequent/extended spot coverage.
CVnet: HS2331, or V455 And as we now know it - is an object you discovered from the HQ survey. You had time on the WHT as it went into outburst (an incredible coincidence). How did this affect your planned timetable on the WHT, and can you give us an insight as to what you have learned up to now from this outburst, and have we learned anything new from it? Also it seems unusual that we haven't yet seen any echo outbursts from this system. Why do some short P_orb systems show rebrightening, whilst others do not?
Boris: Ah, I will not that easily forget that night of September 4, 2007! I was on the WHT, in my second night out of a total three, on a programme to observe CV progenitors. Just before evening twilight, I got the vsnet massage from Hiroyuki Maehara that HS2331+3905 was brightening, and a quick follow-up from Gary Poyner a few minutes later. As soon as it got dark, we spent about 2h on HS2331+3905, which still showed emission lines, though much weaker than in quiescence, and we could see it brighten during that time on the slit-view camera. I then had to knock off a few of the other program stars, as my collaborators would have had my ears for breakfast had I given up the pre-CV programme altogether. Another quick look at HS2331+3905 in the middle of the night suggested that it settled into the "optically thick" state, i.e. a spectrum with smooth absorption lines. As those lines typically don't reveal much information, I went back to our programme stars. However, when I had a third look at HS2331+3905, about 2 hours before sun rise, it was back with emission lines, in particular tremendous Helium II emission. So I spent the last 2 hours on it, chewing my hat that I did not totally override our pre-CV observing programme... The next night, things had stabilised in an emission-line state, and I got another ~3.5 hours of high-speed spectroscopy.
So, what have we learned? At least two things: (1) the onset of the outburst is apparently a very complicated story. I am not aware of any spectroscopy that early during the rise of a UGWZ, and we are only at the beginning of working out what those spectra tell us. (2) the white dwarf in HS2331+3905 is indeed rotating at a spin period of about 60sec. This is confirmed by the detection of that period in the wings of the emission lines during the outburst peak, which implies that material very close to the white dwarf is flung around on its spin period. All in all, I think that we can be pretty certain that HS2331+3905 is a weakly magnetic CV. Such a field could truncate the inner disc - could that be a reason for the lack of echo outburst? I must admit that I have no definite answer to this question.
Just a quick note on HS2331+3905, aka V455 And nowadays: Please keep on monitoring this star for as long as possible, as it will be important for the understanding of the system to know how quickly it fades towards quiescence.
CVnet: It seems to me that amateurs spend a lot of time characterising UGSU's by time resolved photometry. But with the in-depth attention given to UGSU’s in recent years is it still worthwhile to observe new UGSU stars?
Boris: In principle, yes. The work initiated by Joe Patterson on the epsilon-q relation of superhumping systems has the potential of adding some quantitative measure to our understanding of CV evolution, and increasing the known set of superhumpers seems still valuable. This being said, I think that CV research urgently needs amateur support to compile high-quality long-term light curves of non-eruptive CVs, i.e. Polars and novalike variables, for the reasons outlined above. I understand that observing UGSU systems is probably more fun, as you lie waiting for an outburst, and once it starts, the real fun begins... For Polars and novalikes, it is the opposite: patiently waiting for the rare low states, and once they happen, the systems become too faint for most amateurs to be followed.
CVnet: The UGWZ subgroup is rather small when compared to the number of known UG stars. Are they really that uncommon, or do you think there are still a lot to discover? It seems to be like the eclipsing dwarf nova quandary. There should be more than we know about!
Boris: Yes, the number of confirmed UGWZ is currently still fairly small... If we look at optical spectra of UGWZ, we note that they are typically dominated by the white dwarf, with no hint of the secondary star (e.g. WZ Sge or GW Lib), i.e. they have the appearance that one would expect for systems with very low mass transfer rates. Now, browsing through the SDSS spectra that Paula Szkody has published over the past six years, it is apparent that there are ~35 new systems which match the spectroscopic UGWZ fingerprint. And one of them, SDSSJ0804+5103, already proved itself as a genuine UGWZ. Thus, another project that I would really like to promote in the community is the detailed monitoring of the new CVs from SDSS, and I am pretty sure that follow-up of SDSS CVs will increase the number of UGWZ by a factor two to three.
CVnet: You spent many years waiting for V455 And to outburst. Now that it's finally happened, which object would you like to nominate as the next 'one to watch'?
Boris: On the top of my "most wanted" is SDSSJ1035+0551, which is a very likely UGWZ. A detailed analysis of high-speed eclipse light curves by Stuart Littlefair unambiguously demonstrated that this CV harbours a brown dwarf donor, so one can be relatively sure that this is a very evolved CV. An outburst study, including the measurement of its superhump period would be fantastic, but who knows, maybe we will have to wait for another 20 years...
Hamburg Survey CVs- A prioritized list can be found on this web site at: http://deneb.astro.warwick.ac.uk/phsdaj/HQS_Public/HQS_Public.html
BAAVSS Polar Programme- Set up to monitor AM Her type stars in need of further investigation. Details here: http://www.garypoyner.pwp.blueyonder.co.uk/vsspolar.html
Effective March 1, 2005, Dr. Arne Henden will assume the Directorship of the AAVSO.
The AAVSO's relationship with Henden began in 1997 when he provided photometry for variable star charts. Soon after, he became chief adviser to the AAVSO International High Energy Network. Since then, his association with the organization has grown to include advisory roles for the AAVSO Eclipsing Binary and RR Lyrae committees, three High Energy Astrophysics Workshops for Amateur Astronomers, the AAVSO Chart Team and also the publication of CCD Views.
As a scientist, Henden has been a co-author on over a hundred refereed publications, including 3 papers in the journal Nature. He has several papers in progress, mostly related to gamma-ray burst afterglow observations with various worldwide collaborations, and cataclysmic variables for the Sloan Digital Sky Survey (SDSS). In collaboration with Dr. R. H. Kaitchuck, he wrote the textbook Astronomical Photometry, widely regarded as a fundamental text for learning photometry.
Anyone who observes cataclysmic variables using AAVSO charts is aware of the collaboration of Arne and Bruce Sumner in providing hundreds of sequences upon which the latest AAVSO CV charts are based.
