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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.