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 ^ { 13 } CO ( J =3–2 ) and C ^ { 18 } O ( J =3–2 ) at a resolution of \sim 120 \times 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 \sim 600 \times 300 AU feature elongated approximately perpendicular to the [ Fe II ] jet from the southern protostellar component suggestive of a circumbinary disk .
The ^ { 13 } CO and C ^ { 18 } O images confirm that the circumbinary continuum feature is indeed a rotating disk ; furthermore , the C ^ { 18 } O channel maps can be well modeled by a geometrically-thin disk exhibiting Keplerian rotation .
We estimate a mass for the circumbinary disk of \sim 0.03-0.12 M _ { \odot } , compared with an enclosed mass of \sim 0.8 M _ { \odot } 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 ( \sim 91 K ) , highest envelope mass ( \sim 0.39 M _ { \odot } ) , and the lowest ratio in stellar mass to envelope + disk + stellar mass ( \sim 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 .