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Boris Gaensicke

posted Mar 27, 2009, 9:50 AM by Mike Simonsen

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

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