New Astronomy Reviews 44 (2000) 3–8www.elsevier.nl / locate /newar

Cataclysmic variables: some lacunaeBrian Warner

Department of Astronomy, University of Cape Town, 7700 Rondebosch, South Africa

When I arrived in Oxford, almost exactly one third sophistication to stellar photometry. Out of this came´of a century ago, it was as a coude spectroscopist. In the beginning of a renaissance of CV studies, to be

the absence of a nearby telescope I worked on followed in succession by polarimetry, UV, IR andatomic transition probabilities and stellar abundances X-ray observations and ever increasing sensitivity– among other things, David Lambert and I rede- and time resolution in spectroscopy. The CV com-termined all the solar abundances for those elements munity’s achievements in the past 30 years arewhere better equivalent widths and atomic data could remarkable. Studies of CVs are still a growth indus-be given. In retrospect I find that I filled nearly 250 try: for example, every worthwhile university in thepages of the Monthly Notices in the two years that I UK has a CV research group (this is a matter ofwas in Oxford, none of them on the topic of CVs! definition). We are entering another era of rapidThat is the first lacuna. advances: 8-metre class terrestrial telescopes, larger

Leaving Oxford and moving to Austin, Texas X-ray and Gamma Ray satellites, the prospect of atowards the end of 1967, hired as a high resolution New Generation Space Telescope, possibilities of

´spectroscopist to use the two coude spectrographs at dedicated networks of modest sized terrestrial tele-McDonald Observatory, in less than a year I had scopes, ever improving supercomputers, the possi-converted to high time resolution photometry. This bilities of gravitational wave detectors. With betterrebirth has been described elsewhere (Warner, 1993); signal-to-noise in almost every part of the electro-suffice it to say that my professional interest in magnetic spectrum, and more powerful computingcataclysmic variable stars started with a photometric facilities, more detailed models with inclusion ofrun at full moon on the 82-inch McDonald telescope more physical processes will lead to what is alwaysin August 1968. our ultimate goal: better understanding.

At that time work on CVs was relatively quiet: With so much already accomplished, it is perhapsBob Kraft had completed all that he was able to do churlish of me to comment on some areas wherewith the prime focus spectrograph of the 200-inch more progress is clearly needed – but that is whattelescope; photometrists such as Smak, Paczynski, the Conference organisers have asked me to do.Mumford and Krzeminski had pushed standard UBV A good place to look for help in understandingphotometry to its limits; no high energy satellites CVs is among the non-CVs, where circumstancescapable of observing CVs had yet been launched. But may be less complicated. The isolated white dwarfsMerle Walker had shown that there is much high are of limited value where mass or spin distributionspeed flickering in CVs, and had long before found is concerned, because they haven’t been through athe 71 sec signal in DQ Herculis. common envelope nor a mass accretion phase, but

The late 1960s marked a time of rapid advances in their magnetic and pulsational properties may be ofapplication of more sensitive detectors and on-line assistance. In particular, the field geometries found

1387-6473/00/$ – see front matter 244 Elsevier Science B.V. All rights reserved.PI I : S1387-6473( 00 )00004-X

4 B. Warner / New Astronomy Reviews 44 (2000) 3 –8

from rotating magnetic white dwarfs should be nature of winds, angular momentum loss and evolu-indicators of what to expect for accretion structures tion which will be of value to CV studies.in polars. Almost all magnetic white dwarfs appear An exciting discovery among the detached sys-to have complicated field structures (Putney & tems is Gl 781, a subdwarf M1.5e with a white dwarfJordan, 1995), in the sense of obvious departure primary and an orbital period of 11.8 h. This is thefrom centred dipole geometry, but it is not yet only known population II pre-CV; it has [M/H] |possible to distinguish among the alternative models 21.2 and a white dwarf mass of | 0.35 M . In this(

of offset dipole or higher multipole structures (Mar- and other low metal abundance M dwarfs, chromos-tin & Wickramasinghe, 1984). (Note, however, that pheric activity is no lower than in solar compositionrecent work shows that the rapidly rotating REJ dwarfs (Gizis, 1988). Although no metal deficient0317-853 must have a predominantly quadrupole CV has been identified, some CVs have large spacefield (Jordan, 1999).) It is possible, therefore, that motions and must be sampling an old disc or halodespite the added complications of accretion flows, population.the variations in threading region and resultant The multikilogauss fields measured in late Mchanges in accretion columns that occur as polars dwarfs (e.g. Saar, 1990), which lead to magnetic

