The Nearest PMS Multiple Stars

C.A. Grady
Eureka Scientific and Goddard Space Flight Center

The past decade has seen a rapid expansion in our knowledge of exoplanetary
systems, including many with architectures differing significantly from that
of our Solar System. Theoretical studies have indicated that migration of
giant exoplanets must occur prior to central clearing of the disk, increasing
the need to understand the evolution of protoplanetary disks and the timescale
required for the majority of these systems to turn into planetary systems.
A key component in these studies is the establishment of both an accurate
relative dating sequence for these stars and an understanding of the role
played by the environment of the star on the evolution of the planetary system.
Currently the most complete multiwavelength data are available for nearby,
minimally reddened, and hence optically bright, Herbig Ae stars.  The nearest such
stars in environments ranging from classical, albeit low-density
star formation regions, to  ostensibly  isolated stars apparently located
far from molecular cloud material. While isolated Herbig Ae stars lack many of
the stellar activity signatures that facilitate dating of lower mass stars,
they can be securely dated if they are associated with low mass companions.
Revealing companions located within a few seconds of arc of the primary, where
they can significantly affect the disk survival and architecture typically
requires a variety of multi-wavelength and high contrast imaging techniques.
We review progress in identifying Herbig Ae stars which are members of binary
or small multiple-star  systems, the implications for the dating of these PMS
stellar aggregates, and the potential effects of the companions on the evolution
of the protoplanetary disks of the A stars.