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Session 65 - New Views of the Magellanic Clouds - II.
Oral session, Wednesday, June 12
Historical Society,

[65.03] Variable Stars from Microlensing Surveys

K. H. Cook (LLNL)

Variable star astronomy is being revolutionized by the photometric time-series available from microlensing surveys. There are two major surveys which are using Magellanic Cloud stars as sources to search for the gravitational microlensing signature of baryonic dark matter in the Milky Way's halo. The Clouds provide millions of relatively bright, resolved stars with lines of sight through much of the halo. The two surveys, MACHO and EROS, have amassed huge databases of photographic and CCD photometry in order to search for exceedingly rare microlensing events. These surveys have probed the photometric behavior of about 10 million stars on time scales from a few minutes to a few thousand days. Light curves from these surveys can have over 15,000 points with just a few percent errors. Although the surveys have detected only about 10 microlensing events, they have discovered more than 40,000 variables in the Clouds.

The survey parameters will be reviewed with emphasis on the techniques used to identify variable stars. The detection efficiency, magnitude limits and spatial extent of the surveys have created essentially complete catalogs of many of the brighter or large amplitude variables in the central region of the LMC, such as RR Lyrae, Cepheids and LPVs. These catalogs have been used to examine in detail the physics of stellar pulsation as well, as the formation and chemical evolution of the Magellanic Clouds. The surveys also probe infrequent variability such as novae, rare types of variability such as R Coronae Borealis stars, aperiodic variability such as found in massive young stars, and low-level periodic behavior in an aperiodic variable such as supersoft X-ray source, RX J0513-69. The combination of wide area coverage, dense temporal sampling, and uniformity of data product found in these surveys is yielding a new perspective on stellar pulsation physics, chemical and dynamical evolution of the Clouds, and may solve the discrepant RR Lyrae and Cepheid distance scales.

Program listing for Wednesday