9 / 4~change from high to low M states may ultimately moments for CV secondaries of m | 2.8 P33 orb

provide the most sensitive probes of field geometry. (Warner, 1996), have important implications for (a)What is required here is application of the modern the interaction between the field of the secondary andpowerful techniques (e.g. doppler tomograms) the accretion disc (e.g. Meyer-Hofmeister et al.,

~throughout transitions between M states. 1996) and (b) the interaction between the secondaryIn the area of CV evolution I think that isolated and a magnetic primary, especially as there is

low mass main sequence stars, and detached binaries increasing evidence that an accretion disc shields(of the pre-cataclysmic variable type), have much to very little of the magnetic moment of the primaryteach us. Single M dwarfs with rotation periods as (Warner, 1996; Spruit, 1999). Can field reversals inshort as 0.2 d are now known (Stauffer et al., 1998; the secondary star (on time scales of decades)Barnes et al., 1999; Singh et al., 1999). The mag- prevent the primary in polars from becoming exactlynetic, X-ray and chromospheric properties of these locked?stars are of direct interest to studies of magnetic An interesting way to study the interaction be-braking in CVs. So too are the pre-cataclysmic stars, tween the fields of the secondaries and primaries iswhere orbital angular momentum loss should occur among the near-synchronous polars: - V1432 Aql,through the same mechanism(s) as in CVs, but BY Cam, V1500 Cyg and RXJ2115-58. If thewithout the added complication of mass and angular synchronization time scales can be obtained for allmomentum transfer between components. In this four (they are 185 y for V1500 Cyg and 890 y forconnection it is reassuring to find that HW Vir shows BY Cam: Schmidt et al., 1995; Mason et al., 1998),(Kilkenny et al., 1994) the same large rate of change the fields and masses of the primaries determined,of orbital period (i.e. orders of magnitude greater and the field structures found by the behaviour overthan predicted from magnetic braking) as do some the spin-orbit beat period, then they will form aCVs – which may be ascribed to changes in quad- useful testbed for theories of interaction. As I haverupole moment of the secondary caused by magnetic previously pointed out (Warner (1995), p. 366), thecycles (Applegate, 1992). But other similar systems fraction of desynchronized polars, if nova eruptions

~show no detectable P (Kilkenny et al., 1999) – as are the only cause, gives the average ratio of thealso is the case in some CVs (e.g. WZ Sge), which synchronization time scale to the time betweenmay be the effect of a prolonged ‘‘Maunder Mini- eruptions. With 4 desynchronized out of a total ofmum’’ as seen in, e.g., SS Cyg (Warner, 1988). about 50 polars, and an average synchronization time

Studies of period changes and structures in more of | 500 y, the interval between eruptions is | 644interactive types such as W UMa, Algols and RS y. More evidence for nova eruptions in three of theseCVn stars should also throw light on the general stars (i.e. other than V1500 Cyg) is required: BY

B. Warner / New Astronomy Reviews 44 (2000) 3 –8 5

Cam does not show any nova shell, the others need are VY Scl stars. This is one of the strongestto be examined. The temperatures of the primaries, if correlations in CV statistics, and it is tempting to sayobviously higher than the generally low values found (as in the paper that first drew attention to thisfor polars, would be indicative of nova eruptions in phenomenon: Robinson et al., 1981) that the periodthe recent past. gap occurs because a system approaching P | 3 horb

The general area of interaction between field(s) eventually ends in a low state from which it cannotand plasmas is in a state of infancy. There are many be resurrected until orbital shrinkage enables thegaps in our knowledge – what, for example, is the secondary again to fill its Roche Lobe near P 5 2orb

interaction between the rotating (unshielded portion) h. The points here are that (i) low states are on offerfield lines in an intermediate polar and the stream? Is just above the period gap and (ii) the time scales ofit really possible to have sufficient magnetic inter- switching between high and low states (weeks,action between primary and secondary to provide months or years) are orders of magnitude shortersynchronous rotation of the primary in some systems than the orbital evolution time scale. Whatever thewith discs (Warner, 1996)? Obtaining solutions in mechanism that causes the low states – e.g. instabili-these area is certainly deserving of the first few ty in irradiation induced mass transfer (Wu et al.,milliseconds of time on the first quantum computer. 1995b) or starspots (Livio & Pringle, 1994) – it has

