2012ApJ...754...52T

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 ∼120x80 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 ∼600x300 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 M_☉, compared with an enclosed mass of ∼0.8 M_☉ 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 M_☉), 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.