Amateur Astronomers and Dwarf Novae
L. T JENSEN (Denmark); G. POYNER (United Kingdom); P VAN CAUTEREN (Belgium); T VANMUNSTER (Belgium)
1. Introduction the cataclysmic binaries.ln a cataclysmic superhumps, are observed in most binarya mass-Iosing secondary (often a SUUMa stars. The superhumps have a On December 15, 1855 the English late main-sequence dwarf) is in close orbit period of a few per cent longer than the astronomer John Russell Hind (1823 with a white dwarf primary. The orbit is so orbital period. Thus the detection of these 1895) was searching for new minor close that the secondary star fills its superhumps is very important in order to planets in the constellation of Gemini. Roche-volume and hence it loses determine the orbital period of systems During this search he found a new star at material through the inner Lagrange point where this is otherwise very difficult to approximately 9th magnitude. A new L1. The lost gas is accumulated in an obtain. asteroid? He observed the starforseveral accretion disk around the white dwarf. WZ Sge type (UGWZ): This special days and found that it didn't move - thus This material then accretes onto the type of dwarf nova has outbursts very not an asteroid! The next few daysthe star surface of the white dwarf from the disko At seldom. The typical recurrence time is became fainter and fainter and it was the point where the stream of gas from the several decades. Clearly, the detection soon invisible in Hinds telescope. Hethus secondary impacts the disk, a shock front of all outbursts of these rare objects is classified the star as a faint nova. But a is formed which results in a hot spot. The extremely important. few months later in March 1856 the star luminosity of the disk and the hot spot The origin of dwarf nova outbursts is was seen again, this time by another accounts for most of the luminosity of the due to a brightening of the accretion disko English astronomer Norman Robert entire system. The orbital periods of The mass-transfer burst model proposed Pogson (1829-1891). From this moment cataclysmic binaries are very short, a little in the early 1970's by Geoffrey Bath the star was monitored more systemati over 1 hour to about 15 hours. As the explains the outbursts by semi-periodic cally and more outbursts were observed. orbital periods indicate, cataclysmic enhancements of the mass transfer from The period between the outbursts was binaries are very small systems, and often the secondary. calculated at approximately 100 days. A the dimensions are comparable with solar The disk-instability model proposed nova with numerous outbursts was a new diameter. Dwarf novae are cataclysmic independently by Jozef Smak and Yoji phenomenon, so the first member, U binaries exhibiting quasi-periodic erup Osaki in the mid-1970's is nowadays the Geminorum, of a new class of variable tions of several magnitudes (2-8). Most of most favoured model of an outburst stars, dwarf novae, was discovered! 40 the time they stay in a minimum state, but mechanism. This model explains the years passed before the next member of now and again this state is interrupted by dwarf nova outbursts as folIows: The this class, SS Cygni, was discovered in abrupt outbursts. The outbursts last from accretion disk can accumulate a certain 1896. Since then a few hundred dwarf a few days to about 14 days. The time amount of gas before it gets unstable. novae or possible dwarf novae ([Downes between the outbursts, the recurrence When instability is reached, the accretion and Shara, 1993] list 349) have been time, ranges from 10 days to months, of matter to the white dwarf increases discovered. Most of these are faint and and in some cases several years. The dramatically. We see this as an increase haveonly been observed very infrequent recurrence time for an individual dwarf in luminosity - an outburst. When the Iy. In several cases the classification of nova is not at all constant. It is only accretion disk has lost enough mass, it dwarf novae is based on very few possible to indicate an average recur becomes stable again, the increased observations. rence time, e.g. the recurrence time for accretion stops and the system returns to The observation of dwarf novae (and SS Cyg ranges from 15 to 95 days with minimum magnitude. The disk is now other cataclysmic variables) is one of the 50 being the average. The same un ready for a new fill-up and a new outburst few fields of astronomy in which amateur predictability holds for other dwarf nova cycle can begin. astronomers still can deliver substantial characteristics as outburst amplitude The classical novae (N) and the contributions to the work of professional and outburst duration. recurrent novae (RN) are also cataclys astronomers. The unpredictable behav Dwarf novae are divided