2020A&A...635A.188L

Context. The deuterium fractionation in starless cores gives us a clue to estimate their lifetime scales, thus allowing us to distinguish between dynamical theories of core formation. Cores also seem to be subject to a differential N₂ and CO depletion, which was not expected from the models.
Aims. We aim to create a survey of ten cores to estimate their lifetime scales and depletion profiles in detail. After describing L 183, located in Serpens, we present the second cloud of the series, L 1512, from the star-forming region Auriga.
Methods. To constrain the lifetime scale, we performed chemical modeling of the deuteration profiles across L 1512 based on dust extinction measurements from near-infrared observations and nonlocal thermal equilibrium radiative transfer with multiple line observations of N₂H⁺, N₂D⁺, DCO⁺, C¹⁸O, and ¹³CO, plus H₂D⁺ (1₁₀-1₁₁).
Results. We find a peak density of 1.1x10⁵ cm^–3 and a central temperature of 7.5±1K, which are higher and lower, respectively, compared with previous dust emission studies. The depletion factors of N₂H⁺ and N₂D⁺ are 27_–13⁺¹⁷ and 4_–1⁺² in L 1512, which are intermediate between the two other more advanced and denser starless core cases, L 183 and L 1544. These factors also indicate a similar freeze-out of N₂ in L 1512, compared to the two others despite a peak density one to two orders of magnitude lower. Retrieving CO and N₂ abundance profiles with the chemical model, we find that CO has a depletion factor of ∼430-870 and the N₂ profile is similar to that of CO unlike that toward L 183. Therefore, L 1512 has probably been living long enough so that N₂ chemistry has reached steady state.
Conclusions. N₂H⁺ modeling is necessary to assess the precise physical conditions in the center of cold starless cores, rather than dust emission. L 1512 is presumably older than 1.4Myr. Therefore, the dominating core formation mechanism should be ambipolar diffusion for this source.