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We present APEX observations of C17O(2-1), N2H+(3-2), and N2D+(3-2) towardsthe subfragments inside the prestellar core SMM 6 in Orion B9. We combinedthese spectral line data with our previous SABOCA 350-{\mu}m dust continuum mapof the source. The subfragments are characterised by subsonic internalnon-thermal motions ({\sigma}NT~0.5cs), and most of them appear to begravitationally bound. The dispersion of the N2H+ velocity centroids among thecondensations is very low (0.02 km/s). The CO depletion factors we derive,fD=0.8+/-0.4 - 3.6+/-1.5, do not suggest any significant CO freeze-out but thismay be due to the canonical CO abundance we adopt. The fractional abundances ofN2H+ 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+, or theN2D+/N2H+ column density ratio, lies in the range 0.30+/-0.07 - 0.43+/-0.09.The detected substructure inside SMM 6 is likely the result of cylindricalJeans-type gravitational fragmentation. We estimate the timescale for thisfragmentation to be ~1.8x10^5 yr. The condensations are unlikely to be able tointeract with one another and coalesce before local gravitational collapseensues. Moreover, significant mass growth of the condensations viacompetitive-like accretion from the parent core seems unfeasible. The highlevel of molecular deuteration in the condensations suggests that gas-phase COshould be strongly depleted. It also points towards an advanced stage ofchemical evolution. The subfragments of SMM 6 might therefore be near the onsetof gravitational collapse, but whether they can form protostellar or substellarobjects (brown dwarfs) depends on the local star formation efficiency andremains to be clarified.

Dynamics, CO depletion, and deuterium fractionation of the dense condensations within the fragmented prestellar core Orion B9-SMM 6