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Some Thoughts on the Analysis and Interpretation of Lightcurves - Part Two - Periodicities

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Lee Anne Willson

Erwin W. Fick Observatory, Iowa State University, Ames, IA 50010

Abstract

Part I of this series showed the consequences of the fact that magnitudes are logarithmic quantities. In this contribution the question of variable star periods is discussed, in particular the cases where multiple periods and /or period changes are present.

The O-C diagram can be a powerful tool for determining periods, period changes, and even the presence of multiple periods. Some superficially similar light curves can easily be distinguised by comparison of the O-C diagrams: examples include "AM" vs. "FM" modulation.

Fourier analysis is the classic method for establishing the presence of periodicities. This method is powerful, but has the drawback taht if the light curv eis not strictly sinusoidal, the Fourier results will include extra periods to account for the shape of magnitude variation or the flux variation is analyzed.

Recently a number of relatively straightforward techniques for determining the periods present in a varying source have become popular among astronomers. These techniques include "phase dispersion minimization" and "auto-correlation." The basic assumptions of these techniques are described, examples, shown, and further references given.