tokens: of the blue curves with the (ortho+para) HO abundance profile deduced by Keto et al. (2014) (black curve in Fig. 3 ) shows that the total NH is about 30 times more abundant than HO within the central 1000 AU, while it is two orders of magnitude less abundant than HO from about 8000 AU to 20 000 AU. Fig. 4 MOLLIE radiative transfer results using the o-NH fractional abundance profiles from Fig. 3 , overlapped with the observed spectrum: ( top ) blue solid and dashed curves refer to the Crapsi et al. (2007) profiles, assuming an ortho-to-para NH ratio of 0.7 and 1.0, respectively. ( Bottom ) The red thick curve refers to the Sipilä et al. (2015a , b) profile; the red thin curve is the same as the thick red curve, but divided by 2. The o-NH profile has a very different shape when the chemical model of Sipilä et al. (2015a , b) , which includes spin-state chemistry, is considered (red curve in Fig. 3 ). The chemical abundance gradients in L1544 were simulated using the same procedure as described in Sipilä et al. (2016) : the physical model for L1544 ( Keto Caselli 2010 ; Keto et al. 2014 ) was separated into concentric shells and the chemical evolution was calculated separately in each shell. The gas was initially atomic with the exception of H (all in H) and D (in HD). The o-NH abundance profile (Fig. 3 ) was extracted from the model at the time when the CO column density was comparable to the observed value (; see Caselli et al. 1999 ), taking into account telescope beam convolution. Other details of the chemical model are extensively discussed in Sipilä et al. (2015a , b) . The predicted o-NH profile (red curve in Fig. 3 ) presents a broad peak between 5000 AU and 15 000 AU, it drops at larger radii because of photodissociation and at lower radii because of freeze-out. The o-NH() spectra simulated with MOLLIE are shown in Fig. 4 for the three o-NH abundance profiles in Fig. 3 , together with the observed spectrum. This figure shows that the empirical abundance profile of Crapsi et al. (2007) reproduces the observed spectrum better than the abundance profile predicted by Sipilä et al. (2015a , b) , although the simulated spectrum underestimates the brightness temperature at the lowest velocities of all groups of hyperfine components, as also found in the case of o-HO ( Caselli et al. 2012 ), which suggests that the contraction velocity within the core centre is slightly higher than the velocity used in our model. The red spectrum (based on the chemical model of Sipilä et al. 2015a , b ) is significantly brighter than the detected line and the ratios between the low- and high-velocity peaks of the various hyperfine components are not well reproduced (as better shown by the red thin curve in Fig. 4 , which is the red thick profile scaled down by a factor of two). It is interesting to note that the red curve is shifted to lower velocities than the blue curves. This is due to the large abundance of NH predicted at radii in the range of 400012
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