Potential Variations in the Interstellar N I Abundance

Abstract

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^-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^'₄¹Σ⁺_u-X¹Σ⁺_g 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.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer, which is operated for NASA by the Johns Hopkins University under NASA contract NAS 5-32985, and the NASA/ESA Hubble Space Telescope, obtained from the Multimission Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under the NASA contract NAS 5-26555.