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[110.16] The Search for Interstellar N₂

D. C. Knauth, B-G. Andersson, S. R. McCandliss, H. W. Moos (The Johns Hopkins University)

Interstellar N₂ is of considerable interest because both models of steady-state gas-phase chemistry (Viala 1986) and millimeter wave observations of N₂H⁺ (Womack, Ziurys, & Wyckoff 1992) predict that N₂ should be the dominant nitrogen-bearing molecule in interstellar space. However, the search for interstellar N₂ has been unsuccessful. Using far-ultraviolet Copernicus data, Lutz, Snow, & Owen (1979) reported N₂ upper limits of log[N(N₂)] ~ 12.5 toward two lightly reddened lines of sight [E(B-V) \leq 0.16]. More recently, Sandford et al. (2001) report upper limits for N₂ ice absorption using observations from the Infrared Space Observatory.

With the launch of the Far Ultraviolet Spectroscopic Explorer (FUSE), it is now possible to probe denser environments than was possible with Copernicus. We utilized archival FUSE data to search for the strongest ground state transition of N₂ at 958 Å\ toward several moderately-reddened lines of sight [E(B-V) \geq 0.3]. Despite the low signal-to-noise (S/N \leq 20) below 1000 Å, N₂ was not detected to a level of about 10¹⁴ cm^-2. Here, we report our N₂ upper limits that are almost two orders of magnitude lower than predicted by Viala (1986) toward a stellar sample with A_v \lesssim 2. We discuss these findings in the context of the observed N~{\small I} deficiency seen by Knauth et al. (2003). Our results indicate the need for a better understanding of nitrogen chemistry.

This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission. Financial support has been provided by NASA contract NAS5-32985.

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.