Depletion of 15N in the center of L1544: Early transition from atomic to
  molecular nitrogen?
  We performed sensitive observations of the N15ND+(1-0) and 15NND+(1-0) lines
toward the prestellar core L1544 using the IRAM 30m telescope. The lines are
not detected down to 3 sigma levels in 0.2 km/s channels of around 6 mK. The
non-detection provides the lower limit of the 14N/15N ratio for N2D+ of
~700-800, which is much higher than the elemental abundance ratio in the local
ISM of ~200-300. The result indicates that N2 is depleted in 15N in the central
part of L1544, because N2D+ preferentially traces the cold dense gas, and
because it is a daughter molecule of N2. In-situ chemistry is unlikely
responsible for the 15N depletion in N2; neither low-temperature gas phase
chemistry nor isotope selective photodissociation of N2 explains the 15N
depletion; the former prefers transferring 15N to N2, while the latter requires
the penetration of interstellar FUV photons into the core center. The most
likely explanation is that 15N is preferentially partitioned into ices compared
to 14N via the combination of isotope selective photodissociation of N2 and
grain surface chemistry in the parent cloud of L1544 or in the outer regions of
L1544 which are not fully shielded from the interstellar FUV radiation. The
mechanism is the most efficient at the chemical transition from atomic to
molecular nitrogen. In other words, our result suggests that the gas in the
central part of L1544 already went trough the transition from atomic to
molecular nitrogen in the earlier evolutionary stage, and that N2 is currently
the primary form of gas-phase nitrogen.
