A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A
  Context: Rotationally supported disks are critical in the star formation
process. The questions of when do they form and what factors influence or
hinder their formation have been studied but are largely unanswered.
Observations of early stage YSOs are needed to probe disk formation. Aims:
VLA1623 is a triple non-coeval protostellar system, with a weak magnetic field
perpendicular to the outflow, whose Class 0 component, VLA1623A, shows a
disk-like structure in continuum with signatures of rotation in line emission.
We aim to determine whether this structure is in part or in whole a
rotationally supported disk, i.e. a Keplerian disk, and what are its
characteristics. Methods: ALMA Cycle 0 Early Science 1.3 mm continuum and
C$^{18}$O (2-1) observations in the extended configuration are presented here
and used to perform an analysis of the disk-like structure using PV diagrams
and thin disk modelling with the addition of foreground absorption. Results:
The PV diagrams of the C$^{18}$O line emission suggest the presence of a
rotationally supported component with a radius of at least 50 AU. Kinematical
modelling of the line emission shows that the disk out to 180 AU is actually
rotationally supported, with the rotation being well described by Keplerian
rotation out to at least 150 AU, and the central source mass to be $\sim$0.2
M$_{sun}$ for an inclination of 55$^{\circ}$. Pure infall and conserved angular
momentum rotation models are excluded. Conclusions: VLA1623A, a very young
Class 0 source, presents a disk with an outer radius $R_{\rm out}$ = 180 AU
with a Keplerian velocity structure out to at least 150 AU. The weak magnetic
fields and recent fragmentation in this region of rho Ophiuchus may have played
a lead role in the formation of the disk.