into 4 mic variables. The outburst of classical iour of most cataclysmic variables makes subclasses according to their specific novae as weil as some recurrent novae it very difficult for professional astrono behaviour. are generated by thermonuclear runaway mers to monitor these variables system SS Cyg type (UGSS): This class reactions at the su rface of the white dwarf, atically. Our current knowledge and contains the classical dwarf novae, i.e., following long periods of accretion from understanding of the physical processes, stars like U Gem and SS Cyg. the secondary. The outburst of classical that form the basis of the dwarf novae Z Cam type (UGZ): In addition to the novae are always accompanied by an outburst mechanism, are still subject to a behaviour shown by the SS Cyg subtype, expulsion of a shell of material, whereas lot of controversy. In this article the work stars of the Z Cam type are characterised for the recurrent ones this seems not of a world-wide network of amateur dwarf by standstills in their light curves. The to be the case. A more thorough and novae observers is described. We eruption light curve is occasionally comprehensive review of dwarf novae present some examples of the important interrupted by periods of standstilllasting and other cataclysmic variables can be results achieved by these observers. from days to several years. During the found in [Patterson 1984] and the Moreover we describe the instruments, standstills the star remains at abrightness references therein. the charts and the observational methods between the normal maximum and used. minimum magnitudes. 3. The Observers, Their SU UMa type (UGSU): This subtype is Instruments and Techniques 2. Dwarf Novae characterised by frequent narrow out bursts, but in addition super-outbursts The major part of the visual dwarf Dwarf novae are associated with a occasionally occur. During a super novae observations are made by a few special type of interacting binary stars - outburst, short-period light variations, very active and enthusiastic observers.
43 EF Peg: October 1991 (Schmeer, Ger) SS UMi: August 1991 (Mitchell, UK) HV Vir: April 1992 (Schmeer, Ger) AK Cnc: January 1992 (Kato, Japan) 11 V1113 Cyg: August 1993 (Szentasko, Hungary) 12 V493 Lyr: October 1993 (Bortle, USA/Van 13 Cauteren, B) LL And: December 1 (Vanmunster, B) 14 :fY : 15 Also many stars which have been monitored by observers have been re 16 classified after observations had shown 17 that outburst activity wasn't quite as 18.1.- ...... ---'- ---' "- ...... predicted. If a programme star is found to go in outburstfrequently, it isdropped from 2449580 2449585 2449590 2449595 2449600 2449605 2449610 the programme, but only after monitoring JD it intensely through several outbursts. Figure 1. This has recently happened with the UGSU star SS UMi. Most of them are very experienced observers contribute to the dwarf nova Figure 1 shows an example of a light variable star observers, with proven skills section of the AAVSO Circular. Often an curve obtained for a programme star: in 'traditional' variable star work. Lot of outburst is only observed by a few UGWZ type dwarf nova UZ Bootis. The practice and perseverance has turned observers. Nearly 400 variables are light curve documents arecent superout them into well-trained amateurs, that are currently classified as UG stars. For many burst (August 1994). The last observed capable of memorising tens of star fields. possible UG stars nearly no characteris outburst of UZ Boo was in 1978. On every clear night, they visually check tics are known, because they are very Occasionally stars which are not erup these fields, hunting for 'new' stars. No faint even in outburst or they have very tive in nature, but were originally thought sophisticated instruments are required: long recurrence times. Some of them to be are revealed. Four of these rogue an alt-azimuth dobsonian type of reflector have only been observed on very few stars have recently been identified and is quite common, with apertures in the occasions, for some the identification is dropped from the programme. Three, HN range from 20 cm to 50 cm and more. unknown, or the subclass classification is Cyg, UYVul and UZVul, were infactsemi Using this technique, some amateurs are very tentative. The task of monitoring regular type variables catalogued as capable of visually inspecting between 50 such stars is largely taken over by possible cataclysmic stars. However, ob and 100 star fields in one single night! On amateurs, who join their efforts in world servations from the programme's observ such nights typically 3-10 dwarf novae wide networks, that in total coverover 100 ers have revealed their real nature. When are seen in outburst. peculiar objects. All over the world, only a activity of