We're going to ask Arne about the role of visual and CCD observers in cataclysmic variable research, and the prospects of professional-amateur collaboration in the foreseeable future.
CVnet: Congratulations on your being appointed Director of the AAVSO! Two questions come to mind immediately. How will this impact your ability to contribute photometry for sequences for AAVSO charts, and will your being Director mean a shift in AAVSO policy towards CCD observing and away from the traditional role visual observers have played in the history of AAVSO?
Arne: Thank you for the congrats! Being Director will impact my ability to personally obtain photometry for sequences, though probably not as much as you might think. I will be moving my TASS camera down to our Flagstaff house (which we will keep for retirement) and setting it up for bright all-sky calibration. I am planning observing runs at my old NOFS stomping grounds, as well as submitting proposals to the national observatories for doing calibration work. Brian Skiff has implied that he is willing to do some all-sky photometry for the AAVSO. Of course, we intend to continue mentoring amateurs in becoming good all-sky photometrists.
As for shifting policy, my main goal has been and always will be to make amateur and small-college observatories more productive and generating higher quality, scientifically useful data. I've spent thousands of hours in calibrating fields, so that we have good sequences for both visual and CCD observers. I don't care whether an observer uses an expensive CCD camera or a pair of binoculars to measure a star; I just want the best results. There are many areas of variable-star astronomy where visual observations are valuable, and I don't see that changing much in the future. At the same time, CCD observations are still in their infancy and will be given a lot of attention since their complexity requires more care. So the shift will be in the sense of "more" rather than "less."
CVnet: Recently, the AAVSO seems to be more proactive in initiating observing campaigns and producing scientific papers based on observations of unusual or scientifically rewarding CV systems. Is this something you will be encouraging the AAVSO and its members to pursue in the future?
Arne: You bet. Campaigns have many beneficial aspects. Amateurs see real-time involvement with other observers (Aaron's chat room for the past BZ UMa outburst is a great example). Professionals come on-board with specific projects, get the data they need as well as an understanding of how valuable a resource amateurs can be. Papers highlight AAVSO results, giving free advertising to our organization. In addition, some science really does require the 24/7 type of monitoring, and campaigns provide that important database. Amateurs have the advantage of "numbers" (there are thousands out there) and worldwide geographic distribution.
Again, this is in addition to our normal monitoring activities, but I think campaigns are a lot of fun.
CVnet: As a co-author of the Sloan Digital Sky Survey (SDSS) CV papers, do you see follow up observations and long term monitoring of any or all of these systems as fertile ground for pro-am collaboration? Is this an area of research you will be encouraging AAVSO observers to get involved with?
Arne: Something over 100 new CVs have come out of the SDSS. Most of these are faint, low-accretion-rate systems. In quiescence, they are difficult for most amateurs to measure. However, most of the AAVSO CVs are also faint in quiescence; it is their bright outbursts that amateurs generally follow. I think it is a very important monitoring project to observe the SDSS CVs and determine their outburst behavior. I have so many projects clamboring for my attention that I never know which ones to highlight, but monitoring the SDSS CVs is an obvious new project for the AAVSO.
CVnet: What other areas of CV research do you see as fertile ground for pro-am collaboration, and how can visual and CCD observers contribute?
Arne: I have a few pet CV projects, such as predicting outbursts, looking at "stunted" outbursts, and following eclipsing systems. These would be primarily monitoring projects. However, I'm just one researcher, and the ideal future is to get many more professionals involved with the AAVSO and CV research. Outburst alerts for target-of-opportunity satellite observations have always been an important part of both visual and CCD observing programs. My goal here is to continue these opportunities, and to get more amateurs and professionals alike involved.
CVnet: As the new Director of the AAVSO, is there anything you would like to add regarding the future of CV research or the role CVnet can play?
Arne: Ah, the open-ended question! CVs are hot topics right now because interacting binaries are so prevalent in many areas of astrophysics. I think the future is bright (so to speak) and full of opportunity for the small-telescope observer. Surveys keep producing more and more lists of new objects that need followup observations, and with more objects come the rare cases that require even greater attention. CVnet is a useful invention at the right time, and I wish it well in disseminating information about CVs along with being a clearing-house for projects and providing instruction for observers. I'm glad that it has strong ties to the AAVSO.
For literally hundreds of references on the web, Google Search on "Arne Henden":http://www.google.com/search?q=Arne+Henden&hl=en&lr=&start=0&sa=N
For links to abstracts and full papers, arXiv.org Search Results by author: http://arxiv.org/find/astro-ph/1/au:+Henden_A/0/1/0/all/0/1
Steve Howell is an astronomer at the WIYN Observatory located on Kitt Peak near Tucson, Arizona. Steve pursues research on interacting binaries, extra-solar planets, and observational techniques with CCDs.
Steve has authored so many papers on cataclysmic variables that a Google search results in hundreds of hits. He is also author of the 'Handbook of CCD Astronomy' (2nd Edition coming out this fall), and he's got his own space rock named after him, asteroid 15091, Howell. By the way, this asteroid is eccentric (in its orbit), a fact unlikely to be random.
We're going to discuss polars, magnetic fields and the nature of the secondary stars in these systems.
CVnet: Hi, Steve. To begin with, can you give us a working definition of a polar?
Steve: A Polar is a binary star containing a white dwarf and a more-or-less normal late type star. The two stars are very close (a solar diameter or less) and the white dwarf has a very strong magnetic field. The field strength on the white dwarf can be 10 up to 250 MegaGauss. The gravitational field of the white dwarf, a star of 1/2 to ~1 times the mass of the sun but of a size about equal to the Earth, is so strong that material is pulled from the secondary star. This material falls onto the white dwarf at one or both of its magnetic poles, releasing tremendous amounts of energy. The light from this energy release outshines both the white dwarf and the secondary star in the optical and infrared.
These binaries are short orbital period systems, 80 minutes to about 8 hours, and tend to be discovered via their copious x-ray emission or optical brightness. Oh, by the way, they are called polars as the first to be discovered, AM Herculis, showed (as they all do) highly polarized optical light. This group of cataclysmic variable stars are also called AM Her stars.
CVnet: Where does the intense magnetic field in the white dwarf come from?
Steve: The intense magnetic field of the white dwarf is believed to come from the original star which produced it. A class of A stars with higher than normal magnetic fields are thought to be the progenitors of highly magnetic white dwarfs. The connection is far from proven, but seems logical.
