We present SubMillimeter-Array observations of a Keplerian disk around the
Class I protobinary system L1551 NE in 335 GHz continuum emission and
submillimeter line emission in 13CO (J=3-2) and C18O (J=3-2) at a resolution of
~120 x 80 AU. The 335-GHz dust-continuum image shows a strong central peak
closely coincident with the binary protostars and likely corresponding to
circumstellar disks, surrounded by a ~600 x 300 AU feature elongated
approximately perpendicular to the [Fe II] jet from the southern protostellar
component suggestive of a circumbinary disk. The 13CO and C18O images confirm
that the circumbinary continuum feature is indeed a rotating disk; furthermore,
the C18O channel maps can be well modeled by a geometrically-thin disk
exhibiting Keplerian rotation. We estimate a mass for the circumbinary disk of
~0.03-0.12 Msun, compared with an enclosed mass of ~0.8 Msun that is dominated
by the protobinary system. Compared with several other Class I protostars known
to exhibit Keplerian disks, L1551 NE has the lowest bolometric temperature (~91
K), highest envelope mass (~0.39 Msun), and the lowest ratio in stellar mass to
envelope + disk + stellar mass (~0.65). L1551 NE may therefore be the youngest
protostellar object so far found to exhibit a Keplerian disk. Our observations
present firm evidence that Keplerian disks around binary protostellar systems,
``Keplerian circumbinary disks', can exist. We speculate that tidal effects
from binary companions could transport angular momenta toward the inner edge of
the circumbinary disk and create the Keplerian circumbinary disk.Comment: 27 pages, 8 figure
