2013A&A...555A..41M

Low-mass prestellar cores are rarely found to be fragmented into smaller condensations, but studying any substructure, where present, is essential for understanding the origin of multiple stellar systems. We attempt to better understand the kinematics and dynamics of the subfragments inside the prestellar core SMM 6 in Orion B9. Another goal of the present study is to constrain the evolutionary stage of the condensations by investigating the levels of CO depletion and deuterium fractionation. We used the APEX telescope to observe the molecular lines C¹⁷O(2-1), N₂H⁺(3-2), and N₂D⁺(3-2) towards the condensations. We used the line data in conjunction with our previous SABOCA 350-µm dust continuum map of the source. The condensations are characterised by subsonic internal non-thermal motions (σ_NT≃0.5c_s), and most of them appear to be gravitationally bound. The dispersion of the N₂H⁺ velocity centroids among the condensations is very low (0.02km/s). The CO depletion factors we derive, f_D=0.8±0.4-3.6±1.5, do not suggest any significant CO freeze-out, but this may be due to the canonical CO abundance we adopt. The fractional abundances of N₂H⁺ and N₂D⁺ with respect to H₂ are found to be ∼0.9-2.3x10^–9 and ∼4.9-9.9x10^–10, respectively. The deuterium fractionation of N₂H⁺ lies in the range 0.30±0.07-0.43±0.09. The detected substructure inside SMM 6 is most likely the result of cylindrical Jeans-type gravitational fragmentation. We estimate the timescale for this fragmentation to be ∼1.8x10⁵yr. The condensations are unlikely to be able to interact with one another and coalesce before local gravitational collapse ensues. Moreover, significant mass growth of the condensations via competitive-like accretion from the parent core seems unfeasible. The high level of molecular deuteration in the condensations suggests that gas-phase CO should be strongly depleted. It also points towards an advanced stage of chemical evolution. The subfragments of SMM 6 might therefore be near the onset of gravitational collapse, but whether they can form protostellar or substellar objects (brown dwarfs) depends on the local star formation efficiency and remains to be clarified.