CVnet: Doesn't the presence of a strong magnetic field complicate our understanding of these systems? What kind of computing power is necessary to model polars?
Steve: YES!! True, three-dimensional, fully correct models are yet to be realized. Jennifer Cash, an astronomer at the South Carolina State University, has about the most detailed models to date and runs these smooth-particle hydrodynamics on supercomputers. Each "run", to model one part of one system, may take weeks to compute fully.
CVnet: When a polar is in its low state you are better able to study the secondary. Why is this, and what have you learned about the nature of the secondaries in these interacting binaries?
Steve: First off, lets talk about low states and the corresponding high states. Polars have times of high rates of mass accretion (mass pulled from the lower mass secondary star to the white dwarf) and times of low to nearly zero rates of mass accretion. These high and low states are apparently random and can cause the polar to change by 2 to 5 magnitudes in optical brightness. The cause is thought to be related to stellar activity on the late type star (star spots etc.) but absolute proof of this is hard to come by.
OK, low states... During these times, the mass transferred is often very low to nearly zero, and the two component stars now become the dominant light sources in the optical (white dwarf) and infrared (secondary star). Thus, during low states one can actually observe the stars themselves.
During the low states, study of the secondary stars has led to an amazing discovery. Theory had predicted that if a secondary star lost mass to its white dwarf companion for a long time, maybe up to nearly the age of the Galaxy, it would become a very low mass, non-normal star like object. The polar EF Eri has not had mass transfer for over 9 years now and detailed study of its secondary reveals an object best described as a cross between a former star, an odd brown dwarf, and an extra-solar planet.
The secondary has become a small (Jupiter size) low mass (about 40-50 Jupiter masses) very cool (T=1100K) object. This former star is not like anything else known in the universe. Four other candidates are in the works including the recent campaign on the polar VV Pup which included a large participation by the AAVSO. Initial results are that the secondary in VV Pup is similar to that in EF Eri.
An additional interesting observational fact about EF Eri is that at present, optical observations only detect the white dwarf and IR observations only detect the secondary. It'd be very hard today to know this star is a polar let alone even that it is a binary star.
CVnet: Polars appear to be a small sub-set of cataclysmic variables. Is this a reality, or due to some observational selection effect?
Steve: Single, highly magnetic white dwarfs make up about 20-25% of all single white dwarfs. Polars make up about 20-25% of all cataclysmic variables. So maybe the numbers are about correct.
However, every time a new X-ray mission or large area optical survey occurs, more polars seem to be discovered.
CVnet: So far, we've talked about research done with large telescopes, spectroscopes and super-computers. What can observers with modest telescopes contribute to researching polars?
Steve: Here is where we can get serious about observing. The AAVSO has a wonderful polar program they are starting in connection with myself.
The idea is two-fold:
1) Look at polars as often as possible to try to get better statistics on the times of high and low states and how often they occur. Do these measurements in colors to extract physics as well.
2) Obtain time series observations, that is, stare at one polar as often as you can for as long as you can, and collect a light curve. Again do this in color and do it at high states and , if you can, at low states.
Both of these projects are not generally doable by professionals due to the amount of telescope time required. Check out the AAVSO web pages to get the details of this program.
Although not comprehensive, this astrophysics pre-print search result will bring up a list of papers authored or co-authored by Steve Howell. http://arxiv.org/find/astro-ph/1/au:+Howell_S/0/1/0/all/0/1
Guy Hurst has been an active observer of Variable Stars for over 35 years. He is past President of the British Astronomical Association and has been Editor of the UK based magazine 'The Astronomer' since 1975.
Guy has become the 'central hub' for UK and on some occasions worldwide discoveries. He has been instrumental in providing Professional Astronomers with data on Variable Stars and indeed many other objects too. Here CVnet discusses his role in Pro-Am collaboration.
CVnet: How did you get involved with Pro-Am collaboration?
Guy: I have always supported the cause of Pro-Am. However upon taking the role as editor of the monthly UK magazine 'The Astronomer' in 1975, I found that because observations were published so rapidly we began receiving enquiries for the latest news on a wide variety of objects in the sky. Variable stars were of particular interest to professional astronomers even in those days!
At the about the same time as I took on the role of editor, I also began to exchange notes with Dr. Brian Marsden at the Central Bureau for Astronomical Telegrams. He told me that there were so many discovery claims sent direct to him without always being independently checked, that subscribers to 'The Astronomer', who number some of the most active observers in the world, could assist the Central Bureau in checking these reports.
From 1975 Brian referred many such reports to me which I relayed to a 'checking team' who were willing to be on 24 hour call. Gradually many observers routed their claims through us for checking before relay to the Bureau and in a typical year such discovery reports might number over 100. Inevitably many such reports did not result in a confirmed discovery. Ironically many of the nova claims turned out to be genuine objects in the field of view but later were found to be existing variable stars, many of which had not reached a listing in the main catalogues such as the GCVS.
CVnet: From personal experience, how do Professionals generally regard amateur data?
Guy: Most professionals regard data from amateur astronomers of considerable value. Again results on variable stars seem to be in frequent demand. There are so many variables in the sky and with the difficulties of professionals obtaining telescope time, amateurs are well placed to assist with the monitoring of such objects.
However a second important aspect is the databases maintained by groups all around the world. Often variables which have been regarded as obscure objects suddenly gain prominence due to research by professionals who then need to know of the activity of the star in the past. Organisations such as the AAVSO and BAAVSS have lengthy series of results on many variables which enables an 'instant history' to be supplied on a star which has suddenly gained prominence.
CVnet: What is the current status of Pro-Am collaboration in the UK?
Guy: I recently served a two year term as President of the British Astronomical Association and one of the important tasks each year is the presidential address. In my second year I gave my lecture in London on the theme of Pro-Am. Of course this covered many objects other than variables but it was clear that collaborations remain healthy. However I have always looked at this from an international point of view and requests, whether from the UK or further afield, should be treated in the same way.
One of the reasons we should continue this policy is the friendship we have built up with professionals and their amazing help given in answer to many enquiries we send to them. It is still quite rare that I do not receive a reply, almost instantly, to requests for help.
