2003ApJ...596L..51K

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph observations of the weak interstellar N I λ1160 doublet toward 17 high-density sight lines [N(H_tot)≥10²¹ cm^–2]. When combined with published data, our results reveal variations in the fractional N I abundance showing a systematic deficiency at large N(H_tot). At the FUSE resolution (∼20 km/s), the effects of unresolved saturation cannot be conclusively ruled out, although O I λ1356 shows little evidence of saturation. We investigated the possibility that the N I variability is due to the formation of N₂in our mostly dense regions. The 0-0 band of the c'₄¹Σ⁺_u-X¹Σ⁺_gtransition of N₂at 958 Å should be easily detected in our FUSE data; for 10 of the denser sight lines, N₂is not observed at a sensitivity level of a few times 10¹⁴ cm^–2. The observed N I variations are suggestive of an incomplete understanding of nitrogen chemistry.