Date: Thu, 1 Feb 1996 22:37:37 +0900 From: tkato@kusastro.kyoto-u.ac.jp To: vsnet@sanma.kusastro.kyoto-u.ac.jp Subject: [vsnet 517] "Why observe Z Cam stars?" (Ringwald) This article first appeared on the Cataclysmic Variables Home Page (http://www.cv.psi.edu) operated by Tonny Vanmunster. Reproduced for VSNET under permission by Fred Ringwald and Tonny Vanmunster. [...] ---------------------------------------------------------------------------- Why Observe Z Cam stars? F. A. Ringwald Planetary Science Institute Tucson, Arizona U. S. A. Internet: far@psi.edu ---------------------------------------------------------------------------- I am pleased to have received from Tonny Vanmunster and Eric Broens a summary of photometry done by members of VVS, Vss-Belgium on several Z Cam stars, listed at the end of this article. Fine, you might say, but why would anyone be interested in Z Cam stars? The Z Cam stars are a subclass of dwarf novae. Dwarf novae, in turn, are a subclass of cataclysmic variables (CVs). In addition to the normal outbursts that dwarf novae show, the Z Cam stars have standstills. During these standstills, they remain for months or even years at a brightness of about one magnitude fainter than at outburst maximum. Dwarf novae are natural laboratories for accretion disk physics. Accretion disks are common throughout the Universe. They occur any time both gravity and angular momentum act on matter in space, which is almost always. (Angular momentum can be thought of as the amount of rotational motion a body has: it is what keeps a spinning object spinning.) All stars are thought to form with accretion disks, and this is why all the planets in the Solar System are nearly in the same plane: they formed from what once was the Sun's accretion disk. That most stars are in binary systems is probably because the angular momentum throws gas into a preferred plane, where it forms into another star. Accretion disks make us aware of some of the more fascinating objects in the Universe, neutron stars and black holes: after all, without matter falling into it, a black hole is invisible. Supermassive black holes are thought to reside in the centers of all galaxies and explain the complex behavior of active galaxies and quasars. Since galaxies rotate, it is hard to escape the conclusion that their central engines are fed by accretion disks. A problem with studying star formation, neutron stars, black holes, and active galaxies is that all these objects are complex, hard to observe, or so bizarre they defy comprehension---and often all three. This is why dwarf novae are useful: they teach us the basic physics that must govern these more exotic systems. Dwarf novae do this because they vary on human timescales of minutes to decades, not centuries to millenia. CVs also often eclipse, so their basic geometry is well understood---insofar as it isn't, precisely---but then the geometry of protostars and quasars is often not understood at all. Nearly all CVs, dwarf novae included, consist of a K -- M dwarf that spills gas onto a white dwarf, which this red dwarf orbits. Because of the sideways motion of the orbit, the gas stream does not fall onto the white dwarf directly, but settles into orbit around it. This resulting ring spreads out into a disk, which settles onto the white dwarf. Aside from dwarf novae, the two other general classes of CVs are the classical novae and the nova-likes. Classical novae have nuclear-powered eruptions, of amplitude 10 -- 15 magnitudes or more, which usually occur only once in many centuries, and often last for years. These eruptions are caused by the buildup of gas on the surface of of the white dwarf: sooner or later, the mass, pressure, and temperature build up and the gas detonates. Nova-likes have spectra that resemble those of classical novae many years after an eruption, but are not known ever to have erupted. Many nova-likes have spectra that resemble those of dwarf novae in outburst: these nova-likes might therefore be thought of as dwarf novae stuck in outburst all the time. Dwarf novae normally have outbursts of 2 -- 5 magnitudes' amplitude and days-to-weeks' duration. These outbursts occur only quasi-periodically: it not easy to guess in advance when they will occur. There is no substitute, therefore, for regular monitoring. Studies of eclipsing dwarf novae show that their outbursts occur in the disks. Unlike classical nova eruptions, they are not nuclear-powered: they are powered by gravity, by the luminosity of a large amount of gas heating up as it falls into the strong gravity field of the white dwarf. Why dwarf nova outbursts occur is still unclear. In 1974, Yoji Osaki (Tokyo) presented the theory that is still the favorite, in which the outbursts are caused by thermal instabilities in the disk. In this picture, gas accumulates in the disk