Even the cause of orbital angular momentum loss to increase in strength as P approaches 3 h, and toorb

– at least, that in excess of gravitational radiation – be sufficient to prevent the secondary, once detachedis still only hypothetically due to magnetic braking. from its Roche Lobe, from filling it again. The very

~This certainly gives, from available models of fields high value of M observed in the VY Scl stars whenand winds, braking torques of the correct order of in their high states is also important here – and thismagnitude, even for fully convective secondaries is an integral part of the above mentioned irradiation(Tout & Pringle, 1992) – but all models of field model (which was first introduced by Osaki (1985)generation in low mass dwarfs are to a large extent in a different application).scaled versions of the av model for the Sun, and are Continuity in orbital evolution requires that thosenot successful in producing ab initio the general CV subtypes that possess a period gap must eitherproperties of fields and cycling seen among these race through the period range 2.3 h & P & 2.8 horb

stars – so can they be trusted to deliver correct or almost disappear from view and amble through~results for braking? And the supposed reason for the with M | 0. The Tout & Pringle result mentioned

orbital period gap – that of a large decrease in above indicates that magnetic braking should still bebraking when the secondary becomes fully convec- present, and may well dominate over that of gravita-tive – is, as a result, a hypothesis within a hypoth- tional radiation. The putative population of detachedesis! Nevertheless, there is evidence from other kinds CVs in the period gap still has to be detected – theyof interacting binary that magnetic braking is proba- should not be difficult to identify when found: theybly the dominant torque provider. will all have highly chromospherically active (flaring

I think that there is a strong case for believing that and spotted) M dwarf secondaries with rotationa dramatic change in the mechanism of orbital periods | 2.5 h; even if their white dwarfs haveangular momentum loss is not called for. The clue is cooled to | 1044 K, there will be some reflection

1in the VY Scl stars – all of which are now known to effect detectable in systems of at least moderatelie in the range 3.2 , P , 4.05 h – and it is inclination; about a quarter of them will be eclipsingorb

probably correct to say that all nova-likes in the binaries. In principle, although faint (M | 11), theyv2range 3.0–4.0 h which have been sufficiently studied should be easy to identify. A measured space

1 2The two stars which were earlier suspected to have P | 6 h It is worth noting that a suitable search for such systems, whichorb

are now known to have P 5 4.05 h (V751 Cyg: Thorstensen, will need to reach m | 21 or fainter in order to gather sufficientorb v

private communication) and P 5 3.68 h (V794 Aql: Honeycutt numbers, should also reach the large population of post periodorb

& Robertson, 1998). minimum CVs predicted by Politano et al. (1998).

6 B. Warner / New Astronomy Reviews 44 (2000) 3 –8

density for this population will place valuable con- tion (e.g., irradiation of the secondary: see Wu et al.,straints on the nature of, and evolution within, the 1995a) then some other mechanism of loss of orbitalperiod gap. At present, the only detached systems in angular momentum is being overlooked.the correct period range have either (HW Vir, MT Some bounds on CV evolution may be set bySer, PG1336-018) hot subdwarf primaries expected membership of CVs in clusters. Many CVs are nowof recent emergence from a common envelope phase, known in globular clusters, but it is in the youngeror have M dwarfs so low in mass (GD448) that they galactic clusters that more stringent limits on rate ofcan never have been CVs. evolution (e.g. in the pre-cataclysmic era) may be set.

