What Can Be Derived from Eclipsing Binaries

The analysis of photometric light curves alone cannot provide absolute dimensions of binary stars or their orbits. The reason is a scaling property with respect to the relative orbital semi-major axis a: If all linear dimensions are increased by a certain factor, the associated light curve changes can be canceled by shifting the binary to a larger distance. A light curve can provide the orbital inclination and, among other parameters, relative quantities such as the radii in units of a, ratio of luminosities, stellar figures, and perhaps the photometric mass ratio.

Radial velocity curves can provide the mass ratio and the scaling factor a in physical units if the inclination is known from another kind of observation. With a and the period P known, the masses can be found unambiguously from (4.4.14) and (4.4.16). Similarly, i must be known to derive orbital dimensions [cf. (4.4.14, 4.4.15, 4.4.16 and 4.4.17)] from a radial velocity curve. Combining these rules, we find the following:

The full determination of absolute eclipsing binary parameters requires both a light curve and a radial-velocity curve for each component. EBs are informative objects because they allow photometry and spectroscopy to be combined effectively. Eclipsing, double-lined systems are rare but very valuable. If the data quality is high and the binary configuration is well conditioned, we have a fundamental source of information about sizes, masses, luminosities, and distances or parallaxes of stars. Many other parameters can be determined from precise light curve data if the configuration fulfills certain requirements, for example, by having complete eclipses. Because such stars may be found over the full range of ages, they also tell how stars evolve - at least in binary stars.

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