Why Observe Z Cam Stars?

This website and the Z Cam List created and maintained by Mike Simonsen. 


Why Observe Z Cam stars?    (2009 edition)

F. A. Ringwald


Why would anyone be interested in observing Z Cam stars?

The Z Cam stars are a subclass of dwarf novae.  Dwarf novae 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 in accretion disks. 

This is why the planets in the Solar System are all nearly in the same plane: they formed by gravity in what was once the Sun’s accretion disk, called “The Solar Nebula.”  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, such as neutron stars and black holes, since 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.


Accretion disks around super-massive black holes are believed to power active galaxies

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 that 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 millennia.  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 orbits a white dwarf, close enough that it spills gas onto the white dwarf. 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.   Two book-length reviews of cataclysmic variables are Cataclysmic Variable Stars by Brian Warner (1995) and Cataclysmic Variable Stars: How and Why They Vary by Coel Hellier (2001). Coel Hellier’s book was written specifically for undergraduates and amateur astronomers: the first chapter describes how to make useful observations of CVs.

 Aside from dwarf novae, the two other general classes of CVs are the classical novae and the nova-like variables.  Classical novae have nuclear-powered eruptions, of amplitude 10 − 15 magnitudes or more.  These usually occur only once in many centuries, and often last for years.  Nova eruptions are caused by the buildup of gas on the surface of the white dwarf: eventually, the mass, pressure, and temperature build up and the gas detonates, in a thermonuclear runaway.  Nova-like variables do not have eruptions or outbursts, but do have spectra that resemble those of classical novae many years after an eruption.  Many nova-like variables have spectra that resemble those of dwarf novae in outburst: these nova-likes might therefore be thought of as dwarf novae that are 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.  Observations 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.

In 1974, Yoji Osaki (Tokyo) presented the theory that dwarf nova 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.  We now have good evidence that dwarf nova outbursts are indeed caused by thermal instabilities: the accretion disk radii in eclipsing cataclysmic variables grow and shrink to the sizes predicted by disk instability theory (Harrop-Allin and Warner 1996). 

With a high-enough mass-transfer rate into the disk, the disk can resemble a dwarf nova stuck in outburst all the time.  This is what nova-like variables are thought to be.  If the flow is just at the critical rate that separates the dwarf novae from the nova-like variables, the disk may settle into a standstill.  Standstills are the defining characteristic of the Z Cam stars.

The most detailed observational study of standstills has been for Z Cam itself.  It used 51,086 observations made by the AAVSO, between 1928 and 1995 (Oppenheimer, Kenyon, & Mattei 1998).  Kent Honeycutt (Indiana University) and collaborators also published detailed studies of five other Z Cam stars (RX And, Z Cam, SY Cnc, AH Her, and HX Peg) in 1998.  Their observations were done between 1990 and 1996 with RoboScope, an automated 16-inch telescope, as well as by the AAVSO, in 1975-1996. 


AAVSO Light curve of RX Andromedae standstill

The outbursts of Z Cam stars are of special interest to observers, because they’re constantly doing something.  They tend not to spend long periods between outbursts in a faint, quiescent state, the way some other dwarf novae do: most of the time, they are either on the rise to outburst, or on the decline from outburst.  Shafter, Cannizzo, & Waagen (2005) used AAVSO light curves to study the outbursts, aside from the standstills, in Z Cam stars.  They found that Z Cam stars with longer orbital periods have longer outbursts.  This may be since they have larger sizes: more heat is required to light up the disk during an outburst, but detailed models that explain this have not been done.

Even with the observations and analyses of recent years, we still know relatively little about standstills, not even the fundamental observational properties of how often they occur or how long they last.  Oppenheimer et al. (1998) found that the standstills of Z Cam could last between 9 days and 1020 days: Z Cam was in standstill almost continuously between 1977 and 1981.  In contrast, HX Peg has much shorter standstills, about 30 to 90 days long, which recur yearly (Honeycutt et al. 2005).  Standstills are not completely static:  in their compilation of the statistics of dwarf nova outbursts published in 1984, Paula Szkody (Washington) and Janet Mattei (AAVSO) showed there can appear erratic flareups with amplitudes of several tenths of a magnitude.

