Transition from the Infalling Envelope to the Keplerian Disk around L1551 IRS 5

We present combined SubMillimeter Array (SMA) + Atacama Submillimeter Telescope Experiment (ASTE) images of the Class I protobinary L1551 IRS 5 in the CS ($J$ = 7--6) line, the submillimeter images of L1551 IRS 5 with the most complete spatial sampling ever achieved ($0''.9$ -- $36''$). The SMA image of L1551 IRS 5 in the 343 GHz dust-continuum emission is also presented, which shows an elongated feature along the northwest to southeast direction ($\sim$160 AU $\times$ 80 AU), perpendicular to the associated radio jets. The combined SMA+ASTE images show that the high-velocity ($\gtrsim$1.5 km s$^{-1}$) CS emission traces the structure of the dust component and shows a velocity gradient along the major axis, which is reproduced by a geometrically-thin Keplerian-disk model with a central stellar mass of $\sim$0.5 $M_{\odot}$. The low-velocity ($\lesssim$1.3 km s$^{-1}$) CS emission shows an extended ($\sim$1000 AU) feature that exhibits slight south (blueshifted) to north (redshifted) emission offsets, which is modeled with a rotating and infalling envelope with a conserved angular momentum. The rotational motion of the envelope connects smoothly to the inner Keplerian rotation at a radius of $\sim$64 AU. The infalling velocity of the envelope is $\sim$three times lower than the free-fall velocity toward the central stellar mass of 0.5 $M_{\odot}$. These results demonstrate transition from the infalling envelope to the Keplerian disk, consistent with the latest theoretical studies of disk formation. We suggest that sizable ($r\sim$50--200 AU) Keplerian disks are already formed when the protostars are still deeply embedded in the envelopes.