One of the cases which will remain in my mind forever was on the morning of 1996 October 23 when Mark Armstrong of Rolvenden, England telephoned at 04.05UT (the 'dead of night' here!) to report a possible supernova found in NGC 673. After gradually waking up (!) I realised that if this was true it was the first ever supernova to be found from England in living memory!
As the sky became light here, messages to Robert McNaught and Gordon Garradd in Australia brought forth optical confirmation the same day. However the spectrum was crucial for confirmation of the object. One of my appeals was sent to the ESO in Paranal and they used one of the telescopes almost immediately to confirm it was a type Ia event. Quite remarkably, IAUC 6497 was issued by Dan Green carrying details of the discovery, less than 24 hours after the object was first reported by Mark to me! Pro-Am Co-operation at its very best!
On an International basis I was also much encouraged during my trip to Huntsville, Alabama in 2000 at the kind invitation of the late Janet Mattei. Through my membership of the AAVSO since the 1970's, we had known each other for a great many years and met on several occasions at various international conferences. She assured me attendance at the HEA Workshop to meet NASA officials and other amateurs and professionals from around the world was vital. One of the main tasks was to monitor optical counterparts to Gamma Ray Bursters. But meeting so many professionals for informal chats during the three days was not only memorable but an assurance that there remained much scope for joint projects involving the two groups.
CVnet: Which area's of Pro-Am collaboration provide the most useful results, and which area's could amateurs get more involved with?
Guy: Within the collective umbrella of variable stars, past records suggest that 'eruptive' objects are ideal candidates. Alerting professionals to the start of a climb to maximum avoids valuable professional telescope/satellite time being wasted in monitoring. Simultaneous monitoring by professionals and amateurs over an agreed time schedule and in various wavelengths still remains useful in understanding the physics of these stars.
I still regard visual observations as important even though I fully acknowledge that the development of imaging and associated CCD equipment has allowed amateurs to move on to more precise measurement and monitoring of some variables. We must not underrate the visual observer and their ability to detect sudden change in stars which may have been at minimum for a year or longer. I initiated a 'Recurrent Objects programme' many years ago for just this reason. This is still run by Gary Poyner and is, I believe, a worthwhile project to continue.
My main interest though remains in the discovery and monitoring of novae and supernova. The UK Nova/Supernova Patrol was formed by John Hosty and myself in 1976 for just this purpose. More than 100 supernovae and many novae have been found by its members. This is vital work which should continue but we need more help in monitoring old novae which have already been mentioned from time to time on CVnet. Many such objects are totally neglected after the fade from the bright phase and older objects, long forgotten, may still be active near quiescence.
CVnet: Generally Professionals tend to approach the AAVSO first for amateur data on Variable Stars. How can groups outside of the AAVSO (such as the BAAVSS) compete with this, if indeed they should try, and might CVnet have a role to play here with regard to raising the profile of your work in CV related subjects?
Guy: I would like to see BOTH professionals and amateurs describe their special interests through the medium of CVnet. Do members of each group really know how they can help each other or the ideal target variables?
There is no real need for organisations such as the AAVSO and BAAVSS to compete and I would like to see systems where the databases of such worldwide organisations could be linked even if each observation was tagged so information on the observers' organisational membership was retained.
There is also more scope for co-authorship of papers by amateur and professional astronomers. Such publication is a clear declaration of intent that the two sides will continue to work together.
Pro-Am is alive and well and we can continue to help it flourish through many areas including the valuable addition of CVnet.
Related links: The Astronomer Online The British Astronomical Association BAAVSS Recurrent Objects Programme
Koji Mukai was born in Osaka, Japan, and graduated from University of Tokyo
with a Rigakushi (B.Sc.) degree.
For his graduate study, he chose University of Oxford. There, he worked with
Phil Charles, Robin Corbet and Alan Smale, among others, on interacting
binaries, completing his Ph.D. thesis on AM Her type systems in less than 3
years and 1 month. He then spent 3 years at Mullard Space Science
Laboratory, working with Keith Mason, Alan Smale, Simon Rosen and Coel
During his 6 years in England, he was mostly an optical astronomer, and had
6 observing trips to La Palma, 3 to South Africa, 2 to Australia, and 1 to
Hawaii, using world-class telescopes with state-of-the-art instruments
located at the best sites in the world.
After moving to the United States, Koji spent 2 years at UC Berkeley's
Center for EUV Astrophysics, working on preparations for the EUVE all-sky
survey. In January 1992 Koji joined NASA Goddard Space Flight Center as an
USRA research scientist. Initially, his work was on the Japanese-US ASCA
mission. More recently he has worked on the ill-fated ASTRO-E mission, and
now the Astro-E2 mission, renamed Suzaku after launch.
In recent years, Koji has focused his research efforts into the X-ray observations of magnetic cataclysmic variables and other accreting binaries.
It is these magnetic CVs, specifically intermediate polars (IPs) that will
be the topic of out interview.
CVnet: Hello, Koji. Thank you for agreeing to participate in this CVnet
IPs are the CV class with the most ferocious debates about membership or
not. On your web site you refer to yourself as the 'Curmudgeon', referring
to your strict requisites for inclusion as a member of the IP class, so let'
s begin with the definition of an intermediate polar.
Intermediate polars are systems between non-magnetic CVs and the AM Her
systems. The accretion process in IPs is through a disc with a disrupted
inner radius, or an accretion stream (as in the polars), or both. By virtue
of a lower magnetic field (as compared to polars) the accreted area on the
white dwarf is larger, and typically extends over a hemisphere. The white
dwarf spin is not synchronous with the orbital period, as in the AM Her
systems. The spin period of the white dwarf is shorter than the orbital
What other characteristics would you include to define a "genuine IP"?
Mukai: Nothing at all. The definition is fine, I think pretty much
everybody agrees. Any differences of opinion are in how to apply
this definition to real-life data. What I'm trying to do, as the
Curmudgeon, is to curb the enthusiasm of some observers. I mean,
if you see a peak in the periodogram of a three-hour stretch
of photometry of a CV, you shouldn't jump to the conclusion that
you've discovered an IP. After all, all CVs vary on a variety of
timescales, and that will produce a peak in the periodogram somewhere.
You need more than just half a night's photometry to be sure you're
looking at a persistent, periodic, characteristic of the system.
