Potential Variations in the Interstellar N I Abundance

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(Htot) ≥ 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(Htot). At the FUSE resolution (~20 km s-1), 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 N2 in our mostly dense regions. The 0-0 band of the c 1Σ-X 1Σ transition of N2 at 958 Å should be easily detected in our FUSE data; for 10 of the denser sight lines, N2 is 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.