  The light curves of the pre-main-sequence star KH 15D from the years
1913--2003 can be understood if the star is a member of an eccentric binary
that is encircled by a vertically thin, inclined ring of dusty gas. Eclipses
occur whenever the reflex motion of a star carries it behind the circumbinary
ring; the eclipses occur with period equal to the binary orbital period of 48.4
days. Features of the light curve--including the amplitude of central reversals
during mid-eclipse, the phase of eclipse with respect to the binary orbit
phase, the level of brightness out-of-eclipse, the depth of eclipse, and the
eclipse duty cycle--are all modulated on the timescale of nodal regression of
the obscuring ring, in accord with the historical data. The ring has a mean
radius near 3 AU and a radial width that is likely less than this value. While
the inner boundary could be shepherded by the central binary, the outer
boundary may require an exterior planet to confine it against viscous
spreading. The ring must be vertically warped to maintain a non-zero
inclination. Thermal pressure gradients and/or ring self-gravity can readily
enforce rigid precession. In coming years, as the node of the ring regresses
out of our line-of-sight toward the binary, the light curve from the system
should cycle approximately back through its previous behavior. Near-term
observations should seek to detect a mid-infrared excess from this system; we
estimate the flux densities from the ring to be 3 mJy at wavelengths of 10--100
microns.

Astrophysics