DETECTION OF INTERSTELLAR NH TOWARDS ZETA-OPHIUCHI BY MEANS OF ULTRA-HIGH-RESOLUTION SPECTROSCOPY

We have obtained an ultra-high-resolution (0.38 km s(-1) FWHM) spectru m of the R-1(0) (3358.053 Angstrom) line of interstellar NH towards ze ta Oph. These observations comprise the first detection of NH towards zeta Oph, and the third reported detection of this molecule by absorpt ion-line spectroscopy towards any star. The equivalent width (0.36 +/- 0.05 m Angstrom) corresponds to a column density of 8.8 +/- 1.2 x 10( 11) cm(-2). This is more than an order of magnitude higher than that e xpected on the basis of equilibrium gas-phase models, but is consisten t with models incorporating grain-surface chemistry. The line profile is fully resolved, and has an intrinsic velocity dispersion characteri zed by b = 1.15 +/- 0.12 km s(-1) This is sufficiently narrow to exclu de the possibility of hot NH formation through the endothermic reactio n N + H-2 --> NH + H. There is evidence that the NH Line is split into two velocity components (separation 1.26 +/- 0.17 km s(-1)), with rel ative strengths mirroring those of the two CN components towards this star. We argue that this correlation also supports the case for grain surface reactions dominating the nitrogen chemistry of diffuse molecul ar clouds.