a suspected star is noticed, Often the magnitude of a dwarf nova in few dozens of amateurs participate in other members of the network are in minimum is in the range magnitude 17 these dwarf novae alert networks. formed (overtelephone) in order to obtain 21. Most dwarf novae are hence only immediate confirmation. Positive identifi observed in maximum and naturally the cations of outbursts result in alert calls observers get a lot of negative (fainter 3. Interest from Professional (telegrams and electronic circulars), that than) observations. Negative observa Astronomers are issued to amateurs all over the world tions are often as important as the positive and to interested professional astrono ones to get a continuous Iightcurve. What The Astronomer organisation (UK) mers. makes dwarf novae hunting so challeng has set up a programme specifically Examples of such services are The ing to amateur astronomers? There is no to monitor poorly studied-Iong period Astronomer Electronic Circulars (UK) and single answer to this question. Most dwarf novae. The Recurrent Objects the Cataclysmic Variables Circular (B). amateurs are driven by the unpredictable Programme contains approximately 75 Photometric observations, that cover the nature of the dwarf novae: no one can objects of various types, ranging from outburst activity, are passed between anticipate the outcome of a nightly dwarf novae, recurrent and old novae, to professional and amateurastronomers by observation session. Others are attracted suspected UG, UGSU and UGWZ stars. electronic mail, mainly through Internet by the 'scientific' contribution of their Gary Poyner has been coordinator of the and CompuServe. The availability of e observations. Variable star charts and programme since 1991. The main criteria mail to many amateurs has meant that sequences come from various organisa in the object selection were the following: professional astronomers can be alerted tions, such as the AAVSO (American 1. The star must have an outburst to an outburst very quickly, thus making Association of Variable Star Observers), period (or suspected period) of at least possible extremelyval uableobservations The Astronomer and the BAAVSS (UK), one year. early in the outburst phase and in various RASNZ (Royal Astronomical Society of 2. Little (if any) information on the wavelengths.This can provide valuable New Zealand), BVVS (Belgium Vereni precise cycle length or amplitude is information into the mechanisms behind ging voor Sterrenkunde), and from available. these cataclysmic events. Summarising reprints of articles in professional journals Gradually more stars were added to results, complemented with contributions [Vogt and Bateson, 1982], [Bruch et al. , the list, as research uncovered many from professional astronomers, are pub 1987] and [Downes and Shara, 1993]. more of these long-period objects where lished in various magazines. Monthly tables, listing all dwarf novae just one or perhaps two outbursts had outburst activities are generated and been recorded. Among recent successes 4. 1. HV Virginis distributed by organisations like the we note the first ever visual observations AAVSO (The AAVSO Circular). Approxi of the following stars (discoverers' names The HV Vir outburst deserves special mately 40-50 more or less active following). mention here, as it proves just how
44 valuable amateur observations can be. programme, complementary to the TA observer, before the astronomical world is HV Vir was photographed by Schneller in programme. It mainly consists of poorly informed. Confirmation via a CCD image 1929 in outburst at magnitude 11.5. observed objects, for wh ich professional is much more 'safe' than a second visual Following aseries of observations of it at astronomers have shown interest and sighting. Having something on file/paper maximum light and in decline, it was have requested continuous monitoring. also makes it possible to analyse and re classified as a classical nova. The object The CVAP currently includes about 13 check the result later on. was added to the recurrent objects objects, of wh ich the identification and/or programme in 1988 primarily because of the subclass is uncertain or unknown. (3) Astrometry/photometric sequence it's high galactic latitude, and the possibil These objects are now monitored on a ity that it may be a rare type WZ Sge dwarf very regular basis by European variable With CGO images an accurate position nova, although no other outbursts had star amateur observers. Oue to the measurement may be obtained. At been detected. On April 20, 1992, a unavailability of professional search present, too many inaccurate charts are German amateur, Patrick Schmeer, made charts and sequences for these objects, used, resulting in 'false' outburst-alerts or his usual