CVnet: The intermediate polars with the shortest spin periods and weakest
magnetic fields are called DQ Herculis stars, although there has been some
discussion concerning whether they really deserve to be called a separate
class of object. Are all DQ Her systems IPs, are all IPs DQ Hers, or is
there a distinction?
Mukai: I personally don't think there are two distinct classes - there
are quantitative differences, yes, but not qualitative one. So, to me,
they are all IPs. You can call them all DQ Her systems, like Joe Patterson
prefers to do.
CVnet: How does the relative stability of the spin period provide evidence
for white dwarf, as opposed to neutron star, nature of the compact object.
Mukai: Speaking of Joe, I think he was the first person to point this out,
way back in 1981 in his Nature paper. Basically, if you compare the radius
of a white dwarf versus that of a neutron star, a white dwarf is about a
thousand times bigger. And that makes them more stable - it's much harder
to change the spin of a white dwarf than the spin of a neutron star. So,
observationally, if you see the spin of an X-ray pulsar changing quickly,
then you suspect it's a neutron star. If the spin doesn't change much,
either you were unlucky or the system has a white dwarf.
CVnet: Will you explain what a spin up or down is, and how it relates to the
study of these systems?
Mukai: If you measure the spin period of an IP accurately one year,
and come back the following observing season, you might find that
the spin period is now slightly shorter - that's spin up - or slightly
longer - that's spin down. Since white dwarf spin is stable, it takes
years of collecting a lot of photometry to really see if an IP is spinning
up or spinning down. In some cases, one system might do both!
IPs probably last hundreds of millions of years as IPs, gradually
changing its orbital period and such. If IPs never deviate from such long
term average, it should be exceedingly difficult to detect any spin ups
or downs. The fact that we do see them mean that IPs do deviate from
their long-term average on timescales shorter than, say, millions of
years. In simple terms, if the accretion rate goes up, the torque
exerted by the accreting gas wins and the white dwarf spins up.
If the accretion rate goes down, the braking effects of the rotating
magnetic field wins and the white dwarf spins down. But the physics
is rather complicated - any time you have an interaction of plasma and
magnetic field, it's hard to figure out exactly what will happen (just
ask the people trying to build nuclear fusion reactors!). So, getting
numbers out of spin ups and spin downs is not easy - or rather, you can
extract numbers but you never know how much you can trust them.
CVnet: Some IPs, [GK Per, DO(YY) Dra, HT Cam, and EX Hya] , exhibit
outbursts. These are probably the systems most familiar to CVnet observers
and participants. What is the current thinking on the cause of these
outbursts, and why are these systems different than the IPs that don't
Mukai: You would think that the mechanism is the same for these IPs
as in ordinary (non-magnetic) dwarf novae. By that logic, the IP
outbursts must also be due to instability in the accretion disk.
CVnet: Some IPs have shown occasional low states in archival plate
photometry, but probably not as frequently as in AM Her type systems. What
are the possible causes for this behavior?
Mukai: I'm mostly going to punt on this question, because nobody really
knows why some CVs go into low states. Yes, some clever people have ideas,
but there are far more questions than answers on this topic, in my opinion.
There are some ideas why AM Her type systems go into low state more often
- perhaps because they don't have a reservoir of mass in the form of an
accretion disk, perhaps because the magnetic field would force the plasma
to climb up the gravitational potential (that's hard to do!) under certain
But mostly, I'd like to appeal to the readers of CVnet to watch out for
IPs going into low states. Because, as far as I know, the only known
examples of IPs going into low states are from archival plates, discovered
many years after the fact, and nobody has caught one in action. Low states,
if you can catch one, are a great opportunity to learn about the underlying
stars uncontaminated by accretion. That would be great! In contrast, AM Her
type systems go into low states so often that many of us now consider low
states to be an annoyance as far as AM Her systems are concerned.
CVnet: Let's talk about the evolutionary track of these systems a bit. The
commonly proposed evolutionary history of non-magnetic and magnetic CVs has
been assumed to be similar. Recent results, infrared spectroscopy of two
dozen CVs in Harrison et al (2004, 2005) and UV spectroscopy by Gaensicke et
al 2003, find evidence for peculiar abundance ratios in the secondaries of
non-magnetic CVs; specifically deficits of carbon and enhancements of
nitrogen. If magnetic CVs follow the same evolutionary path, one would
expect to find similar abundances in the secondary stars of magnetic
systems. In Harrison et al, Astrophys.J. 632 (2005), several
polars were examined and the spectra of these secondaries are consistent
with normal late type dwarfs, suggesting that the evolution of secondaries
in magnetic systems is different than that of non-magnetic systems.
Have there been similar studies of the secondaries in IPs, and if so, are
the secondaries of these systems normal late type dwarfs or do they exhibit
the same peculiar abundance ratios as non-magnetic systems?
Mukai: By and large, the secondary of IPs have never been convincingly
detected - with the exception of GK Per and YY Dra, I think. Accretion
rates are so high in IPs, it's hard to see the secondary (and believe me,
I've tried). That's another reason to want to see them go into low states.
CVnet: One of the goals of CVnet is to bring professional and amateur
astronomers together to the mutual benefit of both. What can interested
amateur observers with modest telescopes do to help in the understanding of
Mukai: CVnet observers can watch out for outbursts and low states of
IPs - at the moment, I don't have anything specifically set up to observe
outbursts, but it's always useful to know. As I said, if an IP is found
in a low state, that can be a gold mine of information. The other main
thing is to follow the spin ups and spin downs - the best way may be to
join Joe Patterson's CBA network, which always have campaigns set up to
make sure there are enough data on IPs to be able to track their spin
CVnet: Thank you very much, Koji. I hope you enjoyed this and will agree to
talk with us again in the future.
Mukai: Sure, it's been a pleasure.
