Systematic Molecular Differentiation in Starless Cores

We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by at least one or two orders of magnitude with respect to the value in the outer core. At the same time, the N2H + molecule has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a systematic study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mapped in CO(1–0), CS(2–1), N2H (1–0), NH3(1,1) and (2,2) and the 1.2 mm continuum (complemented with CO(1–0) and CS(2–1) data for some systems). For each core we have built a spherically symmetric model in which the density is derived from the 1.2 mm continuum, the kinetic temperature from NH3, and the abundance of each molecule is derived using a Monte Carlo radiative transfer code which simultaneously fits the shape of the central spectrum and the radial profile of integrated intensity. Regarding the cores for which we have CO(1–0) and CS(2–1) data, the model fits these observations automatically when the standard isotopomer ratio is assumed. As a result of this modeling, we also find that the gas kinetic temperature in each core is constant at approximately 10 K. In agreement with previous work, we find that if the dust temperature is also constant, then the density profiles are centrally flattened, and we can model them with a single analytic expression. We also find that for each core the turbulent linewidth seems constant in the inner 0.1 pc. The very strong abundance drop of CO and CS toward the center of each core is naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 ×104 cm. N2H seems unaffected by this process up to densities of several 10 or even 10 cm, while the NH3 abundance may be enhanced by its lack of depletion and reactions triggered by the disappearance of CO from the gas phase. With the help of the Monte Carlo modeling, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence. Subject headings: ISM: abundances— ISM: clouds—ISM: molecules—stars: formation