tokens: the exact influence of radiation pressure on nitrogen atoms would require a dynamical model, such a low value for β indicates that it should have a negligible effect. Despite these very weak effects from radiation pressure, we do not find an overabundance of nitrogen in the β Pictoris disc. Other papers have also shown no clear deficiency in radiation-sensitive species such as Na and Ca ( Lagrange et al. 1998 ), which also seems to disfavour the preferential depletion scenario. The preferential production scenario, also proposed by Xie et al. (2013) , suggests that the overabundance is the result of gas being produced from materials rich in C and O. This includes photodesorption from C and O-rich icy grains ( Grigorieva et al. 2007 ) and collisional vaporisation of the dust in the disc ( Czechowski Mann 2007 ). Unlike preferential depletion, this scenario does not infer high N abundances and is thus more compatible with the near-solar NI abundance measured here. 5.2 Origin and dynamics of the NI gas The absorption components that we used to model the shape of the NI absorption lines may provide us with hints at both the origin and dynamics of the NI gas. The two absorbing circumstellar components, CS 0 and CS X , were not constrained to particular radial velocity values, but provided the best fit when their radial velocities matched that of the star. This indicates an absence of detectable amounts of NI falling in towards the star. The clearly asymmetric HI Ly- α line presented in Wilson et al. (2017) , on the other hand, indicates that HI gas is falling in towards the star. The HI gas is thought to be originating from the dissociation of water molecules from evaporating exocomets. The fact that we do not see similar asymmetric line profiles in NI is perhaps not surprising. In solar system comets, nitrogen-bearing molecules are only minor constituents ( Eberhardt et al. 1987 ), which means that if β Pic comets are similar, we would not expect the NI lines to be asymmetric. This does not exclude the possibility that NI might originate from exocomets, however, as we argue in Sect. 5.3 . 5.3 Role of exocomets The NI column density measurements are dominated by the narrow absorption feature, CS 0 , which has a line width of km s −1 . With a similar line width observed in FeI ( Vidal-Madjar et al. 2017 ), this absorption feature likely traces a stable band of circumstellar material that orbits β Pic. The origin of the broad absorption feature, CS X , with km s −1 , which absorbs seven times less strongly, but clearly governs much of the absorption line shape, is less clear. The feature might consist of a number of individual absorption components that are beyond the resolving powers of the COS instrument. The broad component could be related to NI absorption lines in the stellar atmosphere of β Pic. This would certainly explain that the bulk velocity matches that of the star. However, we would then expect the lines to be much broader and shallower
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