The Curmudgeon's IP Home Page: http://lheawww.gsfc.nasa.gov/users/mukai/iphome/iphome.html
Kuntz, K.D., Gruendl, R.A., Chu, Y.-H., Chen, H.-C. R., Still, M., Mukai, K., Mushotzky, R.F. 2005, "The optical counterpart of M101 ULX-1," ApJLett 620, L31-L34
Mukai, K., Orio, M. 2005, "X-ray Observations of the Brigt Old Nova V603 Aquilae," ApJ 622, 602-612
Belle, K.E., Howell, S.B., Mukai, K., Szkody, P., Nishikida, K., Ciardi, D.R., Fried, R.E., Oliver, J.P. 2005, "Simultaneous X-ray and Optical Observations of EX Hydrae," AJ 129, 1985-1992
de Martino, D., Matt, G., Mukai, K., Bonnet-Bidaud, J.-M., Gaensicke, B.T., Gonzalez Perez, J.M., Haberl, F., Mouchet, M., Solheim, J.-E. 2005, "X-ray Confirmation of the Intermediate Polar HT Cam," A&A 437, 935-945
Parker, T.L., Norton, A.J., Mukai, K. 2005, "X-ray Orbital Modulations in Intermediate Polars," A&A, 439, 213-226
Mukai, K., Still, M., Corbet, R.H.D, Kuntz, K.D., Barnard, R. 2005, "The X-ray Properties of M101 ULX-1 = CXOKM101 J140332.74+542102," ApJ 634, 1085-1092
Joe Patterson is a professor of astronomy at Columbia University. One of his
major research goals is to understand the structure and evolution of cataclysmic variables, particularly WZ Sge, AM CVn and DQ Her (intermediate polar) type stars. Up until the early 90's Joe's observing was done primarily at the National Observatories and with various X-ray and other satellites.
In 1991, Joe formed the Center for Backyard Astrophysics (CBA), a network of
primarily amateur astronomers spanning the globe and collaborating on
observation of variable stars, using CCDs mounted on backyard telescopes.
Since then, they have conducted observing campaigns and long-term
photometric studies of several systems and published papers with the results
(a selected list follows this interview).
We're going to ask Joe about the CBA's collaboration between professional
and amateur astronomers.
CVnet: After more than a decade of observing superhumps in SU UMa systems, don't we know just about everything there is to know about UGSU type CVs? What is left to understand, and how can observations with small telescopes help in unraveling the remaining mysteries?
Joe: The dwarf-nova outburst itself has become generally well understood -- in the sense that there's a theory which successfully reproduces the observed phenomena. But it's noteworthy that all of that theory was crafted to fit previously known data -- it has never actually predicted something not known in advance. So that higher standard of scientific worthiness has never been met. In addition, the theory breaks down when applied to dwarf novae in quiescence -- it predicts about a thousand times less accretion than is actually observed! Plenty of thought needed there.
The UGSUs are of high interest because of the very regular periodic signals they show -- superhumps and their relatives. These are caused by the (gravitational) tugging of the secondary star on the outer accretion disk. Their periods, if measured with sufficient precision, reveal the mass of the secondary pretty accurately. That is quite hard to do in any other way, since the secondary is usually invisible in the spectrum.
Professional astronomers typically observe the UGSUs for a a few days at one site - their data is very limited in time and very afflicted by aliasing (since they are "off the air" for ~20 out of every 24 hours). They go very faint (of course) but their data on periodic signals is often of low accuracy and/or aliased. Single large telescopes proudly performing their hijinks for a few days at a time... that just doesn't produce enough accuracy (or reliability, because of the aliasing problem) for time-series studies. The best time-series data sets are nearly always those of amateur astronomers.
CVnet: What makes nova-like variables (and old novae) such rewarding targets for CBA?
Joe: Oh, we like these because they don't show much night-to-night variability. This makes the time-series studies extremely sensitive. They also are the only CVs you can confidently model with the "steady disk" assumption.
CVnet: FY Per and FS Aur seem to be frustrating your attempts to understand them.
What makes them so interesting, and what has made them so difficult to
Joe: Well, at present "annoying" would be a better word. Both show stable photometric periods greatly discrepant from their spectroscopic periods -- yet not fitting any interpretation previously recognized in other CVs (superhumps, rotation, precession). They're not difficult to study - we just haven't figured 'em out! When a good idea arrives, that will be a reason to upgrade "annoying" to "interesting".
CVnet: What are the CBA's current observing targets, and why have they been selected for study?
Joe: We change targets every month or so, shown on the homepage (and detailed more in the "News" category). The reasons are varied. Some of the targets are selected for very long-term study (studying a periodic signal as it evolves over years), others are more "object of the month" (new dwarf novae).
CVnet: How can CCD observers contribute to CBA? How do you sign up or get involved?
Joe: If you have an 8" or larger scope, a CCD, appropriate software for time-series photometry (repititive images, >100 for a night), and a love of doing science, then we want you! Best to write to firstname.lastname@example.org
; this goes to myself and Jonathan Kemp. You should then select a current target, get a few long nights of differential photometry, and send the data (Var-Comp versus JD) to email@example.com
(same destinations). I'll write back with comments. The CBA site also has emails of members; you might want to write them for advice.
CVnet: How can visual observers contribute? Are there any particular stars you are waiting for an outburst or unusual activity that you would like to be notified of?
Joe: The recognized UGWZ stars are of huge interest - every outburst is of high importance. Since they erupt so rarely, the astronomical community needs a lot of help in finding the outbursts. I couldn't stress that enough! In addition, there are a few dozen faint CVs thought to be UGWZ stars, but which have never actually erupted. Any eruption of theirs would be an even bigger deal.
This is awfully tough work - staring at a patch of sky and seeing nothing for years or decades on end. Yet VSOers do it. Amazing.
CVnet: What has been the biggest advantage to collaborating with mostly amateurs on observing projects like CBA?
Joe: Well, after collaborating with professional astronomers for 15 years, I couldn't help but notice a few things: they get rained on a lot, they're very nervous about money, there aren't many of them, and their life stories and skills are pretty redundant with mine. Amateur astronomers seemed like a better and more interesting investment...
