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

The abundance of interstellar molecular nitrogen (N-2) is of considerable importance: models of steady-state gas-phase interstellar chemistry(1,2), together with millimetre- wavelength observations(3,4) of interstellar N2H+ in dense molecular clouds predict that N-2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet(5) or infrared(6) (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N-2 at far-ultraviolet wavelengths towards the moderately reddened star HD 124314 in the constellation of Centaurus. The N-2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds(1). Moreover, the N-2 abundance does not explain the observed variations(7) 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 incomplete(8).