  Exploring the structure and dynamics of cold starless clouds is necessary to
understand the different steps leading to the formation of protostars. Because
clouds evolve slowly, many of them must be studied in detail to pick up
different moments of a cloud's lifetime. We study here L1506C in the Taurus
region, a core with interesting dust properties which have been evidenced with
the PRONAOS balloon-borne telescope. To trace the mass content of L1506C and
its kinematics, we mapped the dust emission, and the line emission of two key
species, C18O and N2H+ (plus 13CO and C17O). This cloud shows peculiar
features: i) a large envelope traced solely by 13CO holding a much smaller core
with a strong C18O depletion in its center despite a low maximum opacity (Av~20
mag), ii) extremely narrow C18O lines indicating a low, non-measurable
turbulence, iii) contraction traced by C18O itself (plus rotation), iv)
unexpectedly, the kinematical signature from the external envelope is opposite
to the core one: the 13CO and C18O velocity gradients have opposite directions
and opposite profiles (C18O blue peaked, 13CO red peaked). The core is large (r
= 3E4 AU) and not very dense (n(H2) ? 5E4 cm-3 or less). This core is therefore
not prestellar yet. All these facts suggest that the core is kinematically
detached from its envelope and in the process of forming a prestellar core.
This is the first time that the dynamical formation of a prestellar core is
witnessed. The extremely low turbulence could be the reason for the strong
depletion of this core despite its relatively low density and opacity in
contrast with undepleted cores such as L1521E which shows a turbulence at least
4 times as high.
