When Is a Variable a Cataclysmic Variable?
By Duane Dunkerson
A cataclysmic variable is an interacting binary with an accretion disk.
A "variable", in astronomical parlance, is a star that has a readily noticeable difference in brightness. The brightness of a star is synonymous with its magnitude. A star that has a varying magnitude has a characteristic apart from the more usual star that has one magnitude listed for it. Star A is of the second magnitude. Star B is of magnitude 3.5 and so on. A variable has a limited range of magnitudes. It varies from its present magnitude to a lower limit or an upper limit depending on when you started observing the star. It will cycle through its high and low magnitudes repeatedly
.Omicron Ceti, or Mira as it was known to the ancients, brightens from a dim magnitude of 8 or 10 to magnitude 2 in 33 days. In 1596 Fabricius saw Mira as a star of magnitude 3. In two months Mira had disappeared. Bayer saw it again in 1603 and put it in a star catalogue. Then it vanished. By 1638 it was known that this disappearance and reappearance was occurring because Mira is a variable star having a minimum that is much fainter than its maximum. Polaris, the Pole Star, varies much less. It changes magnitude from 2.5 to 2.6 in less than 4 days. After Mira the second variable discovered in 1669 is the decidedly irregular Algol. The third variable to be discovered was Chi Cygni. There are now at least 22,650 variable stars that have been catalogued.
It is not surprising to find a variable gravitationally associated with another star. Most stars are binaries. Nor is it unusual to find astronomical accretion disks. These disks as usually gas or dust surrounding a massive central object like a star or black hole. The components of the disk move toward the central object and eventually fall onto the object. Accretion disks are thought to have a role to play in star-making, planet formation, and quasar development.
Disks themselves, of accretion or not, are common throughout the Universe. Random motion of the constituents within a sea of astronomical objects is put in order by gravitational forces that tend to bring them closer. In the spherical form the set of objects also tends to rotate. A rotating, pulled together set, settles into a disk. Friction is generated by the close passage of the objects of a set as they get closer. This viscosity, as the friction is called, involves some unknown physics. This unknown factor is called alpha. The alpha is considered to be a constant in order to model what the disk does.
What a disk can do is help to transport material from one star of an interacting binary to the other. Rarely are the two stars of the binary system of equal size or mass. The smaller or less massive star gives up matter into the disk and then onto the larger and/or more massive star. The detection of such a disk in a binary system was accomplished in 1934 by Arthur Wyse of the Lick Observatory. The star system Wyse studied is called RW Tauri. As it turns out, the RW Tauri disk isn't permanent. It comes and goes since the contributor to the matter of the disk puts out its supplies not at a steady pace.
The steady pace of magnitude change maintained by the usual variables amounts to a change of a few magnitudes. But cataclysmic variables can alter their magnitude by as much as 100 times and only in a matter of days or hours. The two stars are in orbit in a time span of days or hours. In addition, the two stars are in orbit about each other at very close range. Typically a cataclysmic binary has an orbital period of 4 hours, some are less than 2 hours. The entire system of such binaries could be placed within the Sun. The binary is composed of a very small red star that is much cooler than the other star, a very small white star.
The ordinary variable stars are so numerous that they cannot be monitored in total. Amateur astronomers in organizations devoted to variable star observation such as the American Association of Variable Star Observers, help to keep track of what is going on. As for the cataclysmic variables, there are also amateurs doing what the professional cannot do.
That is, amateur astronomers have underway projects to monitor the behavior of cataclysmic variables. One group, the Center for Backyard Astrophysics, has a web site at http:// cba.phys.columbia.edu. The web site itself, as they state, varies periodically. The CBA is a network of small backyard telescopes dedicated to the photometry of cataclysmic variables. Since 1991, mostly via use of charge coupled devices (CCDs) and their telescopes, they have electronically recorded the variance in these variables. Besides the huge fluctuations in magnitude for cataclysmic variables, these amateur astronomers also study periodic changes that can vary from 10 minutes to 4 days. These changes are spins for white dwarf stars, accretion disk precession, orbital period of these binaries, and superhumps. All SU UMa-type dwarf novae, in a subdivision of cataclysmic variables, have repetitious large (superhump) increases in magnitude. The superhumps' outbursts closely exceed the normal change in magnitude for these binaries.
Three to six stars are selected for a month's worth of observing by the network. The selected stars can remain on the observation schedule for 2 to 3 months. At some times during the year, all members of the network gang up on one star for high priority intense observation for weeks at a time.
The most bang per buck, the most data change per time observed can come from cataclysmic variables that are helium stars. There are 6 helium cataclysmic variables. Their fluctuation periods go from 17 to 46 minutes. The helium cataclysmic variables have the accretion characteristics of the more common cataclysmic variable - superhumps, standstills, and eruptions.
The CBA is headquartered in New York City. There are observing stations operated by amateur astronomers using their own equipment in Australia, Belgium, South Africa, USA, Denmark, Finland, Russian, and Italy. The stars become like people to some of these observers. The stars can be cranky or smooth, predictable or not. Some are like friends, so well known do they become.
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by Duane Dunkerson
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