On the internal structure of starless cores. Physical and chemical properties of L1498 and L1517B

We have characterized the internal structure of two close-to-round starless cores in Taurus, L1498 and L1517B, setting constraints on the initial conditions of star formation and on models of core condensation. Our analysis is based on high angular resolution observations in at least two transitions of NH3, N2H+, CS, (CS)-S-34, (CO)-O-18, and (CO)-O-17, together with maps of the 1.2 mm continuum. For both cores, we derive radial profiles of constant temperature and constant turbulence, and density distributions close to those of non-singular isothermal spheres. Using a Monte Carlo radiative transfer model, we derive abundance profiles for all species and find a pattern of strong chemical differentiation. NH3 has a higher abundance toward the core centers while N2H+ has a constant abundance over most of the cores. Both (CO)-O-18 and CS ( and isotopomers) are strongly depleted in the core interiors, most likely due to their freeze out onto cold dust grains. Concerning the kinematics of the dense gas, we find ( in addition to constant turbulence) a pattern of internal motions at the level of 0.05 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 happens in rather irregular manner. A comparison with supersonic turbulence models of core formation shows that our observed cores are much more quiescent than allowed by these models.