Gas-phase CO depletion and N₂H⁺ abundances in starless cores

Context. In the dense and cold interiors of starless molecular cloud cores, a number of chemical processes allow for the formation of complex molecules and the deposition of ice layers on dust grains. Dust density and temperature maps of starless cores derived from Herschel continuum observations constrain the physical structure of the cloud cores better than ever before. We use these to model the temporal chemical evolution of starless cores. Aims. We derive molecular abundance profiles for a sample of starless cores. We then analyze these using chemical modeling based on dust temperature and hydrogen density maps derived from Herschel continuum observations. Methods. We observed the ¹²CO (2-1), ¹³CO (2-1), C¹⁸O (2-1) and N ₂H⁺ (1-0) transitions towards seven isolated, nearby low-mass starless molecular cloud cores. Using far infrared (FIR) and submillimeter (submm) dust emission maps from the Herschel key program Earliest Phases of Star formation (EPoS) and by applying a ray-tracing technique, we derived the physical structure (density, dust temperature) of these cores. Based on these results we applied time-dependent chemical modeling of the molecular abundances. We modeled the molecular emission profiles with a line-radiative transfer code and compared them to the observed emission profiles. Results. CO is frozen onto the grains in the center of all cores in our sample. The level of CO depletion increases with hydrogen density and ranges from 46% up to more than 95% in the core centers of the three cores with the highest hydrogen density. The average hydrogen density at which 50% of CO is frozen onto the grains is 1.1 ± 0.4 × 10⁵ cm^-3. At about this density, the cores typically have the highest relative abundance of N ₂H⁺. The cores with higher central densities show depletion of N₂H⁺ at levels of 13% to 55%. The chemical ages for the individual species are on average (2 ± 1) × 10 ⁵ yr for ¹³CO, (6 ± 3) × 10⁴ yr for C¹⁸O, and (9 ± 2) × 10⁴ yr for N ₂H⁺. Chemical modeling indirectly suggests that the gas and dust temperatures decouple in the envelopes and that the dust grains are not yet significantly coagulated. Conclusions. We observationally confirm chemical models of CO-freezeout and nitrogen chemistry. We find clear correlations between the hydrogen density and CO depletion and the emergence of N ₂H⁺. The chemical ages indicate a core lifetime of less than 1 Myr.