Although there is no doubt of the existence of the I draw attention to SS Aur, for which Eggen (1968)period gap, or at least a much lower space density, gave a Hyades Moving Group distance of 200 pc,for non-magnetic CVs (see Kolb et al., 1998), which has been recently confirmed by the HSTwhether or not one exists for magnetic systems is not parallax (Harrison et al., 1999) and which gives it an

8 9agreed. In my opinion an answer is not possible age in the range 6 3 10 – 1 3 10 y (Eggen, 1998);without a larger sample of polars – a case can be BX Pup, which is possibly in NGC 2482 (Moffat &made for the period distribution of mCVs being not Vogt, 1975), which agrees with the distance obtainedsignificantly different (in a Monte Carlo sense) from from other methods (Warner, 1987); variable B7,either a uniform or a non-magnetic CV distribution. which is probably in NGC 6791 (Liebert et al.,Again, the answer to this problem has important 1994); nova V720 Sco, which is possibly in NGCimplications for the evolution of mCVs – perhaps in 6475 (Kholopov, 1956); and a probable polar in M67the sense of lessened magnetic braking (e.g., Li et (Gilliland et al., 1991). These are all CVs withal., 1995). interesting ages.

Another area in which there are gaps in our With 8-m class telescopes available in the south-knowledge – partly caused by selection effects – is ern hemisphere, the population of quiescent CVs in

~in the M versus P diagram. The magnetic braking the Magellanic Clouds will become accessible. Al-orb

mechanism leads (practically independent of the ready Shara (see these Proceedings) has located novavalue of P at the time of becoming a CV) to the remnants; a systematic search for outbursts of dwarforb

~prediction of quite a tight relationship between M novae is required. Such studies will at the very leastand P . I have drawn attention elsewhere (Warner, provide information on luminosity functions andorb

1998) to the fact that the long-known large spread in absolute magnitudes.~M at each P is in fact underestimated because of Turning now to the physics of accretion discs. It isorb

~(a) the difficulty of discovering high M systems at tempting to think of discs as merely two-dimensionalsmall P (these are permanent superhumpers of stars and therefore simpler in structure – but theorb

small amplitude) and (b) the difficulty of finding low reverse is the case. Discs have:~M systems at large P because of the high lumin-orb

osity of the secondary. Only in the past few years • Outer radial boundaries, complicated by tidalhave the first few stars been recognised in these stresses and mixing of energy and angular

3 ~regions. However, the large values of M (up to momentum from the stream. These regions may28 21| 10 M y ) of nova-likes in the 3–4 h P range be relatively cool and emit long wavelength( orb

do not appear explicable from magnetic braking radiation, which is difficult to disentangle fromalone – another, and dominant, means of producing the flux of the secondary. This may be the reasonmass transfer is required. If this is not the kind of why absolute magnitudes from K magnitudes ofmechanism appealed to that leads to cyclical evolu- secondaries are not as reliable as expected (Har-

rison et al., 1999). Detailed modelled spectra ofthe outer regions of discs need to be comparedwith observed spectra of SS Aur, SS Cyg and U

3 Gem to verify this.‘‘Excellent, Warner’’ cried Holmes, springing to his feet. ‘‘Wat-• A large radial range of photospheric temperatures,son, the gaps are closing rapidly’’. A. Conan Doyle, The

Adventure of Wisteria Lodge. and photospheres in which some viscous energy

B. Warner / New Astronomy Reviews 44 (2000) 3 –8 7

is dissipated – plus chromospheres, coronae and fronts are seen to decelerate as they propagate thiswinds that may be strongly radially dependent. would confirm the thermal-viscous instability model.

• An inner radial boundary which is still not fully The short-lived outbursts in the discs of inter-understood. The standard model of the optically mediate polars such as TV Col and V1223 Sgr arethick boundary layer (BL) (e.g. Frank et al., 1985) different from ordinary dwarf nova outbursts, whichevidently gives a BL that is too narrow, for it must be a result of the magnetized primary, through,

5predicts a temperature | 2 3 10 K. Actual BL e.g., a truncated disc. The mechanism for these5temperatures are 0.5–1.0 3 10 K (Hoare & outbursts is not known: it would be helpful if a

Drew, 1991). Perhaps the first glimpse of a deeply eclipsing IP were available! The mechanismsolution is seen in the recent work of Obach & may be the Spruit–Taam magnetospheric boundaryGlatzel (1999) who find that the emitting area of instability (Spruit & Taam, 1993); if so, does thisthe BL is enlarged because of radial transfer of instability play a role in systems with low fields, notenergy. recognised as magnetic? And is this instability the

