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 \farcs 9 – 36 \arcsec ) .
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 .