The kinematics of NGC1333-IRAS2A - a true Class 0 protostar
  Low-mass star formation is described by gravitational collapse of dense cores
of gas and dust. At some point during the collapse, a disk is formed around the
protostar and the disk will spin up and grow in size as the core contracts
because of angular momentum conservation. The question is how early the disk
formation process occurs. In this paper we aim to characterize the kinematical
state of a deeply embedded, Class 0 young stellar object, NGC1333-IRAS2A, based
on high angular resolution (< 1$''$ $\approx$ 200 AU) interferometric
observations of HCN and H$^{13}$CN J = 4-3 from the Submillimeter Array, and
test whether a circumstellar disk can be detected based on gas kinematic
features. We adopt a physical model which has been shown to describe the object
well and obtain a fit of a parameterized model of the velocity field, using a
two-dimensional axis-symmetric radiation transfer code. The parameterization
and fit to the high angular resolution data characterize the central dynamical
mass and the ratio of infall velocity to rotation velocity. We find a large
amount of infall and very little rotation on all scales. The central object has
a relatively low mass of 0.25 M$_\odot$ . As an object with a low stellar mass
compared to the envelope mass, we conclude that NGC1333-IRAS2A is consistent
with the suggestion that, as a Class 0 object, it represents the earliest
stages of star formation. The large amount of infall relative to rotation also
suggests that this is a young object. We do however find the need of a central
compact component on scales of a few hundred AU based on the continuum data,
which suggests that disk formation happens shortly after the initial
gravitational collapse. The data do not reveal a distinct velocity field for
this 0.1 M$_\odot$ component.
