The interstellar N2 abundance towards HD 124314 from far-ultraviolet observations

Abstract

The abundance of interstellar molecular nitrogen (N2) is of considerable importance: models of steady-state gas-phase interstellar chemistry1,2, together with millimetre-wavelength observations3,4 of interstellar N2H+ in dense molecular clouds predict that N2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet5 or infrared6 (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N2 at far-ultraviolet wavelengths towards the moderately reddened star HD 124314 in the constellation of Centaurus. The N2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds1. Moreover, the N2 abundance does not explain the observed variations7 in the abundance of atomic nitrogen (N I) towards high-column-density sightlines, implying that the models of nitrogen chemistry in the interstellar medium are incomplete8.

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Figure 1: The N2 portion of the normalized FUSE spectra of HD 124314.
Figure 2: The normalized archival STIS/HST spectra of O I, S I and CO.

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Acknowledgements

We thank A. Fullerton, S. R. Federman, P. Feldman, E. B. Jenkins, P. Sonnentrucker and P. Wannier for discussions and G. Stark for sharing his unpublished N2 f-values. This work is based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer, which is operated for NASA by Johns Hopkins University.

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Correspondence to David C. Knauth.

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Supplementary information

Supplementary Figure

Shows the N2 portion for three O6 stars: HD 124314, HD 39680 [E(B-V)=0.34], and HD 165052 [E(B-V)=0.42]. (DOC 319 kb)

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Knauth, D., Andersson, B., McCandliss, S. et al. The interstellar N2 abundance towards HD 124314 from far-ultraviolet observations. Nature 429, 636–638 (2004). https://doi.org/10.1038/nature02614

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