Infrared images of the transiting disk in the ε Aurigae system

Epsilon Aurigae (epsilon Aur) is a visually bright, eclipsing binary star system with a period of 27.1 years. The cause of each 18-month-long eclipse has been a subject of controversy for nearly 190 years(1) because the companion has hitherto been undetectable. The orbital elements imply that the opaque object has roughly the same mass as the visible component, which for much of the last century was thought to be an F-type supergiant star with a mass of similar to 15M(circle dot) (M(circle dot), mass of the Sun). The high mass-to-luminosity ratio of the hidden object was originally explained by supposing it to be a hyperextended infrared star(2) or, later, a black hole(3) with an accretion disk, although the preferred interpretation was as a disk of opaque material(4,5) at a temperature of 500 K, tilted to the line of sight(6,7) and with a central opening(8). Recent work implies that the system consists of a low-mass (2.2M(circle dot)-3.3M(circle dot)) visible F-type star, with a disk at 550K that enshrouds a single B5V-type star(9). Here we report interferometric images that show the eclipsing body moving in front of the F star. The body is an opaque disk and appears tilted as predicted(7). Adopting a mass of 5.9M(circle dot) for the B star, we derive a mass of similar to(3.6 +/- 0.7)M(circle dot) for the F star. The disk mass is dynamically negligible; we estimate it to contain similar to 0.07M(circle plus) (M(circle plus), mass of the Earth) if it consists purely of dust.