Superhumps in Cataclysmic Binaries. XXIV. Twenty More Dwarf Novae
Joseph Patterson, John Thorstensen, Jonathan Kemp, David Skillman, Tonny
Vanmunster, David Harvey, Robert Fried, Lasse Jensen, Lewis Cook, Robert
Rea, Berto Monard, Jennie McCormick, Fred Velthuis, Stan Walker, Brian
Martin, Greg Bolt, Elena Pavlenko, Darragh O'Donoghue, Jerry Gunn, Rudolf
Novák, Gianluca Masi, Gordon Garradd, Neil Butterworth, Thomas Krajci, Jerry
Foote, and Edward Beshorehttp://cba.phys.columbia.edu/results/20dn/
Superhumps in Cataclysmic Binaries. XXIII. V442 Ophiuchi and RX J1643.7+3402
Joseph Patterson, William Fenton, John Thorstensen, David Harvey, David
Skillman, Robert Fried, Berto Monard, Darragh O'Donoghue, Edward Beshore,
Brian Martin, Panos Nirachos, Tonny Vanmunster, Jerry Foote, Greg Bolt,
Robert Rea, Lewis Cook, Neil Butterworth, and Matt Wood
Publications of the Astronomical Society of the Pacific
December 2002, Volume 114, Page 1364http://cba.phys.columbia.edu/results/novalike/
The 2001 Superoutburst of WZ Sagittae
Joseph Patterson, Gianluca Masi, Michael Richmond, Brian Martin, Edward
Beshore, David Skillman, Jonathan Kemp, Tonny Vanmunster, Robert Rea,
William Allen, Stacey Davis, Tracy Davis, Arne Henden, Donn Starkey, Jerry
Foote, Arto Oksanen, Lewis Cook, Robert Fried, Dieter Husar, Rudolf Nov? Tut
Campbell, Jeff Robertson, Thomas Krajci, Elena Pavlenko, Panos Niarchos,
Orville Brettman, and Stan Walker
Publications of the Astronomical Society of the Pacific
July 2002, Volume 114, Page 721http://cba.phys.columbia.edu/results/wzsge-b/
Rapid Oscillations in Cataclysmic Variables. XV. HT Camelopardalis (= RX
Jonathan Kemp, Joseph Patterson, John Thorstensen, Robert Fried, David
Skillman, and Gary Billings
Publications of the Astronomical Society of the Pacific
June 2002, Volume 114, Page 623http://cba.phys.columbia.edu/results/htcam/
Gary is one of the most prolific variable star observers in the world. He has reported over 100,000 visual observations to the AAVSO and BAAVSS databases. Quite an accomplishment, considering the fact that he observes from the heart of light-polluted Birmingham, England.
He is a past Director of the BAAVSS, and is currently assistant coordinator of both the UK Nova Patrol and UK GRB detection programmes. Gary is editor of the monthly Variable Star Pages of 'The Astronomer' and is the current coordinator of the Recurrent Objects Programme; the subject of our interview.
CVnet: What is the Recurrent Objects Programme (ROP)?
Gary: The ROP is an observation programme dedicated to cataclysmic and eruptive stars. It all began in the mid 1980's when Rob McNaught observed the first outburst of DO Dra. This was quickly followed by Stephen Lubbock's first ever outburst detection of RZ Leo. Guy Hurst quickly realised that there were many such objects just waiting to be observed, so compiled the first ROP list of stars. I began to observe all of the objects on the ROP in the late '80's. At that time there were very few observers of faint CVs around the world (all visual), and as I was the only contributor to the program around 1989, Guy asked me if I would like to take it over - which I did in 1990. When I became Director of the BAAVSS in 1995, the first thing I did was to incorporate the ROP into the telescopic programme of the VSS!
CVnet: What are the main goals of the program?
Gary: To monitor for outbursts or unusual behaviour either visually or with a CCD. To determine an outburst cycle, orbital period and of course to establish what sub-group if any the object belongs too! To obtain high quality CCD photometry and astrometry once the outburst has been detected (usually by a visual observer), and to provide the professional community with this information once we have it. Finally to raise the profile of the objects to the amateur community, so that greater observational coverage can be achieved!
CVnet: What types of objects are in the program; and how many are there?
Gary: The objects included in the program are poorly studied stars where outbursts occur at periods greater than once per year, and/or where few or no observations of an outburst have been secured. Currently, there are 81 objects on the programme. These are type UG, UGSU, UGSU+E, UGWZ, N, NR, NL, VY, IP and of course objects which are only suspected of belonging to these sub-groups.
CVnet: How often is the list revised?
Gary: I look at the programme regularly, but only make major revisions on a yearly basis. However, if an object comes up that looks like it may be a ROP candidate, then it goes on immediately. If it turns out to have outbursts frequently, I take it off. Once any object is taken off the list, it doesn't mean all the work is done. The star remains on the BAAVSS telescopic programme for continued observation. Any object which is seen to be outbursting more frequently than once per year is taken off. I'm always on the look out for new objects. VSNET was a major source of new candidates, and Taichi Kato and others were instrumental in bringing to my attention many objects worth consideration. These days the SDSS is throwing up some really interesting stuff, and of course the IBVS is always a valuable source.
CVnet: Can you tell us about some of the successes of the program?
Gary: We have had many successes. First ever visual outbursts of many stars including SS UMi, V1113 Cyg, LL And, UW Tri, QY Per, DI UMa, DV UMa, etc. I could go on. There were some very memorable moments too, like Patrick Schmeer’s detection of the outbursts of EG Cnc (and that fantastic light curve) and HV Vir. Also, I remember Tonny Vanmunster picking up the first ever visual outburst of DV UMa, and ringing me for confirmation. We ended up decscribing the outburst to each other on the phone at the telescope, whilst we were actually observing it in different countries! The same thing happened with LL And and UW Tri if I remember. There is also the other part of the tale where stars thought to be rare outbursters were actually quite frequent - SS UMi and V344 Lyr are good examples. These were originally on the ROP, and because they weren't being observed at all were thought to be long period outbursters. The ROP raised the profile of these two stars, which were eventually found to be very interesting short period UGSU stars! I have many examples of this type of thing.
We have also had stars like HN Cyg, which again was thought to be a very rare outburster. ROP observations clearly showed it be a SR star. And of course V635 Cas, which was observed visually in outburst for the first time along with X-ray outbursts being monitored by professionals at Los Alamos! Another success of the programme has been the cooperation with other like-minded groups. The Belgian VVS started a CVAP programme similar to the ROP in the 1990's. Tonny and I worked together to make sure we didn't duplicate our efforts, and exchanged ideas as to what and what not to observe. It also led to a long term friendship!
CVnet: How can CVnet help the program?