• Stream overflow – which results, among other cause of the anomalously short outbursts in EX Hyathings, in addition of low specific angular – and if so, can we recognise a continuum ofmomentum material at the surface of inner parts outburst behaviours from the | 12 h duration of TVof the disc. Horne (1999) points out that part of Col, through EX Hya to standard outbursts, or issuch a mass flow may be propellered out of the there a distinct break between the two mechanisms?CV system by interaction with the rapidly rotating Incidentally, on statistical grounds it is clear thatmagnetic field lines of the disc. among the known eclipsing nova-likes there should

• Irradiation of the disc by the hot inner disc, BL be at least one, probably several, intermediate polarsand primary. – not recognised as such. The anomalous outburst

observed in RW Tri (Still et al., 1995) gives this starThe implications of these, and other, processes are prime candidature.

that realistic models of discs are probably beyond the The mechanism that produces the supposed discrange of computation at present. A full model tilt in CVs is currently a mystery. It is important for arequires inclusion of realistic viscosities, irradiation, wide range of the astronomical community: althoughBL, wind, stream impact and overflow and tidal and radiation pressure-driven tilted discs are acceptablemagnetic interaction with the secondary. Not all of in high luminosity sources, such as low mass X-raythese are necessarily important – some assistance in binaries (Pringle, 1997), this does not work in CVs.assessing the required physics will come from de- Yet, if the interpretation of negative superhumpsconvolution of eclipses that allow spectra of in- (Patterson et al., 1993) is correct, there exists in CVsdividual disc annuli to be measured (such spectra a quite different means of generating disc tilts. Couldshow, in UX UMa, deep narrow absorption lines in this mechanism be present also in high luminositythe inner parts of the disc, changing to broad systems – albeit merely as a seed for starting theemission lines in the outer regions Baptista et al., irradiation instability?1995). Quantitative modelling of such information is Finally in these ramblings let me refer to exotics. Ia major challenge. have tended to be more interested in the exotic

A further challenge is the inclusion of this realism behaviour of CVs than in their basics, on theinto the disc instability model required to understand philosophy that any proper model of CVs must havedwarf nova outbursts. First indications are appearing the capacity to accommodate the extreme behav-that stream overflow can significantly affect outburst iours. For example, the dwarf nova oscillationsbehaviour (Schreiber & Hessman, 1998), that ir- (DNOs) whose time scales require them to beradiation from the primary is important (Hameury et associated closely with the inner disc /BL carryal., 1999) and that high resolution of transition fronts structural information – e.g. soft X-rays in outburstsis necessary (Menon et al., 1999). On this last point, of dwarf novae are modulated at a large amplitudeobservational resolution and tracking of the transition by the DNOs. Is the DNO phenomenon an accretionfronts is required – Menon et al. state that if the modulation caused by instability inherent in the flow

8 B. Warner / New Astronomy Reviews 44 (2000) 3 –8

Baptista, R., et al., 1995, ApJ, 448, 395.or is it a result of a weak magnetospheric boundary?Barnes, S.A., et al., 1999, MNRAS, in press.I have tended to favour the latter (Warner (1995) p.Collins, T.J.B., et al., 1998, ApJ, 508, L159.5436) on the grounds that the weak fields ( & 5 3 10Eggen, O.J., 1968, ApJS, 16, 97.

G) expected in most white dwarfs seem capable of Eggen, O.J., 1998, AJ, 116, 284.explaining most of the DNO behaviour – see Warner Frank, J., et al., 1985, Accretion Power in Astrophysics, C.U.P.

Gilliland, R.L., et al., 1991, AJ, 101, 541.(1998b) and Mauche (1996). On the other hand, theGizis, J.E., 1988, AJ, 115, 2053,.less coherent quasi-periodic oscillations (QPOs)Hameury, J.-H., et al., 1999, MNRAS, 303, 39.seem more likely to be oscillations in the disc (e.g.Harrison, T.E., et al., 1999, AJ, in press.

