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 30 m telescope. The lines are not detected down to 3σ levels in 0.2 km s⁻¹ channels of ∼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 interstellar medium of ∼200–300. The result indicates that N₂ is depleted in¹⁵N 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 probably not responsible for the ¹⁵N depletion in N₂; neither low-temperature gas phase chemistry nor isotope selective photodissociation of N₂ explains the¹⁵N depletion; the former prefers transferring¹⁵N to N₂, while the latter requires the penetration of interstellar far-ultraviolet (FUV) photons into the core center. The most likely explanation is that¹⁵N is preferentially partitioned into ices compared to¹⁴N 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 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 has previously gone 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.