Variable stars found by PLANET

The investigation of variable stars by PLANET

The 5 arcminute by 5 arcminute fields monitored by PLANET contain about 10,000 stars each. Among these stars, about a percent are variables. Since PLANET samples each field many times over the course of one season, accurate light curves can be built for these variables. Just 8 of these variables are shown in the figure below. Each of these variable stars found by PLANET changes its brightness in a periodic, rhythmic way over a period of a few hours to many days.

Two plots are shown for each variable:

The brightness of each star is measured in ``magnitudes'' through a very red filter called an I-band filter. If the magnitude of a star decreases by 1 magnitude, it means that the star has gotton about 2.5 times brighter. Notice that some of the variable stars detected by PLANET are quite faint (I=19), others vary quite quickly (2-4 hours), while others vary by only a small amount (less than 0.1 magnitude or 10%).

Many of these stars vary in brightness because they are pulsating; their gaseous atmospheres swell and shrink in size in a rhythmic way. Others --- like the ones with the double dips in their brightness cycles --- vary because they are actually two stars orbiting one another (a binary star system). In binary systems, the light from each star can be temporarily blocked as the other passes in front of it. The dips are of different sizes because the stars themselves can be of different sizes, brightness and temperatures.

As a by-product of the studies of the microlensing events, variable star catalogs and studies will be made in the near future.

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