check of the field with his 20-cm the CVAP members currently are using large scatter in magnitude estimates. telescope and saw HV Vir in outburst at charts based on the Guide Star Catalogue Preparing charts with reliable comparison magnitude 12. At first Schmeer thought it (GSC). Example CVAP objects include: stars is of utmost importance. might be a minor planet, but following a AS Psc (last seen in outburst in 1963), search of minor planet positions it soon V358 Lyr (seen in 1965) and SS LMi (seen (4) Photometry became evident that HV Vir was indeed in in 1980, and classified as either N or UG). By taking regular images of the Although the CVAP started only re outburst. Observations of spectra and variable star, reliable photometry may be high-speed photometry have shown that cently, a first result has al ready been performed. The accuracy is much higher HV Vir is indeed a dwarf nova. It shares obtained. The programme star IR Lyr, than the visual estimates. Oetecting c1assified as a possible UG star, has manycharacteristics ofWZ Sge itself, and superhumps in suspected UGSU stars is been observed during activity in recent displays superhumps typical of UGSU an example of useful work that can be months. A first interpretation of photo stars. done. metric observations indicates that the It is our impression that most astron star does probably not belong to the UG 4.2. LL Andromedae omers do not know about the important variables. Paul Van Cauteren and Tonny work some amateurs do in the field of Vanmunster are currently the coordina Another major success of the TA dwarf cataclysmic variables. It is our hope that tors of the Belgian programme. nova network was the detection of an this article will inspire more professional outburst of LL And by Belgian amateur astronomers to make use of the results Tonny Vanmunster on the evening of and observations obtained by the ama 6. Future Perspectives Oecember 7,1993 using a 35-cm dobso teur astronomers. The different recurrent nian reflector. The outburst was con object programmes are always open for As CCO cameras become more widely firmed within minutes by UK and Belgian new interesting cataclysmic variables to available to amateur astronomers, we observers. LLAnd was discovered photo be monitored. Naturally, it is also in would like to point out the important role graphically by astronomer Paul Wild spiring for the amateurs to see that that these instruments can play in the (Bern Observatory) in September 1979 their efforts can be and are used by observation of dwarf novae, even if they [Wild 1979], when it reached V-13th. professional researchers to gain more are mounted on a small telescope. Since then, the dwarf nova had never knowledge about the behaviour of cata been seen again. Oue to an alert, the clysmic binaries. Oecember 1993 (super)outburst could (1) Monitoring of dwarf novae also be monitored by several profession at quiescence al astronomers. Spectroscopic observa References tions of the star at maximum light were Most amateur astronomers, even with Patterson 1984: J. Patterson, The evolution 01 obtained, as weil as a photometric larger instruments, obtain only limiting cataclysmic and low-mass X-ray binaries. outburst light curve based mainly on magnitudes around magnitude 14.5 The Astrophysicat Journat Suppt. Ser.. 54, observations contributed by amateurs. 15.5. But the majority of dwarf novae are p. 443-493, (1984). The outburst amplitude was - 6 mag much fainter when at quiescence. With Vogt 1982: N. Vogt and F.M. Bateson, An atlas 01 southern and equatorial dwarf novae, nitudes. Or. Kato (Kyoto University, Ja the use of a CCO camera, however, many Astron. Astrophys. Suppt. Ser., 48, p. 383 pan) conducted CCD photometry during of these objects are within reach. This 407, (1982). four nights, and detected superhumps, means that daily monitoring may result in Bruch 1987: A. Bruch, F.-J. Fischer and U. hence classifying the star as a UGSU observing the start of the outbursts, and Wilmsen, An atlas and catalogue 01 north dwarf nova, possibly belonging to the WZ early warning of interested observers is ern dwarf novae, Astron. Astrophys. Suppt. Ser., 70, p. 481-516, (1987). Sge subclass. possible. Downes 1993: RA Downes and M.M. Shara, A Catalog and Atlas 01 cataclysmic vari 5. Belgian Cataclysmic Variables (2) Gonfirmation of outburst ables, P.A.S.P., 105, p. 127-245 (1993). Alert Programme discovered by visual observers Wild 1979: lAU Circular No. 3412.
The Belgian Cataclysmic Variables When an outburst is reported by a E-mail address: Alert Programme (CVAP) was initiated in visual observer, it is common practice to Lasse Teist Jensen: May 1994 as a dwarf nova observation obtain confirmation from a second [email protected]
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