For more than a century, the binary star system ε Aurigae has been a riddle, wrapped in a mystery, inside an enigma. But no more — the system's previously inferred but unseen disk of dust has been detected.
The first direct images of the large dark disk component of the bright eclipsing binary system ε Aurigae during eclipse are reported on page 870 of this issue by Kloppenborg et al.1. These observations mark a turning point in the study of this mysterious, long-period (about 27.1 years) binary system, and represent a leap forward in our understanding of a system that has long puzzled astronomers.
Eclipse phenomena have had an amazing impact on the development of astronomy, and of general scientific thought, over the past three millennia. For example, it was probably the sight of Earth's circular shadow, visible during lunar eclipses, that led Pythagoras to deduce that Earth is spherical. Later, around 270 BC, lunar eclipses helped Aristarchus of Samos measure the relative sizes of, and distances between, Earth, the Moon and the Sun. The need for accurate forecasts of eclipses was one of the spurs to the practical development of scientific observations and mathematics. During the eighteenth and nineteenth centuries, observations of the transits of Venus and Mercury across the Sun helped with the first accurate determination of the Earth–Sun distance and absolute size of the Solar System. Over the past 100 years, observations of occultations of stars by planets revealed their diameters and atmospheric compositions, and the existence of ring systems. Even the first test of general relativity — on the bending of space-time by the Sun — was made in 1919 by observing the deflection of starlight near the Sun's limb during a total solar eclipse.
Since the discovery2 of Algol (β Persei) as a 2.73-day eclipsing binary by John Goodricke in 1783, the number of known eclipsing binaries has grown to tens of thousands, and they have become fundamentally important to stellar astrophysics. Although about 60% of stars are members of binary and multiple star systems, only a tiny fraction (less than 0.2%) are eclipsing binaries — that is, have their orbital planes aligned such that eclipses can be seen from Earth. Analyses of brightness and orbital-motion variations yield essential orbital and physical characteristics of the stars — for example, masses, radii, temperatures and luminosities. Containing all manner of astronomical objects (such as stars, black holes, white dwarfs and planets), eclipsing binaries have become the foundation for many tests of stellar structure, atmospheres and evolution, accretion processes, and even the detection of more than 50 extrasolar planets in transiting planet–star systems.
The bright eclipsing binary ε Aurigae is one of the best studied, but most puzzling, binary stars known3. The eclipsing nature of the star, a supergiant, was discovered in the 1820s, and its deep, two-year-long eclipses (with brightness decreases of about 50%) have been observed every 27.1 years since. It is clear from the early studies that ε Aurigae is an extraordinary binary star. Its orbital period and very long eclipse durations imply a very large eclipsing companion, of the order of 1,000 times larger than the Sun. Moreover, the supergiant star's orbital motion indicates that the unseen eclipsing object is more massive. Finally, and most confounding of all, the companion is invisible over a wide range of wavelengths.
Many models have been advanced4,5,6,7 to explain the bizarre properties of the unseen object of the binary. These include a huge, nearly spherical semi-opaque nebula; a thick massive dark disk; a not-so-massive disk hiding a black hole; an inclined thin disk with an embedded binary star at its centre; and, more recently, an extensive, slightly tilted disk (with a possible central opening) surrounding a single hot star. However, despite numerous studies using ever-advancing technologies, a precise physical model of the system has remained elusive.
Kloppenborg and colleagues' study1 sheds new light on the nature of the system and the properties of the disk. Using the CHARA interferometer array8 of six 1-metre telescopes, the team was able to reconstruct images of the ingress of the encroaching dark (probably dusty) disk over the face of the bright supergiant (see Fig. 1 on page 871). Although the disk's presence had been inferred from previous observations, it had never actually been imaged. This is the first time that spatially resolved images have been obtained for an eclipsing binary during an eclipse. The images are consistent with models of a thin, dusty disk surrounding a smaller hot star (Fig. 1). Moreover, the observations made from November to December 2009 show the disk moving about 0.7 milliarcseconds to the northwest.
Estimating the system's distance from Earth to be about 650 parsecs, but with large uncertainty (from data taken with the Hipparcos satellite), and scaling to the angular size of the supergiant star (determined from interferometric measurements), Kloppenborg et al.1 were able to measure the motion of the disk relative to the supergiant and — knowing the orbital motion of the supergiant from spectroscopy — they determined its velocity relative to the binary's centre of mass. This velocity implies that the supergiant is about 60% as massive as its larger disk companion. Most recently, Hoard et al.6 analysed the system's energy distribution from the ultraviolet to the far-infrared part of the electromagnetic spectrum and found that it was best fitted by three components: a cool (about 300 °C) disk approximately four times larger than the Earth–Sun distance; an unusual lower-luminosity supergiant star of spectral type F (surface temperature of about 7,500 °C) about 135 times the radius of the Sun but only two to three times its mass; and a hot B5-type (about 15,000 °C) 'main sequence' star about six times the Sun's mass.
This unusual low-mass supergiant (typical F-type supergiants are more luminous and have masses of 10–20 solar masses) results from the evolution of a 6–7-solar-mass progenitor star through the asymptotic giant branch (AGB) phase — in which much of its original mass is lost — to a post-AGB stage supergiant. Post-AGB supergiants are essentially bloated, dying 1–3-solar-mass stars that are short-lived, and go on to eject more material and end up as white dwarfs9. In this model, the observed large dusty accretion disk is explained by the hotter, originally lower-mass companion 'capturing' the material ejected by the bloated supergiant as it evolved through the AGB and post-AGB stages.
Although the new observations1,6 tip the scale in favour of the low-mass model, as described above, it is still possible that the supergiant star is a 'normal' and very young object of 10–20 solar masses, and that its companion is a massive proto-stellar accretion disk object3,7. Because post-AGB stars typically have lower luminosities than do normal massive supergiants, one way in which to discriminate between the two possibilities would be to determine the exact luminosity of the supergiant from a more accurate measurement of its distance. In addition, detailed spectroscopic analyses to search for the chemical anomalies of post-AGB stars (due to convulsions and 'dredge-ups' of processed nuclear material) could help to determine the nature of the supergiant.
Kloppenborg and colleagues' study1 has demonstrated the vital role that eclipses continue to have in astronomy, and how well new technologies can assist in solving old problems. To help to study ε Aurigae's eclipse, professional astronomers have teamed up with amateurs and organized an international observing programme called Citizen Sky10. Continued observations throughout the current eclipse (the middle of the eclipse will occur in July–August 2010) should more narrowly constrain the properties of the system — for example, is there an opening at the centre of the disk? Then ε Aurigae can be used as a laboratory for exploring and testing current astrophysical concepts and theories, including those concerning rapid stages of stellar evolution, binary-star evolution, and the structure and dynamics of large accretion disks.