Observations of L1521F: A highly evolved starless core

We observed the pre-stellar core L1521F in dust emission at 1.2 mm and in two transitions each of N2H+, N2D+, (CO)-O-18 and (CO)-O-17 in order to increase the sample of well studied centrally concentrated and chemically evolved starless cores, likely on the verge of star formation, and to determine the initial conditions for low-mass star formation in the Taurus Molecular Cloud. The dust observation allows us to infer the density structure of the core and together with measurements of CO isotopomers gives us the CO depletion. N2H+ and N2D+ lines are good tracers of the dust continuum and thus they give kinematic information on the core nucleus. We derived in this object a molecular hydrogen number density n(H-2) similar to 10(6) cm(-3) and a CO depletion factor, integrated along the line of sight, f(D) = 9.5 x 10(-5)/x(obs) (CO) similar to 15 in the central 20", similar to the pre-stellar core L1544. However, the N(N2D+)/N(N2H+) column density ratio is similar to0.1, a factor of about 2 lower than that found in L1544. The observed relation between the deuterium fractionation and the integrated CO depletion factor across the core can be reproduced by chemical models if N2H+ is slightly (factor of similar to2 in fractional abundance) depleted in the central 3000 AU. The N2H+ and N2D+ linewidths in the core center are similar to0.3 km s(-1), significantly larger than in other more quiescent Taurus starless cores but similar to those observed in the center of L1544. The kinematical behaviour of L1521F is more complex than seen in L1544, and a model of contraction due to ambipolar diffusion is only marginally consistent with the present data. Other velocity 'fields, perhaps produced by accretion of the surrounding material onto the core and/or unresolved substructure, are present. Both chemical and kinematical analyses suggest that L1521F is less evolved than L1544, but, in analogy with L1544, it is approaching the "critical" state.