Infrared images of the transiting disk in the Aurigae system

Epsilon Aurigae (Aur) is a visually bright, eclipsing binary star system with a period of 27.1years. The cause of each 18-month-longeclipse has been a subject of controversy for nearly 190years because the companion has hitherto been undetectable. The orbital elements implyt hat 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 super giant star with a mass of ~15Msolar(M solar, 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 or, later, a black hole with an accretion disk, although the preferred interpretation was as a disk of opaque material at a temperature of ~500K, tilted to the line of sight and with a central opening. Recent work implies that the system consists of a low-mass (2.2Msolar-3.3Msolar) visible F-type star, with a disk at 550K that enshrouds a single B5V-type star. 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. Adopting a mass of 5.9Msolar for the B star, we derive a mass of ~(3.6+/-0.7)M solar for the F star. The disk mass is dynamically negligible; we estimate it to contain~0.07M¿ (M¿, mass of the Earth) if it consists purely of dust.