until it heats up and becomes viscous. It therefore avalanches in toward the white dwarf, heating up even more and causing an outburst. However, a minority opinion holds that dwarf nova outbursts come from bursts of mass transfer from the red dwarf. If the red dwarf were suddenly to discharge an unusually large amount of gas into the disk, it would also cause an increase in the mass transfer rate through the disk and onto the white dwarf, and therefore an outburst. This theory, published by Geoff Bath (Oxford) in 1973, has never been conclusively disproved. It does have problems: a way to make a red dwarf pulsate and therefore give a burst of gas is unknown. Still, this theory should not be discounted entirely. It has always been assumed that the red dwarfs in CVs are relatively normal, but since CVs are such close binary stars, their red dwarfs are squashed and rapidly rotating because of tides, and irradiated, by the hot white dwarfs---and in order to be losing mass at all, CV red dwarfs must be out of thermal equilibrium. Annoyingly also, observational tests have so far supported mass-transfer-burst models better than disk-instability models, although only slightly better: neither have been decisively proved or disproved. No one need despair, though, since the behavior of dwarf nova outbursts is complex enough that quite possibly variations of both disk instabilities and mass transfer bursts are present. The standstills in Z Cam stars may be an example of this. Little is known about standstills, not even the fundamental observational properties of how often they occur or how long they last. So far, what is known is vague, anecdotal, and unsystematic, far from being neat, quantitative statistics. We know, for example, that Z Cam itself was in standstill from 1978 to 1981. In contrast, HX Peg has much shorter standstills, about 30 to 90 days long, which recur yearly. Standstills are often not completely static: in their compilation of the statistics of dwarf nova outbursts published in 1984 (and the last time such a compilation was done), Paula Szkody (Washington) and Janet Mattei (AAVSO) showed there can appear erratic flareups with amplitudes of several tenths of a magnitude. I am therefore starting a study of these basic observational properties---the phenomenology---of Z Cam stars. Some effort has been made to explain standstills theoretically, but so far it is sketchy and quite possibly wrong. The theory has not been helped by the sparse data. The leading explanation for standstills in Z Cam stars was published by Friedrich Meyer and Emmi Meyer-Hofmeister (Max Planck Institut fuer Astrophysik, Munich) in 1983, and involves increased mass flow into the disk because of irradiation of the red dwarf. According to this theory, normal outbursts would trigger standstills in dwarf novae that happen to have average mass transfer rates just below the critical rate, above which the disk would be too hot to have outbursts, as in nova-likes. An outburst would heat the side of the red dwarf that faces the white dwarf and accretion disk. This would puff up the red dwarf's atmosphere, causing the mass transfer rate into the disk to be higher. The atmosphere, or outer layers, of the red dwarf would only have so much gas to give up, though, so the mass transfer would eventually reach an equilibrium rate and therefore stand still. This theory runs into a problem in that it is unclear observationally whether standstills really are triggered by normal outbursts: see Figs. 18 and 19 of Lin, Papaloizou & Faulkner (1985), and p. 31 of la Dous (1993), and p. 164 of Warner (1995). Another problem with the theory, that Emmi Meyer-Hofmeister and Hans Ritter (MPIA) discussed in 1993, is that the predicted mass transfer rate increase should last for the diffusion timescale of a red dwarf's atmosphere, about 10^5 years. This prediction, however, is surely too long, since standstills are observed to last for a few years, and often much shorter. It is therefore unclear what stops a standstill. If standstills are outbursts pushed over the critical rate dividing dwarf novae and nova-likes, why then do the standstills not occur at maximum luminosity, but about one magnitude fainter? There may also be insufficient high-energy flux to irradiate the atmosphere of the red dwarf to puff it up enough to increase the mass transfer rate significantly, an essential ingredient of the theory. Standstills are therefore still a mystery, which data collected by amateurs may well help to solve. A major problem with the theory is that it is so schematic: it only tries to explain what is the vaguest outline of the standstill phenomenon. Too much about the actual behavior of standstills is unknown: the theory can't progress, because we don't even know the right questions to ask. The pre-standstill behavior may be an