Detection of interstellar NH towards ζ Ophiuchi by means of ultra-high-resolution spectroscopy

We have obtained an ultra-high-resolution (0.38 km s−1 FWHM) spectrum of the R1 (0) (3358.053 Å) line of interstellar NH towards ζ Oph. These observations comprise the first detection of NH towards ζ Oph, and the third reported detection of this molecule by absorption-line spectroscopy towards any star. The equivalent width (0.36 ± 0.05 mÅ) corresponds to a column density of 8.8 ± 1.2 × 1011 cm−2. This is more than an order of magnitude higher than that expected on the basis of equilibrium gas-phase models, but is consistent with models incorporating grain-surface chemistry. The line profile is fully resolved, and has an intrinsic velocity dispersion characterized by b = 1.15 ± 0.12 km s−1. This is sufficiently narrow to exclude the possibility of hot NH formation through the endothermic reaction N + H2 → NH + H. There is evidence that the NH line is split into two velocity components (separation 1.26 ± 0.17 km s−1), with relative 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 molecular clouds