Depletion of $^15$N in the center of L1544: Early transition from atomic to molecular nitrogen?
Furuya, K., Watanabe, Y., Sakai, T., Aikawa, Y., & Yamamoto, S.
We performed sensitive observations of the N$^15$ND$^+$(1-0) and $^15$NND$^+$(1-0) lines toward the prestellar core L1544 using the IRAM 30 m telescope. The lines are not detected down to 3\ensuremathσ levels in 0.2 km s$^-$1 channels of 6 mK. The non-detection provides the lower limit of the $^14$N/$^15$N ratio for N$_2$D$^+$ of 700-800, which is much higher than the elemental abundance ratio in the local interstellar medium of 200-300. The result indicates that N$_2$ is depleted in $^15$N in the central part of L1544, because N$_2$D$^+$ preferentially traces the cold dense gas, and because it is a daughter molecule of N$_2$. In situ chemistry is probably not responsible for the $^15$N depletion in N$_2$; neither low-temperature gas phase chemistry nor isotope selective photodissociation of N$_2$ explains the $^15$N depletion; the former prefers transferring $^15$N to N$_2$, while the latter requires the penetration of interstellar far-ultraviolet (FUV) photons into the core center. The most likely explanation is that $^15$N is preferentially partitioned into ices compared to $^14$N via the combination of isotope selective photodissociation of N$_2$ 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 N$_2$ is currently the primary form of gas-phase nitrogen.