Kinematics of Gas around HL Tauri Revisited: Free Fall and Rotation in a 1000-AU Radius Disk

Astronomy and Astrophysics – Astronomy

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Scientific paper

Molecular gas around the prototypical young stellar object, HL Tauri, has been the target of multiple aperture synthesis imaging studies in rotational transitions of the isotopomers of CO. Accounts published to date provide contrasting views of the gas velocity structure on scales of 100 to 1000 AU. Here we present new high-resolution (θ = 1.5'') observations of HL Tau that were taken with the Owens Valley Millimeter Array in the 13CO(2->1) transition at 220.4 GHz. The resulting spectral-line images can be simulated with a model of a flat geometric disk and a parametrized velocity field that encompasses both free fall and centrifugally supported states. The images are best matched by a model of circumstellar gas that is predominantly infalling at distances of a few hundred to a thousand AU about a central object with mass 1.75 M\bigodot . Model-fits are also consistent with the presence of a smaller region of Keplerian rotation inside a stellocentric radius of 220 AU. The overall picture suggests that most of the infalling gas accretes further from HL Tau than is commonly assumed for the formation of our own solar system. The preservation of interstellar chemical signatures is favored in this scenario with implications for the evolution of molecules in protoplanetary disks.
Thanks to the National Science Foundation Research Experience for Undergraduates program for their financial support.

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