important clue: often, the minima get brighter, the maxima get fainter, and the amplitudes of the outbursts get smaller. Somehow, a Z Cam star knows it will go into standstill some months before it actually does. Since standstills occur on timescales of weeks to years, a sustained effort by amateur astronomers is exactly what is needed to solve this problem. Indeed, an extended campaign by amateurs is really the only thing to do. Professional astronomers everywhere are finding it more difficult all the time to get funding. This encourages what's called "smash-and-grab" science: quick projects that promise large immediate scientific payoffs, but which tend to be conservative, unimaginative, and prone to ignoring serendipity. Long-term monitoring projects are especially unpopular among professionals these days---foolishly so, since there is so much to be learned from them. An object of special interest to watch is EM Cyg, because it is the only Z Cam star currently known to have eclipses (for coordinates, see below: finding charts are given by Downes & Shara (1993) and Bruch, Fischer, & Wilmsen (1987)). Observing standstills in Z Cam stars is quite literally science that only amateurs can do---and I am very pleased to find people able and willing to do it! ---------------------------------------------------------------------------- Known or Suspected Z Cam stars. Finding charts for all are given by Downes & Shara (1993) Maximum and minimum brightness are also from Downes & Shara (1993): v = visual, p = photographic (blue) 1950.0 coordinates 2000.0 coordinates Max Min RX And * 01 01 46.0 +41 01 53 01 04 35.63 +41 17 57.8 10.9 v- 12.6 v FS Aur 05 44 38.4 +28 34 10 05 47 48.42 +28 35 10.0 14.4 v- 16.2 v Z Cam * 08 19 39.8 +73 16 22 08 25 13.37 +73 06 38.6 10.5 v- 14.8 v AT Cnc * 08 25 37.8 +25 30 02 08 28 36.99 +25 20 01.8 12.3 p- 14.6 p SY Cnc * 08 58 14.3 +18 05 43 09 01 03.41 +17 53 55.1 11.1 v- 13.5 v HL CMa * 06 43 03.1 -16 48 23 06 45 17.00 -16 51 35.0 10.0 v- 14.5 v SV CMi * 07 28 28.2 +06 05 10 07 31 08.47 +05 58 47.4 13.0 p- 16.3 p BP Cra 18 33 26.6 -37 28 27 18 36 50.88 -37 25 53.8 13.5 v- 15.9 v EM Cyg * 19 36 42.3 +30 23 33 19 38 40.21 +30 30 27.4 12.5 v- 14.4 v V868 Cyg 19 27 05.0 +28 48 09 19 29 04.54 +28 54 25.1 14.3 p- >17.8 p AB Dra * 19 51 04.3 +77 36 39 19 49 06.82 +77 44 22.6 12.3 v- 14.5 v AQ Eri * 05 03 44.1 -04 12 07 05 06 13.11 -04 08 08.2 12.5 v- 17.5 V AH Her * 16 42 06.2 +25 20 31 16 44 10.06 +25 15 01.1 11.3 v- 14.7 v TT Ind 20 29 43.4 -56 44 01 20 33 37.20 -56 33 44.6 12.9 V- >16.5 p DO Leo 10 38 11.3 +15 27 14 10 40 51.28 +15 11 32.8 16.0 B- 17.0 B V426 Oph 18 05 24.9 +05 51 19 18 07 51.79 +05 51 47.7 11.6 B- 13.4 B BI Ori 05 21 17.0 +00 57 46 05 23 51.82 +01 00 29.3 13.2 p- 16.7 p V344 Ori * 06 12 26.8 +15 31 59 06 15 19.02 +15 30 58.6 14.2 p- 17.5:p HX Peg 23 37 51.4 +12 21 03 23 40 23.79 +12 37 40.7 12.9 V- 16.6 V FY Per 04 38 06.3 +50 36 53 04 41 56.68 +50 42 35.7 12.3 B- 14.5 B KT Per * 01 34 01.9 +50 42 03 01 37 08.81 +50 57 19.3 11.5 v- 15.4 V PY Per 02 46 51.7 +37 27 03 02 49 59.91 +37 39 27.0 13.8 p- 16.5 p TZ Per * 02 10 18.5 +58 08 51 02 13 51.09 +58 22 51.8 12.3 v- 15.6 v AY Psc 01 34 18.4 +07 01 13 01 36 55.52 +07 16 29.2 14.9 v- 16.6 v BX Pup 07 52 08.4 -24 11 42 07 54 15.65 -24 19 37.4 13.8 V- 16.0 v UZ Ser 18 08 33.4 -14 56 19 18 11 24.96 -14 55 34.9 11.9 v- 16.0 v FY Vul 19 39 30.3 +21 38 53 19 41 39.98 +21 45 59.0 13.4 B- 15.3 B VW Vul 20 55 34.2 +25 18 47 20 57 45.14 +25 30 25.0 13.1 B- 16.3 B * = Monitored by VVS, Vss-Belgium, thanks very much! Further Reading: Bath, G. T. 1973, Nature (Physical Science), vol. 246, p. 84 Bruch, A., Fischer, F.-J., & Wilmsen, U. 1987, Astronomy & Astrophysics Supplement Series, vol. 70, p. 481 Cannizzo, J. K., Kaitchuck, R. H. 1992, Scientific American, vol. 266, p. 92 Downes, R. A., & Shara, M. M. 1993, Publications of the Astronomical Society of the Pacific, vol. 105, p. 127 la Dous, C. 1993, in Cataclysmic Variables and Related Objects, edited by M. Hack and C. la Dous, NASA/CNRS Monograph Series on Non-Thermal Phenomena in Stellar Atmospheres (U. S. Government Printing Office, Washington, DC), p. 15 Lin, D. N. C., Papaloizou, J., & Faulkner, J. 1985, Monthly Notices of the Royal Astronomical Society, vol. 212, p. 105 Meyer, F., & Meyer-Hofmeister, E. 1983, Astronomy & Astrophysics, vol. 121, p. 29 Meyer-Hofmeister, E., & Ritter, H. 1993, in The Realm of Interacting Binary Stars, edited by J. Sahade, G. McCluskey, and Y. Kondo (Kluwer, Dordrecht), p. 143 Osaki, Y. 1974, Publications of the Astronomical Society of Japan, vol. 26, p. 429 Szkody, P., & Mattei, J. A. 1984, Publications of the Astronomical Society of the Pacific, vol. 96, p. 988 Warner, B. 1995, Cataclysmic Variables Stars (Cambridge University Press, Cambridge) ----------------------------------------------------------------------------