14NH, 14ND, and 15NH cometary fluorescence models and D/H measurement in NH

Context. NH, a product of dissociation of ammonia, NH3, produces emission lines in the visible spectral range and its (0–0) band has largely been observed in cometary spectra. An updated fluorescence model of NH allows measurement of the abundance of the NH radical. This fluorescence model can also shed light on the ongoing question of isotopic ratios in comets, particularly the D/H ratio. Aims. The aim of this NH fluorescence model is to have an updated model to access the line positions and intensities in order to identify new lines in the observed cometary spectra, compute new fluorescence efficiencies, and measure the isotopic ratios 14ND/14NH and 15NH/14NH. Methods. New laboratory Einstein coefficients from ExoMol enabled new fluorescence models for14NH, 14ND, and 15NH to be computed. To increase the signal-to-noise ratio, we then coadded selected emission lines to measure the isotopic ratios 14NH/14ND and 15NH/14NH in high-resolution spectra of the bright comets C/2002 T7 (LINEAR), C/2006 F6 (Lemmon), and 73P/Schwassmann-Wachmann obtained with UVES at the ESO Very Large Telescope. Results. Comparisons of the modeled spectrum of 14NH and observed comet spectra taken at various heliocentric distances confirmed the accuracy of our model. New fluorescence efficiencies were calculated for the three isotopologs and found to be about 20% higher than in previous studies. We were able to measure an isotopic ratio 14ND/14NH of (2.7 ± 1.8) × 10^‑3 in comet 73P, while no average ND line was detected in the two other comets. This is the first D/H ratio measured in the NH radical and it is in agreement with the 14ND/14NH upper limit of 0.006 found in Comet Hyakutake, and with the ratio (D/H)_NH3 = 1.1 × 10^‑3 measured in Comet 67P during the Rosetta mission. Our measurement confirms that the D/H ratio is about one order of magnitude larger for cometary nitrogen-bearing molecules compared to water. No measurements could be carried out regarding the 15NH/14NH ratio, due to the limited shift between the lines of both isotopologs.