Depletion of heavy nitrogen in the cold gas of star-forming regions
  We investigate nitrogen isotope fractionation in forming and evolving
molecular clouds using gas-ice astrochemical simulations. We find that the bulk
gas can become depleted in heavy nitrogen (15N) due to the formation of
15N-enriched ices. Around the chemical transition from atomic nitrogen to N2,
N15N is selectively photodissociated, which results in the enrichment of 15N in
atomic nitrogen. As 15N-enriched atomic nitrogen is converted to ammonia ice
via grain surface reactions, the bulk gas is depleted in 15N. The level of 15N
depletion in the bulk gas can be up to a factor of two compared to the
elemental nitrogen isotope ratio, depending on the photodesorption yield of
ammonia ice. Once the nitrogen isotopes are differentially partitioned between
gas and solids in a molecular cloud, it should remain in the later stages of
star formation (e.g., prestellar core) as long as the sublimation of ammonia
ice is inefficient. Our model suggests that all the N-bearing molecules in the
cold gas of star-forming regions can be depleted in 15N, which is at least
qualitatively consistent with the observations toward prestellar core L1544. In
our models, icy species show both 15N and deuterium fractionation. The
fractionation pattern within ice mantles is different between 15N and
deuterium, reflecting their fractionation mechanisms; while the concentration
of deuterium almost monotonically increases from the lower layers of the ice
mantles to the upper layers, the concentration of 15N reaches the maximum at a
certain depth and declines towards the surface.
