Photometric Methods Eclipsing Binaries

Low-mass binary companions can be detected photometrically in three ways: first, through the presence of excess radiation at infrared wavelengths; second, through eclipses of the primary star; and third, through irregularities in the light profile of a microlensing event. In the case of very low-mass/low-luminosity companions, the first of these methods is effective only for white dwarf primaries, and even then can be ambiguous (vide G29-38B). Microlensing is discussed extensively by Bennett elsewhere in this volume. Analyses indicate that a handful of microlensing events are likely to be associated with isolated brown dwarfs (e.g. Poindexter et al, 2005), but, so far, no brown dwarf companions have been detected. The latter observation reinforces the brown dwarf desert inferred for solar-type stars from radial velocity measurements (Gaudi, 2005).

Orbital phase

Fig. 5.8. Visual photometry of the eclipsing binary brown dwarf system 2MASS J05352184-0546085 (Stassun et al., 2006). The data are folded on the orbital period of 9.779621 ± 0.000042 days and phased relative to periastron passage. The ratio of eclipse depths provides a direct measure of the ratio of surface temperatures, with the deeper eclipse corresponding to the eclipse of the hotter component.

Orbital phase

Fig. 5.8. Visual photometry of the eclipsing binary brown dwarf system 2MASS J05352184-0546085 (Stassun et al., 2006). The data are folded on the orbital period of 9.779621 ± 0.000042 days and phased relative to periastron passage. The ratio of eclipse depths provides a direct measure of the ratio of surface temperatures, with the deeper eclipse corresponding to the eclipse of the hotter component.

Eclipsing binaries are binary systems in which the two components orbit each other in a plane that is aligned along the line of sight. The apparent brightness of an eclipsing binary displays a characteristic double-dipped periodic modulation, corresponding to the times when either of the components occults the other (see Fig. 5.8). The dips are generally different in amplitude, their depth depending on the ratio of luminosities of the two components and on the exact viewing geometry.

The first eclipsing binary to be discovered was Algol (p Persei), whose periodicity was noted by Geminiano Montanari in 1667. It was not until more than a century later that an eclipsing mechanism for the variability of Algol was proposed by the British astronomer John Goodricke, for which he was awarded the Royal Astronomical Society's Copley medal in 1783. Edward Pickering proposed a detailed explanation involving two stellar components in 1881. His hypothesis was confirmed in 1889 when the Potsdam astronomer Hermann Vogel discovered Doppler shifts in the spectrum of Algol, confirming variations in the radial velocities of the components. Thus, along with Mizar (Sect. 5.3.2), Algol was one of the first spectroscopic binaries.

Eclipsing binaries are uniquely suited to studying individual binary components. Since the orbital inclination of the binary is known (nearly edge-on), the masses of the components are fully determined in SB2 systems. The ratio of the brightness dips can be used to infer the luminosity ratio, and the duration of the eclipses to estimate their radii. Eclipsing binaries in which at least one of the components is sub-stellar could therefore provide strong constraints on the properties of sub-stellar objects.

Several candidate eclipsing binaries with brown dwarf secondaries have been announced in recent years, though only one, 2MASS J05352184-0546085 (Stas-sun et al., 2006), has been confirmed. 2MASS J05352184-0546085 is located in the young (1-3 million years) Orion Nebula Cluster star-forming region (distance: « 430 parsecs) and is composed of two brown dwarfs orbiting and eclipsing each other (Fig. 5.8). Given its known age and distance, this eclipsing binary brown dwarf provides the first thorough empirical test of evolutionary models of sub-stellar objects.

Planets crossing in front of their host stars, a.k.a. "transiting planets," are also components of eclipsing binaries, where the primary is the star and the secondary is the planet. The characteristics of these systems are discussed in more detail by Irwin (this volume).

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