Centrifugal barrier and super-Keplerian rotation in protostellar disc formation
ABSTRACT

With the advent of ALMA, it is now possible to observationally constrain how discs form around deeply embedded protostars. In particular, the recent ALMA C3H2 line observations of the nearby protostar L1527 have been interpreted as evidence for the so-called ‘centrifugal barrier,’ where the protostellar envelope infall is gradually decelerated to a stop by the centrifugal force in a region of super-Keplerian rotation. To test the concept of centrifugal barrier, which was originally based on angular momentum conserving-collapse of a rotating test particle around a fixed point mass, we carry out simple axisymmetric hydrodynamic simulations of protostellar disc formation including a minimum set of ingredients: self-gravity, rotation, and a prescribed viscosity that enables the disc to accrete. We find that a super-Keplerian region can indeed exist when the viscosity is relatively large but, unlike the classic picture of centrifugal barrier, the infalling envelope material is not decelerated solely by the centrifugal force. The region has more specific angular momentum than its surrounding envelope material, which points to an origin in outward angular momentum transport in the disc (subject to the constraint of disc expansion by the infalling envelope), rather than the spin-up of the envelope material envisioned in the more »

Authors:
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Award ID(s):
Publication Date:
NSF-PAR ID:
10373197
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
517
Issue:
1
Page Range or eLocation-ID:
p. 213-221
ISSN:
0035-8711
Publisher:
Oxford University Press
5. ABSTRACT We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains ($\gt 0.005\!-\!0.1\, \mu$m), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge.more »