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

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

Authors:

Chou et al

Abstract:

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 (∼160 AU × 80 AU), perpendicular to the associated radio jets. The combined SMA+ASTE images show that the high-velocity (≳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 ∼0.5 M⊙. The low-velocity (≲1.3 km s−1) CS emission shows an extended (∼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 ∼64 AU. The infalling velocity of the envelope is ∼three times lower than the free-fall velocity toward the central stellar mass of 0.5 M⊙. 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∼50--200 AU) Keplerian disks are already formed when the protostars are still deeply embedded in the envelopes.