Evidence for disks at an early stage in class 0 protostars
2017
The formation epoch of protostellar disks is debated because of the competing roles of rotation, turbulence, and magnetic fields in the early stages of low-mass star formation. Magnetohydrodynamics simulations of collapsing cores predict that rotationally supported disks may form in strongly magnetized cores through ambipolar diffusion or misalignment between the rotation axis and the magnetic field orientation. Detailed studies of individual sources are needed to cross check the theoretical predictions. We present 0.06 - 0.1 arcsec resolution images at 350~GHz toward B1b-N and B1b-S, which are young class 0 protostars, possibly first hydrostatic cores. The images have been obtained with ALMA, and we compare these data with magnetohydrodynamics simulations of a collapsing turbulent and magnetized core. The submillimeter continuum emission is spatially resolved by ALMA.
Compact structures with optically thick 350~GHz emission are detected toward
both B1b-N and B1b-S, with 0.2 and 0.35 arcsec radii (46 and 80~au at the
Perseus distance of 230~pc), within a more extended envelope. The flux ratio between
the compact structure and the envelope is lower in B1b-N than in B1b-S, in agreement
with its earlier evolutionary status. The size and orientation of the compact
structure are consistent with 0.2 arcsec resolution 32~GHz observations obtained
with the Very Large Array as a part of the VANDAM survey, suggesting that grains
have grown through coagulation. The morphology, temperature, and densities of the
compact structures are consistent with those of disks formed in numerical simulations
of collapsing cores. Moreover, the properties of B1b-N are consistent with those
of a very young protostar, possibly a first hydrostatic core. These observations
provide support for the early formation of disks around low-mass protostars.
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