Letter to the Editor

Depletion of ¹⁵N in the center of L1544: Early transition from atomic to molecular nitrogen?

¹ Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennoudai, 305-8577 Tsukuba, Japan
e-mail: furuya@ccs.tsukuba.ac.jp
² Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
³ Tomonaga Center for the History of the Universe, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
⁴ Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
⁵ Department of Astronomy, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
⁶ Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Received: 11 June 2018
Accepted: 7 July 2018

Abstract

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 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 probably not 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 far-ultraviolet (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 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.