  We have characterized the physical structure and chemical composition of two
close-to-round starless cores in Taurus-Auriga, L1498 and L1517B. Our analysis
is based on high angular resolution observations in at least two transitions of
NH3, N2H+, CS, C34S, C18O, and C17O, together with maps of the 1.2 mm
continuum. For both cores, we derive radial profiles of constant temperature
and constant turbulence, together with density distributions close to those of
non-singular isothermal spheres. Using these physical conditions and a Monte
Carlo radiative transfer model, we derive abundance profiles for all species
and model the strong chemical differentiation of the core interiors. According
to our models, the NH3 abundance increases toward the core centers by a factor
of several while N2H+ has a constant abundance over most of the cores. In
contrast, both C18O and CS (and isotopomers) are strongly depleted in the core
interiors, most likely due to their freeze out onto grains at densities of a
few 10^4 cm^{-3}. Concerning the kinematics of the dense gas, we find (in
addition to constant turbulence) a pattern of internal motions at the level of
0.1 km s^{-1}. These motions seem correlated with asymmetries in the pattern of
molecular depletion, and we interpret them as residuals of core contraction.
Their distribution and size suggest that core formation occurs in a rather
irregular manner and with a time scale of a Myr. A comparison of our derived
core properties with those predicted by supersonic turbulence models of core
formation shows that our Taurus cores are much more quiescent than
representative predictions from these models.
