Modeling the Break in the Specific Angular Momentum within the Envelope–Disk Transition Zone

Abstract The observations of protostellar systems show a transition in the radial profile of specific angular momentum (and rotational velocity), as evolving from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>j</mml:mi> <mml:mo>∼</mml:mo> <mml:mi mathvariant="normal">constant</mml:mi> </mml:math> ( v ϕ ∼ r −1 ) in the infalling-rotating envelope to j ∝ r 1/2 ( v ϕ ∼ r −1/2 ) in the Keplerian disk. We employ global MHD disk simulations of gravitational collapse starting from a supercritical prestellar core, that forms a disk and envelope structure in a self-consistent manner, in order to determine the physics of the envelope–disk transition zone (E n DT ran Z). Our results show that the transition from the infalling-rotating envelope to Keplerian disk happens through a jump in the j − r profile, spanning over a finite radial width, which is characterized by the positive local gravitational torques. The outer edge of the E n DT ran Z is identified where the radial infall speed ( v r ) begins a sharp decline in magnitude and j begins a transition from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>j</mml:mi> <mml:mo>∼</mml:mo> <mml:mi mathvariant="normal">constant</mml:mi> </mml:math> toward j ∼ r 1/2 . Moving radially inward, the centrifugal radius ( r CR ) is defined where v ϕ first transitions to Keplerian velocity at the disk’s edge. Farther inward of r CR , the model disk develops a super-Keplerian rotation due to self-gravity. The inner edge of E n DT ran Z is defined at model centrifugal barrier ( r CB ) where v r drops to negligible values. Inside r CB , a net negative gravitational torque drives mass accretion onto the protostar. On observational grounds, we identify a jump in the observed j − r profile of class 0/I protostar L1527 IRS for the first time using the ALMA Large Program Early Planet Formation in Embedded Disks (eDisk) data. Comparison with our numerical radial behavior suggests the observed j − r jump serves as a kinematical tracer for the existence of E n DT ran Z. Our results offer insights into the observable imprint of angular momentum redistribution mechanisms during star–disk formation.