Dynamics, CO depletion, and deuterium fractionation of the dense
  condensations within the fragmented prestellar core Orion B9-SMM 6
  We present APEX observations of C17O(2-1), N2H+(3-2), and N2D+(3-2) towards
the subfragments inside the prestellar core SMM 6 in Orion B9. We combined
these spectral line data with our previous SABOCA 350-{\mu}m dust continuum map
of the source. The subfragments are characterised by subsonic internal
non-thermal motions ({\sigma}NT~0.5cs), and most of them appear to be
gravitationally bound. The dispersion of the N2H+ velocity centroids among the
condensations 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 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+, or the
N2D+/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 cylindrical
Jeans-type gravitational fragmentation. We estimate the timescale for this
fragmentation to be ~1.8x10^5 yr. 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.
