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.