Variable Stars
In this chapter we review the research that has been carried out on the variable stars in M33. Included in separate sections are the geometrical variables (eclipsing stars) and the various kinds of intrinsic variables.
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Variable Stars
In this chapter we review the research that has been carried out on the variable stars in M33. Included in separate sections are the geometrical variables (eclipsing stars) and the various kinds of intrinsic variables.
10.1
Early Studies of Variable Stars in M33
The first variable stars in M33 were a few novae, discovered with the large telescopes at the Mt. Wilson and the Lick Observatories (see Sect. 10.7). A systematic study of variables did not occur until Hubble’s (1929) thorough observing program, which discovered a number of different types of variable stars and used the Cepheids among them to establish the extragalactic nature of M33 (Chap. 3). Hubble’s data were photographic images taken with blue-sensitive emulsions, giving him no information on the color of the stars. His photometry was based on magnitudes determined by comparison with “standard sequences” of stars in Selected Area 45, which had been measured by fairly crude techniques (compared to modern standards) and which was reliable only to magnitude (pg) = 18.5. His measurements of the magnitudes of the Cepheids reached from pg = 18.3 to >19.5, and so the results depended largely on unreliable magnitudes and were therefore uncertain. Hubble also discovered and studied what became known as Hubble-Sandage variables, but now more commonly are called luminous blue variables. They are discussed in Sect. 10.5. It was a long time before anyone looked again at the variable stars of M33. Van den Bergh, Herbst and Kowal (1975) made the next survey of M33 to discover variable stars. Using the Palomar Observatory 48-inch (1.2 m) Schmidt Telescope, they discovered 38 new variables, increasing the total number of known variables to 86. Periods could not be obtained because of crowding of the images on the relatively small-scale plates. Sandage (1983) attempted to correct Hubble’s photometry by remeasuring the standard sequences in Selected Area 45 photoelectrically. He found large errors for P. Hodge, The Spiral Galaxy M33, Astrophysics and Space Science Library 379, DOI 10.1007/978-94-007-2025-1_10, © Springer Science+Business Media B.V. 2012
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SANDGE CARLSON ' [m-M] M33 LMC -6.3
M33
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MEAN APPARENT BLUE MAGNITUDE < B >
Fig. 10.1 Period-luminosity diagrams for M33 Cepheids. The Sandage-Carlson photometry was the first new data since Hubble’s original paper. The P-L relation for the Large Magellanic Cloud is shown for comparison (From Sandage and Carlson 1983). Used by permission, copyright AAS
Variable Stars
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HUBBLE CORRECTED '[m-M] M33 LMC -6.5 tt
M33
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C
MARTINETAL (1979)
LMC
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1.2 LOG P
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faint magnitudes, claiming, for example, that the corrections were as large as 2.8 magnitudes at B = 22.5. Additionally, 13 new Cepheids in M33 were discovered and studied by Sandage and Carlson (1983), who produced a new period-magnitude relation (Fig. 10.1).
10.2
Current Large-Scale Searches for Variables
The results of a modern large-scale variable star search were published by Macri e
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