I therefore encourage 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 still sketchy and quite possibly wrong.  More observations would of course greatly help.

The leading explanation for standstills was published by Friedrich Meyer and Emmi Meyer-Hofmeister (Max Planck Institut fűr Astrophysik, Munich) in 1983.  It 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 mass-losing star would only have so much gas to give up, though, so the mass transfer would eventually reach an equilibrium rate, and therefore stand still.   

The standstills of Z Cam are observed to be triggered by normal outbursts (Oppenheimer, Kenyon, & Mattei 1998).  It would be of interest to confirm whether all Z Cam stars do this, since this is an essential feature of most theoretical models.  Another problem with the theory, which 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 105 years.  This prediction is too long.  Standstills are observed to last for a few years, and often much shorter. 

Another idea is that standstills are ended by starspots.  Starspots are similar to sunspots, but are on stars other than the Sun.  Sunspots and starspots are areas on a star’s surface that have strong magnetic fields: they push the gas on the surface of the star aside, like Moses parting the Red Sea.  Because of this, inside a starspot the gas pressure will be less than the pressure of the gas surrounding it.  This is why sunspots are dark: with a lower pressure comes a lower temperature.

Suppose a starspot were to move or form over the point on the mass-losing star from where the gas is spilling into the disk.  It might therefore choke off the flow of the gas, from the star into the accretion disk.  This idea has received much attention, both as an explanation for what ends starspots, and also as an explanation of why CVs of all types have erratic, unpredictable low states.  I wonder whether it’s true: plasmas and other hot gases are well known for the ability to escape confinement.  It is therefore still unclear what stops a standstill.

 Models are now sophisticated enough to explain why standstills are about a magnitude fainter than outburst maximum.  It is because the disk is heated by the gas stream from the mass-losing star.  Because of this extra source of heat, the critical mass transfer rate at which a standstill occurs is about 40% less than the mass transfer rate during outburst (Stehle, King, and Rudge 2001).

Somehow, a Z Cam star knows it will go into standstill some months before it actually does.  The minima get brighter, the maxima get fainter, and the amplitudes of the outbursts get smaller.  Is this from a heating wave, spreading though the disk?  If so, it’s an important piece of accretion disk physics, since it would be essential for understanding mass flow and angular momentum transport through the disk.  Since standstills occur on timescales of weeks to years, sustained observations by amateur astronomers will be exactly what is needed for improved understanding.

Two Z Cam stars have eclipses: EM Cyg and AY Psc.  It would be particularly useful to monitor these stars, since theoretical models of standstills make specific predictions about how disk radius varies between quiescence, outburst, and standstill (see Buat-Ménard, Hameury, & Lasota 2001). 

Indeed, extended campaigns by amateurs are still essential.  Professional astronomers everywhere are finding it increasingly difficult 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 missing out on serendipity.  Long-term monitoring projects are especially unpopular among professionals these days—foolishly so, since there is so much to be learned from them. 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!

 

(Fantastic space art from Mark A. Garlick used by permission. Do not reproduce with out permission.) 


Further Reading:

Buat-Ménard, V., Hameury, J.-M., & Lasota, J.-P. 2001, Astronomy & Astrophysics, vol. 369, p. 925

Harrop-Allin, M. K., & Warner, B. 1996, Monthly Notices of the Royal Astronomical Society, vol. 279, p. 219

Hellier, C. 2001, Cataclysmic Variables: How any Why They Vary (Springer-Praxis)

Honeycutt, R. K., Robertson, J. W., Turner, G. W., & Mattei, J. A. 1998, Publications of the Astronomical Society of the Pacific, vol. 110, 676

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), p. 143

Oppenheimer, B. D., Kenyon, S. J., & Mattei, J. A. 1998, The Astronomical Journal, vol. 115, p. 1189

Osaki, Y. 1974, Publications of the Astronomical Society of Japan, vol. 26, p. 429

Shafter, A. W., Cannizzo, J. K., & Waagen, E. O. 2005, Publications of the Astronomical Society of the Pacific, vol. 117, 931

Stehle, R., King, A., & Rudge, C. 2001, Monthly Notices of the Royal Astronomical Society, vol. 323, p. 584

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)

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