Collins et al., 1998). In astronomy, measuringHoare, M.G. & Drew, J.E., 1991, MNRAS, 249, 452.

periods is much easier than determining masses, Honeycutt, R.K. & Robertson, J.W., 1998, AJ, 116, 1961.luminosities, temperatures, etc.; but the precision Jordan, S., 1999, Report at St. Andreasberg Round-Table Discus-

sion.offered in the DNOs and QPOs still awaits interpre-Kholopov, P.N., 1956, PZu, 11, 325.tation and application. There are other exotics thatKilkenny, D., et al., 1994, MNRAS, 267, 535.will eventually find their quantitative applications:Kilkenny, D., et al., 1999, MNRAS, in press.

e.g. QPOs in polars, TeV gamma rays in AE Aqr. Kolb, U., et al., 1998, MNRAS, 298, L29.Non-radial oscillations in the primaries of CVs Li, J., et al., 1995, MNRAS, 276, 255.

might have been thought so exotic as to be a remote Liebert, J., et al., 1994, AJ, 107, 1408.Livio, M. & Pringle, J.E., 1994, ApJ, 427, 956.possibility, but with the discovery of the first of theseMartin, B. & Wickramasinghe, D.T., 1984, MNRAS, 206, 407.(GW Lib: Warner & Van Zyl (1998)) there is theMason, P.A., et al., 1998, MNRAS, 295, 511.hope of finding ones with mode spectra simpleMauche, C.W., 1996, ApJ, 463, L87.

enough to provide the first really accurate masses for Menon, K., et al., 1999, MNRAS. In press.CVs. I could go on for much longer – CVs are such Meyer-Hofmeister, E., et al., 1996, A & Ap., 310, 519.interesting objects, rich with information not only Moffat, A.F.J. & Vogt, N., 1975, A & AS, 20, 85.

Obach, C. & Glatzel, W., 1999, MNRAS, 303, 603.about themselves but also for use in other areas ofOsaki, Y., 1985, A & Ap, 144, 369.astrophysics, such as neutron star and black holePatterson, J., et al., 1993, ApJ, 419, 83.binaries, interacting binaries in general, and every-Politano, M., et al., 1998, ASP Conf. Ser., 137, 207.

where that accretion discs occur. Pringle, J.E., 1997, MNRAS, 292, 136.I look back on the 1975 Cambridge meeting (IAU Putney, A. & Jordan, S., 1995, ApJ, 449, 863.

Symposium No. 73) as seminal in the field of CVs. It Robinson, E.L., et al., 1981, ApJ, 251, 611.Saar, S.H., 1990, IAU Symp., 138, 427.is encouraging to see how far the field has developedSchmidt, G.D., et al., 1995, ApJ, 441, 414.since then. I hope that in another quarter century itSchreiber, M. & Hessman, F.V., 1998, MNRAS, 301, 626.will be possible to look back on the Oxford meetingSingh, K.P., et al., 1999, ApJ, 512, 874.

and recognise that this too marked the beginning of a Spruit, H., 1999, Comments at St. Andreasberg meeting.new era of CV research – one in which the imminent Spruit, H.C. & Taam, R.E., 1993, ApJ, 402, 593.

Stauffer, J., et al., 1998, ASP Conf. Ser., 154, 1331.new facilities and the new generation of theoreticiansStill, M.D., et al., 1995, MNRAS, 273, 849.will have proven themselves and the field of CVsTout, C.A. & Pringle, J.E., 1992, MNRAS, 256, 269.will once again be poised for thrusts into realms nowWarner, B., 1987, MNRAS, 227, 23.almost inconceivable. I must express my gratitudeWarner, B., 1988, Natur, 336, 129.

for the undeserved honour of having this Conference Warner, B., 1993, BaltA, 2, 365.associated with my name and with a time in life so Warner, B., 1995, Cataclysmic Variable Stars, C.U.P.

Warner, B., 1996, Ap&SS, 241, 263.astronomically significant – or so our predecessorsWarner, B., 1998b, ASP Conf. Ser., 85, 343.who were besotted with sexagesimal arithmeticWarner, B., 1998, ASP Conf. Ser., 137, 2.would have thought. I am particularly grateful to PhilWarner, B. & Van Zyl, L., 1998, IAU Symp., 185, p. 321.

Charles, Andrew King and Darragh O’Donoghue forWu, K., et al., 1995a, A&AS Libr., 205, 315.

their efforts. Wu, K., et al., 1995b, Pub. AS Aus. 12, 60.

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