Gary: Simply by raising the awareness of the stars on the programme - which I'm confident it will do. Unlike those dark, distant days of the 1980's, many more visual observers are now monitoring these objects for outbursts, and are exchanging observations through the Internet. Outbursts are still missed I'm sure, but not much gets through unnoticed these days. CVnet will play an important role in announcing outbursts so that others can continue to monitor either visually or with CCDs. Also, feedback is very important. Now that VSNET is off-line, CVnet has a very important role to play here.
CVnet: How could professional astronomers be of assistance?
Gary: We need to know which objects are suitable for inclusion into a programme like the ROP, and professional astronomers are our best source of information. Also, it's important that these guys give feedback to the amateurs who make those valuable observations. I've found that in most cases, professional astronomers have been very helpful and supportive in this respect.
CVnet: Is there anything else you'd like to add?
Gary: Only that the ROP will evolve along with current trends and interests. We may see more IPs on the list for example. Whatever happens the ROP will continue to highlight many of these weird and wonderful objects, where at this moment in time we haven't got a clue as to what they'll do next - that's what's so exciting!
For the complete listing of objects in the ROP: http://www.garypoyner.pwp.blueyonder.co.uk/rop.html
Gary Poyner’s Variable Star Pages: http://www.garypoyner.pwp.blueyonder.co.uk/varstars.html
BAAVSS Website: http://www.britastro.com/vss/
Paula Szkody is a professor of astronomy at the University of Washington. She is well known for her work with dwarf novae and magnetic CVs. Paula has participated in projects with the EUVE, HST, IUE, ROSAT, Ginga, ASCA, RXTE, Chandra, XMM, FUSE and Galex satellites and is an active user of the UW observatories Apache Point Observatory (APO) and Manastash Ridge Observatory (MRO), as well as National Observatories KPNO, CTIO and the MMT and Keck.
We're going to talk with Paula about her work with the Sloan Digital Sky Survey (SDSS), the CVs discovered to date, and the hundreds of new CVs they are currently finding.
CVnet: How does the SDSS differ from previous surveys finding CVs, like the Palomar Green Survey?
Paula: The SDSS reaches much fainter magnitudes than previous surveys, finding CVs down to 21st magnitude whereas the PG survey limit was near 16th and the Hamburg Survey was near 18th. This means we can reach larger distances and also find systems with lower accretion rates that are fainter than those found in previous surveys. The end result is that we have less of the selection effect that is pre-disposed to finding higher accretion rate systems and can get a better idea of the true distribution of CVs. However, it should be kept in mind that SDSS has its own selection biases in that spectra are not taken of all objects. Instead, color selection is used and so some objects may be missed.
CVnet: How will these survey results aid in our understanding of the evolution and distribution of CVs in the Galaxy?
Paula: If we can determine the periods for the large number of CVs coming out of this survey (near 200), we can compare the distribution of orbital period and type of CV with population models in existence to understand the angular momentum losses that drive our picture of evolution in close binaries. Since SDSS covers the sky out of the galactic plane, the survey results can be compared with previous surveys in the galactic plane to obtain a scale height for CVs.
CVnet: How are the CVs sorted out from all the stars, galaxies and quasars SDSS is observing?
Paula: The SDSS CVs are found from their spectra as emission line sources or those showing cyclotron harmonics. We run a program on point sources (thus eliminating galaxies) that selects all systems showing emission or absorption lines of hydrogen near their rest wavelengths (thus eliminating quasars). Some hand searches of plates are also done. To have a spectrum taken, objects are color selected from the SDSS imaging. Quasars and galaxies have the most spectral fibers allocated. Stars obtain a small number of fibers and CVs have one allocated per plate. Since CVs have a large range in color (blue if the accretion disk dominates, red if cyclotron or a secondary star dominates and both blue and red if the accretion disk does not contribute much to the light and the underlying stars are evident), we rely on overlaps with QSO colors, hot white dwarfs and serendipity (very blue or red objects) to pick up most CVs. Our one fiber is used to pick out the objects which have both blue and red colors as these are almost all WD plus M dwarf binaries or CVs.
CVnet: Even though the primary aim is to find faint CVs, you have discovered some systems well within the reach of amateur equipment. Are there any particular systems for which follow-up observations or continued monitoring by amateurs might prove useful or rewarding?
Paula: The bright saturation limit is near 15th magnitude so there are several systems that are bright enough for amateurs. In addition, systems that are faint during the SDSS observations may have brighter outbursts or higher states of accretion. For followup work, we have been concentrating on the systems near 17-19th mag (the lowest accretors). This means that the bright, mostly novalike systems could use photometric light curves. In addition, no outburst records exist for most of the systems. If a dwarf nova outburst can be observed, it would determine the classification of these systems.
CVnet: Is it possible, or even likely, that there may be some UGWZs (or TOADs) lurking amongst these new CVs?
Paula: It is extremely likely there are UGWZs (TOADs) among these systems. Many show the characteristics of very low accretion that result in the extreme outbursts of TOADs. The most likely ones are the systems which show broad absorption surrounding the emission lines - this means that the white dwarf is showing up (the absorption lines are from the white dwarf) and the accretion disk (creating the emission) is very tenuous.
CVnet: What is the most surprising result of your experiment thus far?
Paula: I think the most surprising thing is the number of magnetic systems of extremely low accretion that we have been finding. They are evident from very large, sawtooth appearing cyclotron humps in the SDSS spectra and very strange colors in the photometry. Models that have been fit to the continua by Gary Schmidt and Lilia Ferrario show that the accretion rates are about 1000 times less than most AM Her stars or Polars. There apparently is no mass transfer stream and the accretion likely takes place from a wind. The temperatures of the white dwarfs in these systems are very cold (~5000K), implying they are very old and have never been heated by accretion. Thus, these objects may be pre-Polars, on the way to becoming Polars. The numbers being found may imply a large population of these objects and a different angular momentum loss than for typical CVs as their periods are all above the gap.
CVnet: Will you come back and talk to us again about future results?
Paula: The main part of the survey will end in June 2005, with the final data release soon after. Hopefully, we will have some summary of all the CVs known by 2006 and can make more generalities about what we have found. I am always happy to answer questions about the SDSS CVs at any time.
Further reading: Sloan Digital Sky Survey Website Cataclysmic Variables from SDSS I, First Results Cataclysmic Variables from SDSS II Cataclysmic Variables from SDSS III
Related Links: Apache Point Observatory Manastash Ridge Observatory University of Washington Astronomy